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#ifndef MATH_FRUSTUM_H
#define MATH_FRUSTUM_H

/*
 * View frustum: the six planes of a projection * view matrix. Lets the
 * renderer skip what can't be seen (behind you, off to the side, beyond
 * the far plane) before it is ever sent to the GPU.
 */

#include <stdbool.h>

#include "math/linalg.h"

typedef struct {
    float plane[6][4]; /* nx, ny, nz, d: inside where n . p + d >= 0 */
} Frustum;

/* From a column-major projection * view matrix (Gribb / Hartmann). */
static inline Frustum frustum_from_matrix(Mat4 m) {
    Frustum f;
    for (int k = 0; k < 6; k++) {
        int row = k / 2;
        float sign = (k % 2) ? -1.0f : 1.0f;
        float p[4];
        for (int c = 0; c < 4; c++) p[c] = m.m[3 + c * 4] + sign * m.m[row + c * 4];
        float len = sqrtf(p[0] * p[0] + p[1] * p[1] + p[2] * p[2]);
        if (len < 1e-8f) len = 1.0f;
        for (int c = 0; c < 4; c++) f.plane[k][c] = p[c] / len;
    }
    return f;
}

/* Does a sphere touch the frustum? (May say yes for some just outside a corner.) */
static inline bool frustum_sphere(const Frustum *f, Vec3 center, float radius) {
    for (int k = 0; k < 6; k++) {
        const float *p = f->plane[k];
        if (p[0] * center.x + p[1] * center.y + p[2] * center.z + p[3] < -radius) return false;
    }
    return true;
}

#endif /* MATH_FRUSTUM_H */