walkingsearch.h File Reference

Header file for particle location functions using the walking search algorithm. More...

#include <petsc.h>
#include <stdbool.h>
#include <math.h>
#include "variables.h"
#include "logging.h"
#include "setup.h"

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Functions
PetscErrorCode	GetCellCharacteristicSize (const Cell *cell, PetscReal *cellSize)
	Estimates a characteristic length of the cell for threshold scaling.
PetscErrorCode	ComputeSignedDistanceToPlane (const Cmpnts v1, const Cmpnts v2, const Cmpnts v3, const Cmpnts v4, const Cmpnts cell_centroid, const Cmpnts p_target, PetscReal *d_signed, const PetscReal threshold)
	Computes the signed distance from a point to the plane approximating a quadrilateral face.
PetscErrorCode	CalculateDistancesToCellFaces (const Cmpnts p, const Cell *cell, PetscReal *d, const PetscReal threshold)
	Computes the signed distances from a point to each face of a cubic cell.
PetscErrorCode	DeterminePointPosition (PetscReal *d, PetscInt *result)
	Classifies a point based on precomputed face distances.
PetscErrorCode	GetCellVerticesFromGrid (Cmpnts ***coor, PetscInt idx, PetscInt idy, PetscInt idz, Cell *cell)
	Retrieves the coordinates of the eight vertices of a cell based on grid indices.
PetscErrorCode	InitializeTraversalParameters (UserCtx *user, Particle *particle, PetscInt *idx, PetscInt *idy, PetscInt *idz, PetscInt *traversal_steps)
	Initializes traversal parameters for locating a particle.
PetscErrorCode	CheckCellWithinLocalGrid (UserCtx *user, PetscInt idx, PetscInt idy, PetscInt idz, PetscBool *is_within)
	Checks if the current cell indices are within the local grid boundaries.
PetscErrorCode	RetrieveCurrentCell (UserCtx *user, PetscInt idx, PetscInt idy, PetscInt idz, Cell *cell)
	Retrieves the coordinates of the eight vertices of the current cell.
PetscErrorCode	EvaluateParticlePosition (const Cell *cell, PetscReal *d, const Cmpnts p, PetscInt *position, const PetscReal threshold)
	Determines the spatial relationship of a particle relative to a cubic cell.
PetscErrorCode	LocateParticleInGrid (UserCtx *user, Particle *particle)
	Locates the grid cell containing a particle using a walking search.
PetscErrorCode	UpdateCellIndicesBasedOnDistances (PetscReal d[NUM_FACES], PetscInt *idx, PetscInt *idy, PetscInt *idz, DMDALocalInfo *info)
	Updates the cell indices based on the signed distances to each face.
PetscErrorCode	FinalizeTraversal (UserCtx *user, Particle *particle, PetscInt traversal_steps, PetscBool cell_found, PetscInt idx, PetscInt idy, PetscInt idz)
	Finalizes the traversal by reporting the results.
PetscErrorCode	FindOwnerOfCell (UserCtx *user, PetscInt i, PetscInt j, PetscInt k, PetscMPIInt *owner_rank)
	Finds the MPI rank that owns a given global cell index.
PetscErrorCode	LocateParticleOrFindMigrationTarget_TEST (UserCtx *user, Particle *particle, ParticleLocationStatus *status_out)
	Locates a particle's host cell or identifies its migration target using a robust walk search.
PetscErrorCode	ReportSearchOutcome (const Particle *particle, ParticleLocationStatus status, PetscInt traversal_steps)
	Logs the final outcome of the particle location search.
PetscErrorCode	UpdateCellIndicesBasedOnDistancesTEST (PetscReal d[NUM_FACES], PetscInt *idx, PetscInt *idy, PetscInt *idz)
	Updates the cell indices based on the signed distances to each face.

Detailed Description

Header file for particle location functions using the walking search algorithm.

This file contains declarations of functions that implement the walking search algorithm to locate particles within a computational grid. It includes functions for computing distances to cell faces, determining particle positions relative to cells, and updating traversal parameters.

Definition in file walkingsearch.h.

Function Documentation

GetCellCharacteristicSize()

PetscErrorCode GetCellCharacteristicSize	(	const Cell *	cell,
		PetscReal *	cellSize
	)

Estimates a characteristic length of the cell for threshold scaling.

For a hexahedral cell with vertices cell->vertices[0..7], we approximate the cell size by some measure—e.g. average edge length or diagonal.

Parameters

[in]	cell	A pointer to the Cell structure
[out]	cellSize	A pointer to a PetscReal where the characteristic size is stored.

Returns

PetscErrorCode 0 on success, non-zero on failure.

Definition at line 25 of file walkingsearch.c.

{
    PetscErrorCode ierr = 0;
    if (!cell || !cellSize) SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, 
        "GetCellCharacteristicSize: Null pointer(s).");
 
    // A simple approach: compute the average of the distances between
    // all pairs of adjacent vertices that share an edge. That gives
    // a typical measure of cell dimension. For a uniform grid, you
    // could do something simpler.
 
    PetscInt edges[12][2] = {
        {0,1},{1,2},{2,3},{3,0},  // bottom face edges
        {4,5},{5,6},{6,7},{7,4},  // top face edges
        {0,7},{1,6},{2,5},{3,4}   // vertical edges
    };
 
    PetscReal totalEdgeLen = 0.0;
    for (PetscInt i=0; i<12; i++) {
        PetscInt vA = edges[i][0];
        PetscInt vB = edges[i][1];
        Cmpnts A = cell->vertices[vA];
        Cmpnts B = cell->vertices[vB];
        PetscReal dx = B.x - A.x;
        PetscReal dy = B.y - A.y;
        PetscReal dz = B.z - A.z;
        PetscReal edgeLen = sqrt(dx*dx + dy*dy + dz*dz);
        totalEdgeLen += edgeLen;
    }
 
    // Average edge length
    *cellSize = totalEdgeLen / 12.0;
 
    return ierr;
}

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ComputeSignedDistanceToPlane()

PetscErrorCode ComputeSignedDistanceToPlane	(	const Cmpnts	v1,
		const Cmpnts	v2,
		const Cmpnts	v3,
		const Cmpnts	v4,
		const Cmpnts	cell_centroid,
		const Cmpnts	p_target,
		PetscReal *	d_signed,
		const PetscReal	threshold
	)

Computes the signed distance from a point to the plane approximating a quadrilateral face.

This function calculates the signed distance from a given point p_target to the plane approximating a quadrilateral face defined by points v1, v2, v3, and v4. The plane's initial normal is determined using two edge vectors emanating from v1 (i.e., v2-v1 and v4-v1).

Normal Orientation: The initial normal's orientation is checked against the cell's interior. A vector from the face centroid to the cell centroid (vec_face_to_cell_centroid) is computed. If the dot product of the initial normal and vec_face_to_cell_centroid is positive, it means the initial normal is pointing towards the cell's interior (it's an inward normal relative to the cell). In this case, the normal is flipped to ensure it points outward.

Signed Distance Convention: The signed distance is the projection of the vector from p_target to the face's centroid onto the (now guaranteed) outward-pointing plane's normal vector.

• d_signed < 0: p_target is "outside" and "beyond" the face, in the direction of the outward normal. This is the primary case indicating the particle has crossed this face.
• d_signed > 0: p_target is "outside" but "behind" the face (on the same side as the cell interior relative to this face plane).
• d_signed = 0: p_target is on the face plane (within threshold).

Parameters

[in]	v1	First vertex defining the face (used as origin for initial normal calculation).
[in]	v2	Second vertex defining the face (used for first edge vector: v2-v1).
[in]	v3	Third vertex defining the face (used for face centroid calculation).
[in]	v4	Fourth vertex defining the face (used for second edge vector: v4-v1).
[in]	cell_centroid	The centroid of the 8-vertex cell to which this face belongs.
[in]	p_target	The point from which the distance to the plane is calculated.
[out]	d_signed	Pointer to store the computed signed distance.
[in]	threshold	The threshold below which the absolute distance is considered zero.

Note

• The order of vertices v1, v2, v3, v4 is used for calculating the face centroid. The order of v1, v2, v4 is used for the initial normal calculation.
• The cell_centroid is crucial for robustly determining the outward direction of the normal.

Returns

PetscErrorCode Returns 0 on success, PETSC_ERR_ARG_NULL if d_signed is NULL, or PETSC_ERR_USER if a degenerate plane (near-zero normal) is detected.

Definition at line 102 of file walkingsearch.c.

{
    PetscErrorCode ierr = 0;
    PetscMPIInt rank;
 
    PetscFunctionBeginUser;
 
    if (d_signed == NULL) {
        ierr = MPI_Comm_rank(MPI_COMM_WORLD, &rank); CHKERRQ(ierr);
        LOG_ALLOW(LOCAL, LOG_ERROR, "ComputeSignedDistanceToPlane - Output pointer 'd_signed' is NULL on rank %d.\n", rank);
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "Output pointer 'd_signed' must not be NULL.");
    }
 
    LOG_ALLOW(LOCAL, LOG_DEBUG, "ComputeSignedDistanceToPlane - Target point: (%.6e, %.6e, %.6e)\n", p_target.x, p_target.y, p_target.z);
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Cell Centroid: (%.6e, %.6e, %.6e)\n", cell_centroid.x, cell_centroid.y, cell_centroid.z);
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Face Vertices:\n");
    LOG_ALLOW(LOCAL, LOG_DEBUG, "    v1: (%.6e, %.6e, %.6e)\n", v1.x, v1.y, v1.z);
    LOG_ALLOW(LOCAL, LOG_DEBUG, "    v2: (%.6e, %.6e, %.6e)\n", v2.x, v2.y, v2.z);
    LOG_ALLOW(LOCAL, LOG_DEBUG, "    v3: (%.6e, %.6e, %.6e)\n", v3.x, v3.y, v3.z);
    LOG_ALLOW(LOCAL, LOG_DEBUG, "    v4: (%.6e, %.6e, %.6e)\n", v4.x, v4.y, v4.z);
 
    // --- Calculate Edge Vectors for Initial Normal Computation ---
    PetscReal edge1_x = v2.x - v1.x;
    PetscReal edge1_y = v2.y - v1.y;
    PetscReal edge1_z = v2.z - v1.z;
 
    PetscReal edge2_x = v4.x - v1.x;
    PetscReal edge2_y = v4.y - v1.y;
    PetscReal edge2_z = v4.z - v1.z;
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Edge1 (v2-v1): (%.6e, %.6e, %.6e)\n", edge1_x, edge1_y, edge1_z);
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Edge2 (v4-v1): (%.6e, %.6e, %.6e)\n", edge2_x, edge2_y, edge2_z);
 
    // --- Compute Initial Normal Vector (Cross Product: edge1 x edge2) ---
    PetscReal normal_x_initial = edge1_y * edge2_z - edge1_z * edge2_y;  
    PetscReal normal_y_initial = edge1_z * edge2_x - edge1_x * edge2_z;
    PetscReal normal_z_initial = edge1_x * edge2_y - edge1_y * edge2_x;
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Initial Raw Normal (edge1 x edge2): (%.6e, %.6e, %.6e)\n", normal_x_initial, normal_y_initial, normal_z_initial);
 
    PetscReal normal_magnitude = sqrt(normal_x_initial * normal_x_initial +
                                   normal_y_initial * normal_y_initial +
                                   normal_z_initial * normal_z_initial);
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Initial Normal Magnitude: %.6e\n", normal_magnitude);
 
    if (normal_magnitude < 1.0e-12) {
        ierr = MPI_Comm_rank(MPI_COMM_WORLD, &rank); CHKERRQ(ierr);
        LOG_ALLOW(LOCAL, LOG_ERROR,
                  "ComputeSignedDistanceToPlane - Degenerate plane detected on rank %d. Normal magnitude (%.3e) is too small.\n",
                  rank, normal_magnitude);
        LOG_ALLOW(LOCAL, LOG_ERROR, "  Offending vertices for normal: v1(%.3e,%.3e,%.3e), v2(%.3e,%.3e,%.3e), v4(%.3e,%.3e,%.3e)\n",
                  v1.x,v1.y,v1.z, v2.x,v2.y,v2.z, v4.x,v4.y,v4.z);
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_USER, "Degenerate plane detected (normal vector is near zero).");
    }
 
    // --- Calculate the Centroid of the Four Face Vertices (v1, v2, v3, v4) ---    
    PetscReal face_centroid_x = 0.25 * (v1.x + v2.x + v3.x + v4.x);
    PetscReal face_centroid_y = 0.25 * (v1.y + v2.y + v3.y + v4.y);
    PetscReal face_centroid_z = 0.25 * (v1.z + v2.z + v3.z + v4.z);
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Face Centroid: (%.6e, %.6e, %.6e)\n", face_centroid_x, face_centroid_y, face_centroid_z);
 
    // --- Orient the Normal to Point Outward from the Cell ---
    // Vector from face centroid to cell centroid
    PetscReal vec_fc_to_cc_x = cell_centroid.x - face_centroid_x;
    PetscReal vec_fc_to_cc_y = cell_centroid.y - face_centroid_y;
    PetscReal vec_fc_to_cc_z = cell_centroid.z - face_centroid_z;
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Vec (FaceCentroid -> CellCentroid): (%.6e, %.6e, %.6e)\n", vec_fc_to_cc_x, vec_fc_to_cc_y, vec_fc_to_cc_z);
 
    // Dot product of initial normal with vector from face centroid to cell centroid
    PetscReal dot_prod_orientation = normal_x_initial * vec_fc_to_cc_x +
                                     normal_y_initial * vec_fc_to_cc_y +
                                     normal_z_initial * vec_fc_to_cc_z;
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Dot Product for Orientation (N_initial . Vec_FC_to_CC): %.6e\n", dot_prod_orientation);
 
    PetscReal normal_x = normal_x_initial;
    PetscReal normal_y = normal_y_initial;
    PetscReal normal_z = normal_z_initial;
 
    // If dot_prod_orientation > 0, initial normal points towards cell interior, so flip it.
    if (dot_prod_orientation > 1.0e-9) { // Use a small epsilon to avoid issues if dot product is extremely close to zero
        normal_x = -normal_x_initial;
        normal_y = -normal_y_initial;
        normal_z = -normal_z_initial;
        LOG_ALLOW(LOCAL, LOG_DEBUG, "  Initial normal was inward (dot_prod > 0). Flipped normal.\n");
    } else if (dot_prod_orientation == 0.0 && normal_magnitude > 1e-12) {
        // This case is ambiguous or face plane contains cell centroid.
        // This might happen for highly symmetric cells or if face_centroid IS cell_centroid (e.g. 2D cell).
        // For now, we keep the original normal direction based on v1,v2,v4 ordering.
        // A more robust solution for this edge case might be needed if it occurs often.
        ierr = MPI_Comm_rank(MPI_COMM_WORLD, &rank); CHKERRQ(ierr);
         LOG_ALLOW(LOCAL, LOG_WARNING, "ComputeSignedDistanceToPlane - Rank %d: Dot product for normal orientation is zero. Normal direction might be ambiguous. Keeping initial normal direction from (v2-v1)x(v4-v1).\n", rank);
    }
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Oriented Raw Normal: (%.6e, %.6e, %.6e)\n", normal_x, normal_y, normal_z);
 
 
    // --- Normalize the (Now Outward-Pointing) Normal Vector ---
    // Note: normal_magnitude was calculated from initial normal.
    // If we flipped, magnitude is the same.
    normal_x /= normal_magnitude;
    normal_y /= normal_magnitude;
    normal_z /= normal_magnitude;
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Normalized Outward Normal: (%.6f, %.6f, %.6f)\n", normal_x, normal_y, normal_z);
 
 
    // --- Compute Vector from Target Point to Face Centroid ---
    PetscReal vec_p_to_fc_x = face_centroid_x - p_target.x;
    PetscReal vec_p_to_fc_y = face_centroid_y - p_target.y;
    PetscReal vec_p_to_fc_z = face_centroid_z - p_target.z;
 
    // --- Compute Signed Distance ---
    *d_signed = vec_p_to_fc_x * normal_x +
                vec_p_to_fc_y * normal_y +
                vec_p_to_fc_z * normal_z;
 
    LOG_ALLOW(LOCAL, LOG_DEBUG, "  Raw Signed Distance (using outward normal): %.15e\n", *d_signed);
 
    // --- Apply Threshold ---
    if (fabs(*d_signed) < threshold) {
        LOG_ALLOW(LOCAL, LOG_DEBUG, "  Distance %.15e is less than threshold %.1e. Setting to 0.0.\n", *d_signed, threshold);
        *d_signed = 0.0;
    }
 
    LOG_ALLOW(LOCAL, LOG_DEBUG, "ComputeSignedDistanceToPlane - Final Signed Distance: %.15e\n", *d_signed);
 
    PetscFunctionReturn(0);
}

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CalculateDistancesToCellFaces()

PetscErrorCode CalculateDistancesToCellFaces	(	const Cmpnts	p,
		const Cell *	cell,
		PetscReal *	d,
		const PetscReal	threshold
	)

Computes the signed distances from a point to each face of a cubic cell.

This function calculates the signed distances from a specified point p to each of the six faces of a cubic cell. The cell is defined by its eight vertices, and the distances are stored in the memory location pointed to by d.

Parameters

[in]	p	The target point in 3D space.
[in]	cell	Pointer to a Cell structure that defines the cubic cell via its vertices.
[out]	d	A pointer to an array of six PetscReal values where the computed signed distances will be stored.
[in]	threshold	A PetscReal value that specifies the minimum distance below which a computed distance is considered to be zero.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

This function calculates the signed distances from a specified point p to each of the six faces of a cubic cell. The cell is defined by its eight vertices, and the distances are stored in the memory location pointed to by d. Each distance corresponds to a specific face of the cell, as enumerated by the Face enumeration. A threshold value is used to determine when a distance should be considered effectively zero, accounting for floating-point precision limitations.

Parameters

[in]	p	The target point in 3D space for which distances to the cell's faces are to be calculated. Represented by the Cmpnts structure.
[in]	cell	A pointer to a Cell structure that defines the cubic cell via its eight vertices. The vertices must be ordered consistently to ensure correct normal directions for each face.
[out]	d	A pointer to an array of six PetscReal values where the computed signed distances will be stored. Each index in the array corresponds to a specific face as defined by the Face enumeration: d[LEFT]: Distance to the Left Face of the cell. d[RIGHT]: Distance to the Right Face of the cell. d[BOTTOM]: Distance to the Bottom Face of the cell. d[TOP]: Distance to the Top Face of the cell. d[FRONT]: Distance to the Front Face of the cell. d[BACK]: Distance to the Back Face of the cell.
[in]	threshold	A PetscReal value that specifies the minimum distance below which a computed distance is considered to be zero. This helps in mitigating issues arising from floating-point arithmetic inaccuracies.

Returns

PetscErrorCode Returns 0 if the function executes successfully. If an error occurs, a non-zero error code is returned, indicating the type of failure.

Note

• The vertices of the cell must be ordered in a counter-clockwise manner when viewed from outside the cell. This ordering ensures that the normal vectors computed for each face point outward, which is essential for correctly determining the sign of the distances.
• All four vertices defining a face must lie on a non-degenerate (i.e., non-flat) plane. Degenerate planes can lead to undefined normal vectors and incorrect distance calculations.
• The threshold parameter provides flexibility in handling cases where the point p is extremely close to a face, allowing the user to define what constitutes "close enough" to be treated as zero distance based on the specific requirements of their application.

Definition at line 271 of file walkingsearch.c.

{
    PetscErrorCode ierr;
    // Validate that the 'cell' pointer is not NULL to prevent dereferencing a null pointer.
    if (cell == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "CalculateDistancesToCellFaces - 'cell' is NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "CalculateDistancesToCellFaces - 'cell' is NULL.");
    }
 
    // Validate that the 'd' pointer is not NULL to ensure there is memory allocated for distance storage.
    if (d == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "CalculateDistancesToCellFaces - 'd' is NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "CalculateDistancesToCellFaces - 'd' is NULL.");
    }
 
    // Compute the centroid of the entire cell
    Cmpnts cell_centroid = {0.0, 0.0, 0.0};
    for (int i = 0; i < 8; ++i) {
      cell_centroid.x += cell->vertices[i].x;
      cell_centroid.y += cell->vertices[i].y;
      cell_centroid.z += cell->vertices[i].z;
    }
    cell_centroid.x /= 8.0;
    cell_centroid.y /= 8.0;
    cell_centroid.z /= 8.0;
 
    LOG_ALLOW(LOCAL,LOG_DEBUG, "CalculateDistancesToCellFaces - Cell Centroid: (%.6e, %.6e, %.6e)\n",
          cell_centroid.x, cell_centroid.y, cell_centroid.z);    
 
    
    // Compute the signed distance from point 'p' to the BACK face of the cell.
    // The BACK face is defined by vertices 0, 3, 2, and 1, with its normal vector pointing in the -z direction.
    ierr = ComputeSignedDistanceToPlane(
        cell->vertices[0], // Vertex 0
        cell->vertices[3], // Vertex 3
        cell->vertices[2], // Vertex 2
        cell->vertices[1], // Vertex 1
    cell_centroid,     // Cell centroid 
        p,                  // Target point
        &d[BACK],           // Storage location for the BACK face distance
        threshold           // Threshold for zero distance
    );  CHKERRQ(ierr);
 
    // Compute the signed distance from point 'p' to the FRONT face of the cell.
    // The FRONT face is defined by vertices 4, 7, 6, and 5, with its normal vector pointing in the +z direction.
    ierr = ComputeSignedDistanceToPlane(
        cell->vertices[4], // Vertex 4
        cell->vertices[7], // Vertex 7
        cell->vertices[6], // Vertex 6
        cell->vertices[5], // Vertex 5
    cell_centroid,     // Cell centroid 
        p,                  // Target point
        &d[FRONT],          // Storage location for the FRONT face distance
        threshold           // Threshold for zero distance
    );  CHKERRQ(ierr);
 
    // Compute the signed distance from point 'p' to the BOTTOM face of the cell.
    // The BOTTOM face is defined by vertices 0, 1, 6, and 7, with its normal vector pointing in the -y direction.
    ierr = ComputeSignedDistanceToPlane(
        cell->vertices[0], // Vertex 0
        cell->vertices[1], // Vertex 1
        cell->vertices[6], // Vertex 6
        cell->vertices[7], // Vertex 7
    cell_centroid,     // Cell centroid 
        p,                  // Target point
        &d[BOTTOM],         // Storage location for the BOTTOM face distance
        threshold           // Threshold for zero distance
    );  CHKERRQ(ierr);
 
    // Compute the signed distance from point 'p' to the TOP face of the cell.
    // The TOP face is defined by vertices 3, 4, 5, and 2, with its normal vector pointing in the +y direction.
    ierr = ComputeSignedDistanceToPlane(
        cell->vertices[3], // Vertex 3
        cell->vertices[4], // Vertex 4
        cell->vertices[5], // Vertex 5
        cell->vertices[2], // Vertex 2
    cell_centroid,     // Cell centroid 
    p,                  // Target point
        &d[TOP],            // Storage location for the TOP face distance
        threshold           // Threshold for zero distance
    );  CHKERRQ(ierr);
 
    // Compute the signed distance from point 'p' to the LEFT face of the cell.
    // The LEFT face is defined by vertices 0, 7, 4, and 3, with its normal vector pointing in the -x direction.
    ierr = ComputeSignedDistanceToPlane(
        cell->vertices[0], // Vertex 0
        cell->vertices[7], // Vertex 7
        cell->vertices[4], // Vertex 4
        cell->vertices[3], // Vertex 3
    cell_centroid,     // Cell centroid 
    p,                  // Target point
        &d[LEFT],           // Storage location for the LEFT face distance
        threshold           // Threshold for zero distance
    );  CHKERRQ(ierr);
 
    // Compute the signed distance from point 'p' to the RIGHT face of the cell.
    // The RIGHT face is defined by vertices 1, 2, 5, and 6, with its normal vector pointing in the +x direction.
    ierr = ComputeSignedDistanceToPlane(
        cell->vertices[1], // Vertex 1
        cell->vertices[2], // Vertex 2
        cell->vertices[5], // Vertex 5
        cell->vertices[6], // Vertex 6
    cell_centroid,     // Cell centroid 
        p,                  // Target point
        &d[RIGHT],          // Storage location for the RIGHT face distance
        threshold           // Threshold for zero distance
    );  CHKERRQ(ierr);
 
    LOG_ALLOW(LOCAL, LOG_DEBUG, "CalculateDistancesToCellFaces - Populated d: "
          "d[LEFT=%d]=%.3e, d[RIGHT=%d]=%.3e, d[BOTTOM=%d]=%.3e, "
          "d[TOP=%d]=%.3e, d[FRONT=%d]=%.3e, d[BACK=%d]=%.3e\n",
          LEFT, d[LEFT], RIGHT, d[RIGHT], BOTTOM, d[BOTTOM],
          TOP, d[TOP], FRONT, d[FRONT], BACK, d[BACK]);
    LOG_ALLOW(LOCAL, LOG_DEBUG, "CalculateDistancesToCellFaces - Raw d: "
          "[%.3e, %.3e, %.3e, %.3e, %.3e, %.3e]\n",
          d[0], d[1], d[2], d[3], d[4], d[5]);
    
    return 0; // Indicate successful execution of the function.
}

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DeterminePointPosition()

PetscErrorCode DeterminePointPosition	(	PetscReal *	d,
		PetscInt *	result
	)

Classifies a point based on precomputed face distances.

Given an array of six distances d[NUM_FACES] from the point to each face of a hexahedral cell, this function determines: 0 => inside 1 => on a single face 2 => on an edge (2 faces = 0) 3 => on a corner (3+ faces = 0) -1 => outside

Parameters

[in]	d	Array of six face distances.
[out]	result	Pointer to an integer classification: {0,1,2,3,-1}.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

Definition at line 407 of file walkingsearch.c.

{
    
 
    // Validate input pointers
    if (d == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "DeterminePointPosition - 'd' is NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "DeterminePointPosition - Input parameter 'd' is NULL.");
    }
    if (result == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "DeterminePointPosition - 'result' is NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "DeterminePointPosition - Output parameter 'result' is NULL.");
    }
 
    // Initialize flags
    PetscBool isInside = PETSC_TRUE;
    PetscBool isOnBoundary = PETSC_FALSE;
    PetscInt IntersectionCount = 0; // Counts the number of intersections of the point with various planes of the cell.
 
    // Analyze distances to determine position
    for(int i = 0; i < NUM_FACES; i++) {
        if(d[i] <  0.0) {
            isInside = PETSC_FALSE; // Point is outside in at least one direction
        }
        if(d[i] == 0.0) {
            isOnBoundary = PETSC_TRUE; // Point is exactly at least one face
            IntersectionCount++; 
        }
    }
 
    // Set the result based on flags
    if(isInside) {
        *result = 0; // Inside the cell
        LOG_ALLOW(LOCAL,LOG_DEBUG, "DeterminePointPosition - Particle is inside the cell.\n");
    }
    else if(isOnBoundary) {
      if(IntersectionCount == 1){ 
          *result = 1; // on face
          LOG_ALLOW(LOCAL,LOG_DEBUG, "DeterminePointPosition - Particle is on a face of the cell.\n");
      }
      else if(IntersectionCount == 2){ 
          *result = 2; // on edge
          LOG_ALLOW(LOCAL,LOG_DEBUG, "DeterminePointPosition - Particle is on an edge of the cell.\n");
      }
      else if(IntersectionCount >= 3){ 
          *result = 3; // on corner
          LOG_ALLOW(LOCAL,LOG_DEBUG, "DeterminePointPosition - Particle is on a corner of the cell.\n");
      }
    }
    else {
        *result = -1; // Outside the cell
        LOG_ALLOW(LOCAL,LOG_DEBUG, "DeterminePointPosition - Particle is outside the cell.\n");
    }
 
    return 0; // Indicate successful execution
}

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GetCellVerticesFromGrid()

PetscErrorCode GetCellVerticesFromGrid	(	Cmpnts ***	coor,
		PetscInt	idx,
		PetscInt	idy,
		PetscInt	idz,
		Cell *	cell
	)

Retrieves the coordinates of the eight vertices of a cell based on grid indices.

This function populates the cell structure with the coordinates of the eight vertices of a hexahedral cell given its grid indices (idx, idy, idz).

Parameters

[in]	coor	Pointer to a 3D array of Cmpnts structures representing the grid coordinates.
[in]	idx	The i-index of the cell.
[in]	idy	The j-index of the cell.
[in]	idz	The k-index of the cell.
[out]	cell	Pointer to a Cell structure to store the cell's vertices.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

This function populates the cell structure with the coordinates of the eight vertices of a hexahedral cell given its grid indices (idx, idy, idz). The vertex numbering follows the convention depicted in the provided figure:

         (i,j+1,k)
        3---------------------------2(i+1,j+1,k)
       /|                          /|
      / |                         / |
     /  |                        /  |
    /   |                       /   |
   /    |                      /    |
  /     |                     /     |
 /      |                    /      |
4-------|------------------5        |
|       |                   |       |
|(i,j+1,k+1)                |       |       
|       |                   |       |
|       0-------------------|-------1 (i+1,j,k)
|       /                   |      /
|      /                    |     /
|     /                     |    /
|    /                      |   /
|   /                       |  /
|  /                        | /
| /                         |/
7---------------------------6
(i,j,k+1)                  (i+1,j,k+1)

Vertex Numbering and Positions:

• Vertex 0: (i, j, k) → cell->vertices[0]
• Vertex 1: (i+1, j, k) → cell->vertices[1]
• Vertex 2: (i+1, j+1, k) → cell->vertices[2]
• Vertex 3: (i, j+1, k) → cell->vertices[3]
• Vertex 4: (i, j+1, k+1) → cell->vertices[4]
• Vertex 5: (i+1, j+1, k+1) → cell->vertices[5]
• Vertex 6: (i+1, j, k+1) → cell->vertices[6]
• Vertex 7: (i, j, k+1) → cell->vertices[7]

Parameters

[in]	coor	Pointer to a 3D array of Cmpnts structures representing the grid coordinates.
[in]	idx	The i-index of the cell.
[in]	idy	The j-index of the cell.
[in]	idz	The k-index of the cell.
[out]	cell	Pointer to a Cell structure to store the cell's vertices.

Returns

PetscErrorCode Returns 0 to indicate successful execution. Non-zero on failure.

Note

• It is assumed that the grid indices (idx, idy, idz) are within valid bounds.
• The coor array should be properly initialized with accurate coordinates for all grid points.
• The function assumes unit spacing between grid points. Modify the coordinate assignments if your grid spacing differs.
• The boundary checks have been removed as they are performed before calling this function.

Definition at line 521 of file walkingsearch.c.

{
 
    // Validate input pointers
    if (coor == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "GetCellVerticesFromGrid - 'coor' is NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "GetCellVerticesFromGrid - Input array 'coor' is NULL.");
    }
    if (cell == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "GetCellVerticesFromGrid - 'cell' is NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "GetCellVerticesFromGrid - Output parameter 'cell' is NULL.");
    }
 
    // Assign vertices based on grid indices matching the figure
    // Vertex numbering follows the convention depicted in the figure
    cell->vertices[0] = coor[idz][idy][idx];         // Vertex 0: (i,   j,   k)
    cell->vertices[1] = coor[idz][idy][idx+1];       // Vertex 1: (i+1, j,   k)
    cell->vertices[2] = coor[idz][idy+1][idx+1];     // Vertex 2: (i+1, j+1, k)
    cell->vertices[3] = coor[idz][idy+1][idx];       // Vertex 3: (i,   j+1, k)
    cell->vertices[4] = coor[idz+1][idy+1][idx];     // Vertex 4: (i,   j+1, k+1)
    cell->vertices[5] = coor[idz+1][idy+1][idx+1];   // Vertex 5: (i+1, j+1, k+1)
    cell->vertices[6] = coor[idz+1][idy][idx+1];     // Vertex 6: (i+1, j,   k+1)
    cell->vertices[7] = coor[idz+1][idy][idx];       // Vertex 7: (i,   j,   k+1)
 
    LOG_ALLOW(LOCAL,LOG_DEBUG, "GetCellVerticesFromGrid - Retrieved vertices for cell (%d, %d, %d).\n", idx, idy, idz);
 
    return 0; // Indicate successful execution
}

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InitializeTraversalParameters()

PetscErrorCode InitializeTraversalParameters	(	UserCtx *	user,
		Particle *	particle,
		PetscInt *	idx,
		PetscInt *	idy,
		PetscInt *	idz,
		PetscInt *	traversal_steps
	)

Initializes traversal parameters for locating a particle.

This function sets the initial cell indices based on the local grid boundaries and initializes the traversal step counter.

Parameters

[in]	user	Pointer to the user-defined context containing grid information.
[in]	particle	Pointer to the Particle structure containing its location.
[out]	idx	Pointer to store the initial i-index of the cell.
[out]	idy	Pointer to store the initial j-index of the cell.
[out]	idz	Pointer to store the initial k-index of the cell.
[out]	traversal_steps	Pointer to store the initial traversal step count.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

This function sets the initial cell indices for the grid search.

• If the particle has valid previous cell indices (not -1,-1,-1), the search starts from that previous cell.
• Otherwise (e.g., first time locating the particle), the search starts from the beginning corner (xs, ys, zs) of the local grid domain owned by the current process. It also initializes the traversal step counter.

Parameters

[in]	user	Pointer to the user-defined context containing grid information.
[in]	particle	Pointer to the Particle structure containing its location and previous cell (if any).
[out]	idx	Pointer to store the initial i-index of the cell.
[out]	idy	Pointer to store the initial j-index of the cell.
[out]	idz	Pointer to store the initial k-index of the cell.
[out]	traversal_steps	Pointer to store the initial traversal step count.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

Definition at line 571 of file walkingsearch.c.

{
    PetscErrorCode ierr;
    DMDALocalInfo info;
 
    // Validate input pointers
    if (user == NULL || particle == NULL || idx == NULL || idy == NULL || idz == NULL || traversal_steps == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "InitializeTraversalParameters - One or more input pointers are NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "InitializeTraversalParameters - One or more input pointers are NULL.");
    }
 
    // Get grid information (needed for fallback start and potentially validation)
    ierr = DMDAGetLocalInfo(user->da, &info); CHKERRQ(ierr);
 
    // --- Check if the particle has a valid previous cell ID ---
    // Assuming particle->cell stores the *global* indices
    if (particle->cell[0] >= 0 && particle->cell[1] >= 0 && particle->cell[2] >= 0) {
 
      // It sees a valid cell ID exists.
      // Before using it, it explicitly checks if this cell is accessible.
      PetscBool is_handoff_cell_valid;
      ierr = CheckCellWithinLocalGrid(user, particle->cell[0], particle->cell[1], particle->cell[2], &is_handoff_cell_valid); CHKERRQ(ierr);
 
      if (is_handoff_cell_valid) {
        // FAST PATH: The check passed. The cell is safe. Use it.
        *idx = particle->cell[0];
        *idy = particle->cell[1];
        *idz = particle->cell[2];
 
     LOG_ALLOW(LOCAL,LOG_DEBUG, "InitializeTraversalParameters - Particle %lld has previous cell (%d, %d, %d). Starting search there.\n",
           (long long)particle->PID, *idx, *idy, *idz); // Cast id to long long for printing PetscInt64
    
      } else {
        // SAFE FALLBACK: The check failed! The handoff cell is NOT safe.
        // Discard the unsafe handoff cell and revert to starting at the
        // guaranteed-safe local corner.
        *idx = info.xs;
        *idy = info.ys;
        *idz = info.zs;
      }
    } else {
        // No valid previous cell ID (e.g., -1,-1,-1 or first time).
        // Start from the first cell in the local owned grid domain.
        *idx = info.xs;
        *idy = info.ys;
        *idz = info.zs;
        LOG_ALLOW(LOCAL,LOG_DEBUG, "InitializeTraversalParameters - Particle %lld has no valid previous cell. Starting search at local corner (%d, %d, %d).\n",
                  (long long)particle->PID, *idx, *idy, *idz);
    }
 
    // Initialize traversal step counter
    *traversal_steps = 0;
 
    // Log the chosen starting point
    LOG_ALLOW(LOCAL,LOG_INFO, "InitializeTraversalParameters - Traversal for particle %lld initialized to start at cell (%d, %d, %d).\n",
               (long long)particle->PID, *idx, *idy, *idz);
 
    return 0;
}

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CheckCellWithinLocalGrid()

PetscErrorCode CheckCellWithinLocalGrid	(	UserCtx *	user,
		PetscInt	idx,
		PetscInt	idy,
		PetscInt	idz,
		PetscBool *	is_within
	)

Checks if the current cell indices are within the local grid boundaries.

Parameters

[in]	user	Pointer to the user-defined context containing grid information.
[in]	idx	The i-index of the current cell.
[in]	idy	The j-index of the current cell.
[in]	idz	The k-index of the current cell.
[out]	is_within	Pointer to a PetscBool that will be set to PETSC_TRUE if within bounds, else PETSC_FALSE.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

Checks if the current cell indices are within the local grid boundaries.

This function determines if the provided global cell indices (idx, idy, idz) fall within the range of cells covered by the current process's owned and ghost NODES. A cell C(i,j,k) (origin N(i,j,k)) is considered within this ghosted region if its origin node N(i,j,k) is within the rank's ghosted nodal region, AND node N(i+1,j+1,k+1) is also within it (to ensure the entire cell extent is covered by available node data). More simply, we check if the cell's origin node is within the range of nodes that can form the start of a ghosted cell.

Parameters

[in]	user	Pointer to the user-defined context (needs user->fda for node info).
[in]	idx	The global i-index of the cell's origin node.
[in]	idy	The global j-index of the cell's origin node.
[in]	idz	The global k-index of the cell's origin node.
[out]	is_within	Pointer to a PetscBool that will be set to PETSC_TRUE if within ghosted bounds, else PETSC_FALSE.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

Definition at line 650 of file walkingsearch.c.

{
    PetscErrorCode ierr;
    DMDALocalInfo info_nodes; // Node information from the DMDA that defines ghost regions (user->fda)
 
    PetscFunctionBeginUser; // Assuming this is part of your PETSc style
 
    // Validate inputs
    if (user == NULL || is_within == NULL) {
        LOG_ALLOW(LOCAL, LOG_ERROR, "Input pointer is NULL in CheckCellWithinLocalGrid.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "Input pointer is NULL in CheckCellWithinLocalGrid.");
    }
    if (user->fda == NULL) {
        LOG_ALLOW(LOCAL, LOG_ERROR, "user->fda is NULL in CheckCellWithinLocalGrid. Cannot get ghost info.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_WRONGSTATE, "user->fda is NULL. Cannot get ghost info.");
    }
 
    // Get node info from user->fda (this DMDA has the ghost layer information for nodes)
    ierr = DMDAGetLocalInfo(user->fda, &info_nodes); CHKERRQ(ierr);
 
    // Determine the range of GLOBAL CELL INDICES that are covered by this rank's ghosted NODAL region.
    // A cell C(i,j,k) has origin node N(i,j,k).
    // The ghosted nodal region starts at global node index info_nodes.gxs and has info_nodes.gxm nodes.
 
    // Global starting index of the first cell whose origin node is within the ghosted nodal region.
    PetscInt gxs_cell_global_start = info_nodes.gxs;
    PetscInt gys_cell_global_start = info_nodes.gys;
    PetscInt gzs_cell_global_start = info_nodes.gzs;
 
    // Number of cells that can be formed starting from nodes within the ghosted nodal region.
    // If there are N ghosted nodes (info_nodes.gxm), they can be origins for N-1 cells.
    PetscInt gxm_cell_local_count = (info_nodes.gxm > 0) ? info_nodes.gxm - 1 : 0;
    PetscInt gym_cell_local_count = (info_nodes.gym > 0) ? info_nodes.gym - 1 : 0;
    PetscInt gzm_cell_local_count = (info_nodes.gzm > 0) ? info_nodes.gzm - 1 : 0;
 
    // Global exclusive end index for cells whose origins are in the ghosted nodal region. 
    PetscInt gxe_cell_global_end_exclusive = gxs_cell_global_start + gxm_cell_local_count;
    PetscInt gye_cell_global_end_exclusive = gys_cell_global_start + gym_cell_local_count;
    PetscInt gze_cell_global_end_exclusive = gzs_cell_global_start + gzm_cell_local_count;
 
    // Check if the given global cell index (idx, idy, idz) falls within this range.
    // This means the origin node of cell (idx,idy,idz) is within the rank's accessible ghosted node region,
    // and that node can indeed serve as a cell origin (i.e., it's not the very last node in the ghosted region).
    if (idx >= gxs_cell_global_start && idx < gxe_cell_global_end_exclusive &&
        idy >= gys_cell_global_start && idy < gye_cell_global_end_exclusive &&
        idz >= gzs_cell_global_start && idz < gze_cell_global_end_exclusive) {
        *is_within = PETSC_TRUE;
    } else {
        *is_within = PETSC_FALSE;
    }
 
    LOG_ALLOW(LOCAL, LOG_DEBUG, "Cell (origin node glob idx) (%d, %d, %d) is %s the ghosted local grid (covered cell origins x:[%d..%d), y:[%d..%d), z:[%d..%d)).\n",
              idx, idy, idz, (*is_within) ? "within" : "outside",
              gxs_cell_global_start, gxe_cell_global_end_exclusive,
              gys_cell_global_start, gye_cell_global_end_exclusive,
              gzs_cell_global_start, gze_cell_global_end_exclusive);
 
    PetscFunctionReturn(0);
}

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RetrieveCurrentCell()

PetscErrorCode RetrieveCurrentCell	(	UserCtx *	user,
		PetscInt	idx,
		PetscInt	idy,
		PetscInt	idz,
		Cell *	cell
	)

Retrieves the coordinates of the eight vertices of the current cell.

Parameters

[in]	user	Pointer to the user-defined context containing grid information.
[in]	idx	The i-index of the current cell.
[in]	idy	The j-index of the current cell.
[in]	idz	The k-index of the current cell.
[out]	cell	Pointer to a Cell structure to store the cell's vertices.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

Definition at line 722 of file walkingsearch.c.

{
    PetscErrorCode ierr;
    Vec Coor;
    Cmpnts ***coor_array;
    PetscMPIInt rank;
    DMDALocalInfo info_nodes;
 
    // Validate input pointers
    if (user == NULL || cell == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "RetrieveCurrentCell - One or more input pointers are NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "RetrieveCurrentCell - One or more input pointers are NULL.");
    }
 
    // Get local coordinates
    ierr = DMGetCoordinatesLocal(user->da, &Coor); CHKERRQ(ierr);
    ierr = DMDAVecGetArrayRead(user->fda, Coor, &coor_array); CHKERRQ(ierr);
 
    // Get the local grid information FOR THE GHOSTED ARRAY from user->fda
    // This info contains the mapping between global and local indices.
    ierr = DMDAGetLocalInfo(user->fda, &info_nodes); CHKERRQ(ierr);
 
    PetscInt  idx_local = idx; // - info_nodes.gxs;
    PetscInt  idy_local = idy; // - info_nodes.gys;
    PetscInt  idz_local = idz; // - info_nodes.gzs;
    
    LOG_ALLOW(LOCAL,LOG_DEBUG," [Rank %d] Getting vertex coordinates for cell: %d,%d,%d whose local coordinates are %d,%d,%d.\n",rank,idx,idy,idz,idx_local,idy_local,idz_local);
    
    // Get the current cell's vertices
    ierr = GetCellVerticesFromGrid(coor_array, idx_local, idy_local, idz_local, cell); CHKERRQ(ierr);
 
    // Restore array
    ierr = DMDAVecRestoreArrayRead(user->fda, Coor, &coor_array); CHKERRQ(ierr);
 
    // Get MPI rank for debugging
    ierr = MPI_Comm_rank(PETSC_COMM_WORLD, &rank); CHKERRQ(ierr);
 
    // Debug: Print cell vertices
    LOG_ALLOW(LOCAL,LOG_DEBUG, "RetrieveCurrentCell - Cell (%d, %d, %d) vertices \n", idx, idy, idz);
    ierr = LOG_CELL_VERTICES(cell, rank); CHKERRQ(ierr);
 
    return 0;
}

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EvaluateParticlePosition()

PetscErrorCode EvaluateParticlePosition	(	const Cell *	cell,
		PetscReal *	d,
		const Cmpnts	p,
		PetscInt *	position,
		const PetscReal	threshold
	)

Determines the spatial relationship of a particle relative to a cubic cell.

This function evaluates whether a particle located at a specific point p in 3D space is positioned inside the cell, on the boundary of the cell, or outside the cell. The determination is based on the signed distances from the particle to each of the six faces of the cell. The function utilizes the computed distances to ascertain the particle's position with respect to the cell boundaries, considering a threshold to account for floating-point precision.

Parameters

[in]	cell	A pointer to a Cell structure that defines the cubic cell via its vertices. The cell's geometry is essential for accurately computing the distances to each face.
[in]	d	A pointer to an array of six PetscReal values that store the signed distances from the particle to each face of the cell. These distances are typically computed using the CalculateDistancesToCellFaces function.
[in]	p	The location of the particle in 3D space, represented by the Cmpnts structure. This point is the reference for distance calculations to the cell's faces.
[out]	position	A pointer to an integer that will be set based on the particle's position relative to the cell: 0: The particle is inside the cell. 1: The particle is on the boundary of the cell. -1: The particle is outside the cell.
[in]	threshold	A PetscReal value that defines the minimum distance below which a computed distance is considered to be zero. This threshold helps in mitigating inaccuracies due to floating-point arithmetic, especially when determining if the particle lies exactly on the boundary.

Returns

PetscErrorCode Returns 0 if the function executes successfully. If an error occurs, a non-zero error code is returned, indicating the type of failure.

Note

• It is assumed that the d array has been properly allocated and contains valid distance measurements before calling this function.
• The function relies on CalculateDistancesToCellFaces to accurately compute the signed distances to each face. Any inaccuracies in distance calculations can affect the determination of the particle's position.
• The threshold parameter should be chosen based on the specific precision requirements of the application to balance between sensitivity and robustness against floating-point errors.
• The function includes a debug statement that prints the face distances, which can be useful for verifying the correctness of distance computations during development or troubleshooting.

Definition at line 811 of file walkingsearch.c.

{
    PetscErrorCode ierr;
    PetscReal cellSize;
    PetscReal cellThreshold;
 
    // Validate input pointers to ensure they are not NULL, preventing potential segmentation faults.
    if (cell == NULL || position == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "EvaluateParticlePosition - One or more input pointers are NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "EvaluateParticlePosition - One or more input pointers are NULL.");
    }
  
    // Compute a local cell size
    ierr = GetCellCharacteristicSize(cell, &cellSize); CHKERRQ(ierr);
 
    // scale base threshold by cell size (ex: if threshold = 1e-6 and cell size = 0.01, cellThreshold = 1e-8)
    cellThreshold = threshold*cellSize; 
 
    // Invoke the function to calculate signed distances from the particle to each face of the cell.
    // The distances are stored in the array pointed to by 'd'.
    ierr = CalculateDistancesToCellFaces(p, cell, d, cellThreshold); CHKERRQ(ierr);
    CHKERRQ(ierr); // Check for errors in distance calculation.
 
 
    // Catch degenerate-plane error manually:
    if (ierr == PETSC_ERR_USER) {
        LOG_ALLOW(LOCAL, LOG_WARNING,
                  "EvaluateParticlePosition - Skipping cell due to degenerate face.\n");
        // We can set *position = -1 here
        *position = -1; // treat as outside
        return 0; // not a fatal error, just skip
    } else {
        CHKERRQ(ierr);
    }
 
 
    // Debugging output: Print the computed distances to each face for verification purposes.
    LOG_ALLOW(LOCAL,LOG_DEBUG, "EvaluateParticlePosition - Face Distances:\n");
    if(get_log_level() == LOG_DEBUG && is_function_allowed(__func__)) LOG_FACE_DISTANCES(d); 
    CHKERRQ(ierr); // Check for errors in printing distances.
 
    // Determine the particle's position relative to the cell based on the computed distances.
    // The function sets the value pointed to by 'position' accordingly:
    // 0 for inside, 1 (2 or 3) for on the boundary, and -1 for outside.
    ierr = DeterminePointPosition(d,position); 
    CHKERRQ(ierr); // Check for errors in position determination.
 
    return 0; // Indicate successful execution of the function.
}

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LocateParticleInGrid()

PetscErrorCode LocateParticleInGrid	(	UserCtx *	user,
		Particle *	particle
	)

Locates the grid cell containing a particle using a walking search.

This function implements a walking search algorithm to find the specific cell (identified by global indices i, j, k) that encloses the particle's physical location (particle->loc).

The search starts from an initial guess cell (either the particle's previously known cell or the corner of the local process domain) and iteratively steps to adjacent cells based on the signed distances from the particle to the faces of the current cell.

Upon successful location (position == 0), the particle's cell field is updated with the found indices (i,j,k), and the corresponding interpolation weights are calculated and stored using the distances (d) relative to the final cell.

Handles particles exactly on cell boundaries by attempting a tie-breaker if the search gets stuck oscillating between adjacent cells due to the boundary condition.

Parameters

[in]	user	Pointer to the UserCtx structure containing grid information (DMDA, coordinates) and domain boundaries.
[in,out]	particle	Pointer to the Particle structure. Its loc field provides the target position. On successful return, its cell and weights fields are updated. On failure, cell is set to {-1, -1, -1} and weights to {0.0, 0.0, 0.0}.

Returns

PetscErrorCode 0 on success. Non-zero error codes may indicate issues during coordinate access, distance calculation, or other internal errors. A return code of 0 does not guarantee the particle was found; check particle->cell[0] >= 0 afterward.

Note

Relies on helper functions like InitializeTraversalParameters, CheckCellWithinLocalGrid, RetrieveCurrentCell, EvaluateParticlePosition, UpdateCellIndicesBasedOnDistances, UpdateParticleWeights, and FinalizeTraversal.

Warning

Ensure particle->loc is set correctly before calling.

The function may fail to find the particle if it lies outside the domain accessible by the current process (including ghost cells) or if MAX_TRAVERSAL steps are exceeded.

UpdateCellIndicesBasedOnDistances()

PetscErrorCode UpdateCellIndicesBasedOnDistances	(	PetscReal	d[NUM_FACES],
		PetscInt *	idx,
		PetscInt *	idy,
		PetscInt *	idz,
		DMDALocalInfo *	info
	)

Updates the cell indices based on the signed distances to each face.

This function modifies the cell indices (idx, idy, idz) to move towards the direction where the particle is likely to be located, based on positive distances indicating that the particle is outside in that particular direction.

Parameters

[in]	d	An array of six PetscReal values representing the signed distances to each face: d[LEFT]: Left Face d[RIGHT]: Right Face d[BOTTOM]: Bottom Face d[TOP]: Top Face d[FRONT]: Front Face d[BACK]: Back Face
[out]	idx	Pointer to the i-index of the cell to be updated.
[out]	idy	Pointer to the j-index of the cell to be updated.
[out]	idz	Pointer to the k-index of the cell to be updated.
[in]	info	DMDALocalInfo structure that holds local & global domain bounds.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

FinalizeTraversal()

PetscErrorCode FinalizeTraversal	(	UserCtx *	user,
		Particle *	particle,
		PetscInt	traversal_steps,
		PetscBool	cell_found,
		PetscInt	idx,
		PetscInt	idy,
		PetscInt	idz
	)

Finalizes the traversal by reporting the results.

This function prints the outcome of the traversal, indicating whether the particle was found within a cell or not, and updates the particle's cell indices accordingly.

Parameters

[in]	user	Pointer to the user-defined context containing grid information.
[out]	particle	Pointer to the Particle structure to update with cell indices.
[in]	traversal_steps	The number of traversal steps taken.
[in]	cell_found	Flag indicating whether the particle was found within a cell.
[in]	idx	The i-index of the found cell.
[in]	idy	The j-index of the found cell.
[in]	idz	The k-index of the found cell.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

Definition at line 977 of file walkingsearch.c.

{
    // Validate input pointers
    if (user == NULL || particle == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "FinalizeTraversal - One or more input pointers are NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "FinalizeTraversal - One or more input pointers are NULL.");
    }
 
    if (cell_found) {
      LOG_ALLOW(LOCAL,LOG_INFO, "FinalizeTraversal - Particle located in cell (%d, %d, %d) after %d traversal steps.\n",
            idx, idy, idz, traversal_steps);
    }
    else {
      LOG_ALLOW(LOCAL,LOG_WARNING, "FinalizeTraversal - Particle could not be located within the grid after %d traversal steps.\n", (PetscInt)traversal_steps);
        particle->cell[0] = -1;
        particle->cell[1] = -1;
        particle->cell[2] = -1;
    }
 
    LOG_ALLOW(LOCAL, LOG_INFO, "FinalizeTraversal - Completed final traversal sync across all ranks.\n");
 
 
    return 0;
}

FindOwnerOfCell()

PetscErrorCode FindOwnerOfCell	(	UserCtx *	user,
		PetscInt	i,
		PetscInt	j,
		PetscInt	k,
		PetscMPIInt *	owner_rank
	)

Finds the MPI rank that owns a given global cell index.

This function performs a linear search through the pre-computed decomposition map (user->RankCellInfoMap) to determine which process is responsible for the cell with global indices (i, j, k). It is the definitive method for resolving cell ownership in the "Walk and Handoff" migration algorithm.

If the provided indices are outside the range of any rank (e.g., negative or beyond the global domain), the function will not find an owner and owner_rank will be set to -1.

Parameters

[in]	user	Pointer to the UserCtx structure, which must contain the initialized RankCellInfoMap and num_ranks.
[in]	i	Global i-index of the cell to find.
[in]	j	Global j-index of the cell to find.
[in]	k	Global k-index of the cell to find.
[out]	owner_rank	Pointer to a PetscMPIInt where the resulting owner rank will be stored. It is set to -1 if no owner is found.

Returns

PetscErrorCode 0 on success, or a non-zero PETSc error code on failure.

Definition at line 1026 of file walkingsearch.c.

{
    PetscErrorCode ierr;
    PetscMPIInt size;
 
    PetscFunctionBeginUser;
 
    ierr = MPI_Comm_size(PETSC_COMM_WORLD,&size); CHKERRQ(ierr);
 
    // --- 1. Input Validation ---
    if (!user || !user->RankCellInfoMap) {
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_WRONGSTATE, "UserCtx or RankCellInfoMap is not initialized in FindOwnerOfCell.");
    }
    if (!owner_rank) {
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "Output pointer owner_rank is NULL in FindOwnerOfCell.");
    }
 
    // --- 2. Linear Search through the Decomposition Map ---
    // Initialize to a "not found" state.
    *owner_rank = -1;
 
    // Loop through the map, which contains the ownership info for every rank 'r'.
    for (PetscMPIInt r = 0; r < size; ++r) {
        const RankCellInfo *info = &user->RankCellInfoMap[r];
 
        // A rank owns a cell if the cell's index is within its start (inclusive)
        // and end (exclusive) range for all three dimensions.
        if ((i >= info->xs_cell && i < info->xs_cell + info->xm_cell) &&
            (j >= info->ys_cell && j < info->ys_cell + info->ym_cell) &&
            (k >= info->zs_cell && k < info->zs_cell + info->zm_cell))
        {
            *owner_rank = r; // We found the owner.
            break;           // The search is over, exit the loop.
        }
    }
 
    // --- 3. Logging for Diagnostics ---
    // This is extremely useful for debugging particle migration issues.
    if (*owner_rank == -1) {
        LOG_ALLOW(LOCAL, LOG_DEBUG, "FindOwnerOfCell: No owner found for global cell (%d, %d, %d). It is likely outside the domain.\n", i, j, k);
    } else {
        LOG_ALLOW(LOCAL, LOG_DEBUG, "FindOwnerOfCell: Owner of cell (%d, %d, %d) is Rank %d.\n", i, j, k, *owner_rank);
    }
 
    PetscFunctionReturn(0);
}

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LocateParticleOrFindMigrationTarget_TEST()

PetscErrorCode LocateParticleOrFindMigrationTarget_TEST	(	UserCtx *	user,
		Particle *	particle,
		ParticleLocationStatus *	status_out
	)

Locates a particle's host cell or identifies its migration target using a robust walk search.

This is the core search engine. It starts from a guess cell and walks through the grid. It returns a definitive, actionable status indicating the outcome:

• ACTIVE_AND_LOCATED: The particle was found in a cell on the current rank.
• MIGRATING_OUT: The particle was found to belong to another rank. particle->destination_rank is set.
• LOST: The search failed to find the particle within the global domain.

This function is globally aware and can walk across MPI rank boundaries. It contains robust checks for global domain boundaries and a tie-breaker for numerically "stuck" particles on cell faces.

Parameters

[in]	user	Pointer to the UserCtx containing all grid and domain info.
[in,out]	particle	Pointer to the Particle struct. Its fields are updated based on the search outcome.
[out]	status_out	The final, actionable status of the particle after the search.

Returns

PetscErrorCode 0 on success, or a non-zero PETSc error code on failure.

Definition at line 1094 of file walkingsearch.c.

{
    PetscErrorCode ierr;
    PetscMPIInt    rank;
    
    // --- Search State Variables ---
    PetscInt  idx, idy, idz;           // Current search cell global indices
    PetscInt  traversal_steps;         // Counter to prevent infinite loops
    PetscBool search_concluded = PETSC_FALSE; // Flag to terminate the main while loop
 
    // --- Oscillation/Stuck Loop Detection Variables ---
    PetscInt  repeatedIndexCount = 0;
    PetscInt  prevIdx = PETSC_MIN_INT, prevIdy = PETSC_MIN_INT, prevIdz = PETSC_MIN_INT;
    PetscInt  last_position_result = -999;
 
    PetscFunctionBeginUser;
    ierr = MPI_Comm_rank(PETSC_COMM_WORLD, &rank); CHKERRQ(ierr);
    LOG_FUNC_TIMER_BEGIN_EVENT(EVENT_IndividualLocation, LOCAL);
 
    // --- 1. Initialize the Search ---
    ierr = InitializeTraversalParameters(user, particle, &idx, &idy, &idz, &traversal_steps); CHKERRQ(ierr);
 
    LOG_ALLOW(LOCAL,LOG_INFO, " The Threshold for considering a particle to be at a face is %.16f.\n",DISTANCE_THRESHOLD);
    
    LOG_ALLOW(LOCAL,LOG_DEBUG," [PID %lld]Traversal Initiated at : i = %d, j = %d, k = %d.\n",(long long)particle->PID,idx,idy,idz); 
    
    // --- 2. Main Walking Search Loop ---
    while (!search_concluded && traversal_steps < MAX_TRAVERSAL) {
        traversal_steps++;
 
        // --- 2a. GLOBAL Domain Boundary Check ---
        if (idx < 0 || idx >= (user->IM - 1) || idy < 0 || idy >= (user->JM - 1) || idz < 0 || idz >= (user->KM - 1)) {
            LOG_ALLOW(LOCAL, LOG_WARNING, "[PID %lld]: Walked outside GLOBAL domain boundaries to invalid cell (%d,%d,%d). Search fails.\n",
                      (long long)particle->PID, idx, idy, idz);
            idx = -1; // Invalidate the result to signal failure
            break;    // Exit the loop immediately
        }
 
        // --- 2b. LOCAL GHOST REGION CHECK (PREVENTS SEGV) ---
        // Before trying to access cell data, check if we have it.
        PetscBool is_cell_local;
        ierr = CheckCellWithinLocalGrid(user, idx, idy, idz, &is_cell_local); CHKERRQ(ierr);
        if (!is_cell_local) {
            // We have walked outside the local rank's ghost region.
            // This definitively means the particle belongs to another rank.
            // Conclude the search here; the current (idx,idy,idz) is the handoff cell.
            LOG_ALLOW(LOCAL, LOG_INFO, "[PID %lld]: Walked outside local ghost region to cell (%d,%d,%d). Concluding search for handoff.\n",
                      (long long)particle->PID, idx, idy, idz);
            search_concluded = PETSC_TRUE;
            continue; // Skip the rest of the loop; proceed to finalization.
        }
    
        // --- 2c. Stuck Loop Detection & Enhanced Tie-Breaker ---
        if (idx == prevIdx && idy == prevIdy && idz == prevIdz) {
            repeatedIndexCount++;
            if (repeatedIndexCount > REPEAT_COUNT_THRESHOLD) {
                // Only apply tie-breaker if we are stuck for the right reason (on a boundary)
                if (last_position_result >= 1) {
                    LOG_ALLOW(LOCAL, LOG_WARNING, "LocateOrMigrate [PID %lld]: Stuck on boundary of cell (%d,%d,%d) for %d steps. Applying enhanced tie-breaker.\n",
                              (long long)particle->PID, idx, idy, idz, repeatedIndexCount);
                    
                    // Re-evaluate at the stuck cell to get definitive weights
                    Cell final_cell;
                    PetscReal final_d[NUM_FACES];
                    PetscInt final_position; // Dummy variable
                    
                    ierr = RetrieveCurrentCell(user, idx, idy, idz, &final_cell); CHKERRQ(ierr);
                    ierr = EvaluateParticlePosition(&final_cell, final_d, particle->loc, &final_position, DISTANCE_THRESHOLD);
                    
                    if (ierr == 0) { // If evaluation succeeded
                        ierr = UpdateParticleWeights(final_d, particle); CHKERRQ(ierr);
                        search_concluded = PETSC_TRUE; // Conclude search, accepting this cell.
                    } else { // Evaluation failed (e.g., degenerate cell)
                        LOG_ALLOW(LOCAL, LOG_ERROR, "[PID %lld]: Tie-breaker failed during final evaluation at cell (%d,%d,%d). Search fails.\n",
                                  (long long)particle->PID, idx, idy, idz);
                        idx = -1; // Invalidate result
                        search_concluded = PETSC_TRUE;
                    }
                } else { // Stuck for the wrong reason (not on a boundary)
                    LOG_ALLOW(LOCAL, LOG_ERROR, "[PID %lld]: Search is stuck at cell (%d,%d,%d) but not on a boundary. This indicates a logic error. Failing search.\n",
                              (long long)particle->PID, idx, idy, idz);
                    idx = -1; // Invalidate result
                    search_concluded = PETSC_TRUE;
                }
                if(search_concluded) continue;
            }
        } else {
            repeatedIndexCount = 0;
        }
        prevIdx = idx; prevIdy = idy; prevIdz = idz;
 
        // --- 2d. Geometric Evaluation ---
        Cell      current_cell;
        PetscReal distances[NUM_FACES];
        PetscInt  position_in_cell;
 
        ierr = RetrieveCurrentCell(user, idx, idy, idz, &current_cell); CHKERRQ(ierr);
        ierr = EvaluateParticlePosition(&current_cell, distances, particle->loc, &position_in_cell, DISTANCE_THRESHOLD); CHKERRQ(ierr);
        last_position_result = position_in_cell;
 
        // --- 2e. Decision Making ---
        if (position_in_cell >= 0) { // Particle is INSIDE or ON THE BOUNDARY
            search_concluded = PETSC_TRUE;
            ierr = UpdateParticleWeights(distances, particle); CHKERRQ(ierr);
        } else { // Particle is OUTSIDE
            ierr = UpdateCellIndicesBasedOnDistancesTEST(distances, &idx, &idy, &idz); CHKERRQ(ierr);
        }
    }
 
    // --- 3. Finalize and Determine Actionable Status ---
    if (idx == -1 || (!search_concluded && traversal_steps >= MAX_TRAVERSAL)) {
        if (idx != -1) {
            LOG_ALLOW(LOCAL, LOG_ERROR, "[PID %lld]: Search FAILED, exceeded MAX_TRAVERSAL limit of %d.\n",
                      (long long)particle->PID, MAX_TRAVERSAL);
        }
        *status_out = LOST;
        particle->cell[0] = -1; particle->cell[1] = -1; particle->cell[2] = -1;
    } else {
        // Search succeeded in finding a candidate cell, now determine its owner.
        PetscMPIInt owner_rank;
        ierr = FindOwnerOfCell(user, idx, idy, idz, &owner_rank); CHKERRQ(ierr);
 
    LOG_ALLOW(LOCAL,LOG_DEBUG," [PID %ld] Owner rank : %d.\n",particle->PID,owner_rank);
    
        // Always update the particle's cell index. It's a good guess for the receiving rank.
        particle->cell[0] = idx; particle->cell[1] = idy; particle->cell[2] = idz;
 
        if (owner_rank == rank) {
            *status_out = ACTIVE_AND_LOCATED;
        } else if (owner_rank != -1) {
            // Particle belongs to another rank. Return the direct, actionable status.
            *status_out = MIGRATING_OUT;
            particle->destination_rank = owner_rank;
        } else { // Found a valid index, but no owner in the map.
            *status_out = LOST;
            particle->cell[0] = -1; particle->cell[1] = -1; particle->cell[2] = -1;
        }
    }
 
    LOG_ALLOW(LOCAL,LOG_DEBUG,"[Rank %d][PID %ld] Search complete.\n",rank,particle->PID);
 
    // --- 4. Report the Final Outcome ---
    ierr = ReportSearchOutcome(particle, *status_out, traversal_steps); CHKERRQ(ierr);
    
    LOG_FUNC_TIMER_END_EVENT(EVENT_IndividualLocation, LOCAL);
    PetscFunctionReturn(0);
}

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ReportSearchOutcome()

PetscErrorCode ReportSearchOutcome	(	const Particle *	particle,
		ParticleLocationStatus	status,
		PetscInt	traversal_steps
	)

Logs the final outcome of the particle location search.

Definition at line 1247 of file walkingsearch.c.

{
    PetscFunctionBeginUser;
    switch (status) {
        case ACTIVE_AND_LOCATED:
            LOG_ALLOW(LOCAL, LOG_INFO, "Search SUCCESS [PID %lld]: Located in global cell (%d, %d, %d) after %d steps.\n",
                      (long long)particle->PID, particle->cell[0], particle->cell[1], particle->cell[2], traversal_steps);
            break;
        case MIGRATING_OUT:
            LOG_ALLOW(LOCAL, LOG_INFO, "Search SUCCESS [PID %lld]: Identified for migration to Rank %d. Handoff cell is (%d, %d, %d) after %d steps.\n",
                      (long long)particle->PID, particle->destination_rank, particle->cell[0], particle->cell[1], particle->cell[2], traversal_steps);
            break;
        case LOST:
            LOG_ALLOW(LOCAL, LOG_WARNING, "Search FAILED [PID %lld]: Particle is LOST after %d steps.\n",
                      (long long)particle->PID, traversal_steps);
            break;
        default:
            LOG_ALLOW(LOCAL, LOG_WARNING, "Search ended with unexpected status %d for PID %lld.\n", status, (long long)particle->PID);
            break;
    }
    PetscFunctionReturn(0);
}

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UpdateCellIndicesBasedOnDistancesTEST()

PetscErrorCode UpdateCellIndicesBasedOnDistancesTEST	(	PetscReal	d[NUM_FACES],
		PetscInt *	idx,
		PetscInt *	idy,
		PetscInt *	idz
	)

Updates the cell indices based on the signed distances to each face.

This function modifies the cell indices (idx, idy, idz) to move towards the direction where the particle is likely to be located, based on positive distances indicating that the particle is outside in that particular direction.

Parameters

[in]	d	An array of six PetscReal values representing the signed distances to each face: d[LEFT]: Left Face d[RIGHT]: Right Face d[BOTTOM]: Bottom Face d[TOP]: Top Face d[FRONT]: Front Face d[BACK]: Back Face
[out]	idx	Pointer to the i-index of the cell to be updated.
[out]	idy	Pointer to the j-index of the cell to be updated.
[out]	idz	Pointer to the k-index of the cell to be updated.
[in]	info	DMDALocalInfo structure that holds local & global domain bounds.

Returns

PetscErrorCode Returns 0 on success, non-zero on failure.

Definition at line 882 of file walkingsearch.c.

{
 
  /*
    PetscInt cxm,cxs;  // maximum & minimum cell ID in x
    PetscInt cym,cys;  // maximum & minimum cell ID in y
    PetscInt czm,czs;  // maximum & minimum cell ID in z
 
    cxs = info->xs; cxm = cxs + info->xm - 2;
    cys = info->ys; cym = cys + info->ym - 2;
    czs = info->zs; czm = czs + info->zm - 2; 
  */
 
  //    LOG_ALLOW(LOCAL, LOG_DEBUG, "UpdateCellIndicesBasedOnDistances - Received d: "
  //    "d[LEFT=%d]=%.3e, d[RIGHT=%d]=%.3e, d[BOTTOM=%d]=%.3e, "
  //          "d[TOP=%d]=%.3e, d[FRONT=%d]=%.3e, d[BACK=%d]=%.3e\n",
        //        LEFT, d[LEFT], RIGHT, d[RIGHT], BOTTOM, d[BOTTOM],
        //  TOP, d[TOP], FRONT, d[FRONT], BACK, d[BACK]);
    //  LOG_ALLOW(LOCAL, LOG_DEBUG, "UpdateCellIndicesBasedOnDistances - Raw d: "
  //          "[%.3e, %.3e, %.3e, %.3e, %.3e, %.3e]\n",
  //          d[0], d[1], d[2], d[3], d[4], d[5]);
    
    // Validate input pointers
    if (d == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "UpdateCellIndicesBasedOnDistances - 'd' is NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "UpdateCellIndicesBasedOnDistances - Input array 'd' is NULL.");
    }
    if (idx == NULL || idy == NULL || idz == NULL) {
      LOG_ALLOW(LOCAL,LOG_ERROR, "UpdateCellIndicesBasedOnDistances - One or more index pointers are NULL.\n");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_NULL, "UpdateCellIndicesBasedOnDistances - One or more index pointers are NULL.");
    }
 
    // Debug: Print current face distances
    LOG_ALLOW(LOCAL,LOG_DEBUG, "UpdateCellIndicesBasedOnDistances - Current Face Distances:\n");
    if(get_log_level() == LOG_DEBUG && is_function_allowed(__func__)) LOG_FACE_DISTANCES(d);
 
    // Update k-direction based on FRONT and BACK distances
    if (d[FRONT] < 0.0) {
      LOG_ALLOW(LOCAL,LOG_DEBUG, "UpdateCellIndicesBasedOnDistances - Condition met: d[FRONT] < 0.0, incrementing idz.\n");
        (*idz) += 1;
    }
    else if(d[BACK] < 0.0){
      LOG_ALLOW(LOCAL,LOG_DEBUG, "UpdateCellIndicesBasedOnDistances - Condition met: d[BACK] < 0.0, decrementing idz.\n");
        (*idz) -= 1;
    }
 
    // Update i-direction based on LEFT and RIGHT distances
    if (d[LEFT] < 0.0) {
      LOG_ALLOW(LOCAL,LOG_DEBUG, "UpdateCellIndicesBasedOnDistances - Condition met: d[LEFT] < 0.0, decrementing idx.\n");
        (*idx) -= 1;
    }
    else if (d[RIGHT] < 0.0) {
      LOG_ALLOW(LOCAL,LOG_DEBUG, "UpdateCellIndicesBasedOnDistances - Condition met: d[RIGHT] < 0.0, incrementing idx.\n");
        (*idx) += 1;
    }
 
    // Update j-direction based on BOTTOM and TOP distances
    if (d[BOTTOM] < 0.0) {
      LOG_ALLOW(LOCAL,LOG_DEBUG, "UpdateCellIndicesBasedOnDistances - Condition met: d[BOTTOM] < 0.0, decrementing idy.\n");
        (*idy) -= 1;
    }
    else if (d[TOP] < 0.0) {
      LOG_ALLOW(LOCAL,LOG_DEBUG, "UpdateCellIndicesBasedOnDistances - Condition met: d[TOP] < 0.0, incrementing idy.\n");
        (*idy) += 1;
    }
 
    /*
    // The 'cell' corners you can reference go from [xs .. xs+xm-1], but
    // to form a valid cell in x, you need (idx+1) in range, so max is (xs+xm-2).
    *idx = PetscMax(cxs,               PetscMin(*idx, cxm));
    *idy = PetscMax(cys,               PetscMin(*idy, cym));
    *idz = PetscMax(czs,               PetscMin(*idz, czm));
    */
 
    LOG_ALLOW(LOCAL,LOG_DEBUG, "UpdateCellIndicesBasedOnDistances - Updated Indices  - idx, idy, idz: %d, %d, %d\n", *idx, *idy, *idz);
 
    return 0; // Indicate successful execution
}

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