/*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "SkColorLookUpTable.h"
#include "SkFloatingPoint.h"

void SkColorLookUpTable::interp3D(float dst[3], float src[3]) const {
    // Call the src components x, y, and z.
    const uint8_t maxX = fGridPoints[0] - 1;
    const uint8_t maxY = fGridPoints[1] - 1;
    const uint8_t maxZ = fGridPoints[2] - 1;

    // An approximate index into each of the three dimensions of the table.
    const float x = src[0] * maxX;
    const float y = src[1] * maxY;
    const float z = src[2] * maxZ;

    // This gives us the low index for our interpolation.
    int ix = sk_float_floor2int(x);
    int iy = sk_float_floor2int(y);
    int iz = sk_float_floor2int(z);

    // Make sure the low index is not also the max index.
    ix = (maxX == ix) ? ix - 1 : ix;
    iy = (maxY == iy) ? iy - 1 : iy;
    iz = (maxZ == iz) ? iz - 1 : iz;

    // Weighting factors for the interpolation.
    const float diffX = x - ix;
    const float diffY = y - iy;
    const float diffZ = z - iz;

    // Constants to help us navigate the 3D table.
    // Ex: Assume x = a, y = b, z = c.
    //     table[a * n001 + b * n010 + c * n100] logically equals table[a][b][c].
    const int n000 = 0;
    const int n001 = 3 * fGridPoints[1] * fGridPoints[2];
    const int n010 = 3 * fGridPoints[2];
    const int n011 = n001 + n010;
    const int n100 = 3;
    const int n101 = n100 + n001;
    const int n110 = n100 + n010;
    const int n111 = n110 + n001;

    // Base ptr into the table.
    const float* ptr = &(table()[ix*n001 + iy*n010 + iz*n100]);

    // The code below performs a tetrahedral interpolation for each of the three
    // dst components.  Once the tetrahedron containing the interpolation point is
    // identified, the interpolation is a weighted sum of grid values at the
    // vertices of the tetrahedron.  The claim is that tetrahedral interpolation
    // provides a more accurate color conversion.
    // blogs.mathworks.com/steve/2006/11/24/tetrahedral-interpolation-for-colorspace-conversion/
    //
    // I have one test image, and visually I can't tell the difference between
    // tetrahedral and trilinear interpolation.  In terms of computation, the
    // tetrahedral code requires more branches but less computation.  The
    // SampleICC library provides an option for the client to choose either
    // tetrahedral or trilinear.
    for (int i = 0; i < 3; i++) {
        if (diffZ < diffY) {
            if (diffZ < diffX) {
                dst[i] = (ptr[n000] + diffZ * (ptr[n110] - ptr[n010]) +
                                      diffY * (ptr[n010] - ptr[n000]) +
                                      diffX * (ptr[n111] - ptr[n110]));
            } else if (diffY < diffX) {
                dst[i] = (ptr[n000] + diffZ * (ptr[n111] - ptr[n011]) +
                                      diffY * (ptr[n011] - ptr[n001]) +
                                      diffX * (ptr[n001] - ptr[n000]));
            } else {
                dst[i] = (ptr[n000] + diffZ * (ptr[n111] - ptr[n011]) +
                                      diffY * (ptr[n010] - ptr[n000]) +
                                      diffX * (ptr[n011] - ptr[n010]));
            }
        } else {
            if (diffZ < diffX) {
                dst[i] = (ptr[n000] + diffZ * (ptr[n101] - ptr[n001]) +
                                      diffY * (ptr[n111] - ptr[n101]) +
                                      diffX * (ptr[n001] - ptr[n000]));
            } else if (diffY < diffX) {
                dst[i] = (ptr[n000] + diffZ * (ptr[n100] - ptr[n000]) +
                                      diffY * (ptr[n111] - ptr[n101]) +
                                      diffX * (ptr[n101] - ptr[n100]));
            } else {
                dst[i] = (ptr[n000] + diffZ * (ptr[n100] - ptr[n000]) +
                                      diffY * (ptr[n110] - ptr[n100]) +
                                      diffX * (ptr[n111] - ptr[n110]));
            }
        }

        // Increment the table ptr in order to handle the next component.
        // Note that this is the how table is designed: all of nXXX
        // variables are multiples of 3 because there are 3 output
        // components.
        ptr++;
    }
}