/*
Copyright (c) 2023 Evan Wallace

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
package com.madebyevan.thumbhash;

public final class ThumbHash {
    /**
     * Encodes an RGBA image to a ThumbHash. RGB should not be premultiplied by A.
     *
     * @param w    The width of the input image. Must be ≤100px.
     * @param h    The height of the input image. Must be ≤100px.
     * @param rgba The pixels in the input image, row-by-row. Must have w*h*4 elements.
     * @return The ThumbHash as a byte array.
     */
    public static byte[] rgbaToThumbHash(int w, int h, byte[] rgba) {
        // Encoding an image larger than 100x100 is slow with no benefit
        if (w > 100 || h > 100) throw new IllegalArgumentException(w + "x" + h + " doesn't fit in 100x100");

        // Determine the average color
        float avg_r = 0, avg_g = 0, avg_b = 0, avg_a = 0;
        for (int i = 0, j = 0; i < w * h; i++, j += 4) {
            float alpha = (rgba[j + 3] & 255) / 255.0f;
            avg_r += alpha / 255.0f * (rgba[j] & 255);
            avg_g += alpha / 255.0f * (rgba[j + 1] & 255);
            avg_b += alpha / 255.0f * (rgba[j + 2] & 255);
            avg_a += alpha;
        }
        if (avg_a > 0) {
            avg_r /= avg_a;
            avg_g /= avg_a;
            avg_b /= avg_a;
        }

        boolean hasAlpha = avg_a < w * h;
        int l_limit = hasAlpha ? 5 : 7; // Use fewer luminance bits if there's alpha
        int lx = Math.max(1, Math.round((float) (l_limit * w) / (float) Math.max(w, h)));
        int ly = Math.max(1, Math.round((float) (l_limit * h) / (float) Math.max(w, h)));
        float[] l = new float[w * h]; // luminance
        float[] p = new float[w * h]; // yellow - blue
        float[] q = new float[w * h]; // red - green
        float[] a = new float[w * h]; // alpha

        // Convert the image from RGBA to LPQA (composite atop the average color)
        for (int i = 0, j = 0; i < w * h; i++, j += 4) {
            float alpha = (rgba[j + 3] & 255) / 255.0f;
            float r = avg_r * (1.0f - alpha) + alpha / 255.0f * (rgba[j] & 255);
            float g = avg_g * (1.0f - alpha) + alpha / 255.0f * (rgba[j + 1] & 255);
            float b = avg_b * (1.0f - alpha) + alpha / 255.0f * (rgba[j + 2] & 255);
            l[i] = (r + g + b) / 3.0f;
            p[i] = (r + g) / 2.0f - b;
            q[i] = r - g;
            a[i] = alpha;
        }

        // Encode using the DCT into DC (constant) and normalized AC (varying) terms
        Channel l_channel = new Channel(Math.max(3, lx), Math.max(3, ly)).encode(w, h, l);
        Channel p_channel = new Channel(3, 3).encode(w, h, p);
        Channel q_channel = new Channel(3, 3).encode(w, h, q);
        Channel a_channel = hasAlpha ? new Channel(5, 5).encode(w, h, a) : null;

        // Write the constants
        boolean isLandscape = w > h;
        int header24 = Math.round(63.0f * l_channel.dc)
                | (Math.round(31.5f + 31.5f * p_channel.dc) << 6)
                | (Math.round(31.5f + 31.5f * q_channel.dc) << 12)
                | (Math.round(31.0f * l_channel.scale) << 18)
                | (hasAlpha ? 1 << 23 : 0);
        int header16 = (isLandscape ? ly : lx)
                | (Math.round(63.0f * p_channel.scale) << 3)
                | (Math.round(63.0f * q_channel.scale) << 9)
                | (isLandscape ? 1 << 15 : 0);
        int ac_start = hasAlpha ? 6 : 5;
        int ac_count = l_channel.ac.length + p_channel.ac.length + q_channel.ac.length
                + (hasAlpha ? a_channel.ac.length : 0);
        byte[] hash = new byte[ac_start + (ac_count + 1) / 2];
        hash[0] = (byte) header24;
        hash[1] = (byte) (header24 >> 8);
        hash[2] = (byte) (header24 >> 16);
        hash[3] = (byte) header16;
        hash[4] = (byte) (header16 >> 8);
        if (hasAlpha) hash[5] = (byte) (Math.round(15.0f * a_channel.dc)
                | (Math.round(15.0f * a_channel.scale) << 4));

        // Write the varying factors
        int ac_index = 0;
        ac_index = l_channel.writeTo(hash, ac_start, ac_index);
        ac_index = p_channel.writeTo(hash, ac_start, ac_index);
        ac_index = q_channel.writeTo(hash, ac_start, ac_index);
        if (hasAlpha) a_channel.writeTo(hash, ac_start, ac_index);
        return hash;
    }

    /**
     * Decodes a ThumbHash to an RGBA image. RGB is not be premultiplied by A.
     *
     * @param hash The bytes of the ThumbHash.
     * @return The width, height, and pixels of the rendered placeholder image.
     */
    public static Image thumbHashToRGBA(byte[] hash) {
        // Read the constants
        int header24 = (hash[0] & 255) | ((hash[1] & 255) << 8) | ((hash[2] & 255) << 16);
        int header16 = (hash[3] & 255) | ((hash[4] & 255) << 8);
        float l_dc = (float) (header24 & 63) / 63.0f;
        float p_dc = (float) ((header24 >> 6) & 63) / 31.5f - 1.0f;
        float q_dc = (float) ((header24 >> 12) & 63) / 31.5f - 1.0f;
        float l_scale = (float) ((header24 >> 18) & 31) / 31.0f;
        boolean hasAlpha = (header24 >> 23) != 0;
        float p_scale = (float) ((header16 >> 3) & 63) / 63.0f;
        float q_scale = (float) ((header16 >> 9) & 63) / 63.0f;
        boolean isLandscape = (header16 >> 15) != 0;
        int lx = Math.max(3, isLandscape ? hasAlpha ? 5 : 7 : header16 & 7);
        int ly = Math.max(3, isLandscape ? header16 & 7 : hasAlpha ? 5 : 7);
        float a_dc = hasAlpha ? (float) (hash[5] & 15) / 15.0f : 1.0f;
        float a_scale = (float) ((hash[5] >> 4) & 15) / 15.0f;

        // Read the varying factors (boost saturation by 1.25x to compensate for quantization)
        int ac_start = hasAlpha ? 6 : 5;
        int ac_index = 0;
        Channel l_channel = new Channel(lx, ly);
        Channel p_channel = new Channel(3, 3);
        Channel q_channel = new Channel(3, 3);
        Channel a_channel = null;
        ac_index = l_channel.decode(hash, ac_start, ac_index, l_scale);
        ac_index = p_channel.decode(hash, ac_start, ac_index, p_scale * 1.25f);
        ac_index = q_channel.decode(hash, ac_start, ac_index, q_scale * 1.25f);
        if (hasAlpha) {
            a_channel = new Channel(5, 5);
            a_channel.decode(hash, ac_start, ac_index, a_scale);
        }
        float[] l_ac = l_channel.ac;
        float[] p_ac = p_channel.ac;
        float[] q_ac = q_channel.ac;
        float[] a_ac = hasAlpha ? a_channel.ac : null;

        // Decode using the DCT into RGB
        float ratio = thumbHashToApproximateAspectRatio(hash);
        int w = Math.round(ratio > 1.0f ? 32.0f : 32.0f * ratio);
        int h = Math.round(ratio > 1.0f ? 32.0f / ratio : 32.0f);
        byte[] rgba = new byte[w * h * 4];
        int cx_stop = Math.max(lx, hasAlpha ? 5 : 3);
        int cy_stop = Math.max(ly, hasAlpha ? 5 : 3);
        float[] fx = new float[cx_stop];
        float[] fy = new float[cy_stop];
        for (int y = 0, i = 0; y < h; y++) {
            for (int x = 0; x < w; x++, i += 4) {
                float l = l_dc, p = p_dc, q = q_dc, a = a_dc;

                // Precompute the coefficients
                for (int cx = 0; cx < cx_stop; cx++)
                    fx[cx] = (float) Math.cos(Math.PI / w * (x + 0.5f) * cx);
                for (int cy = 0; cy < cy_stop; cy++)
                    fy[cy] = (float) Math.cos(Math.PI / h * (y + 0.5f) * cy);

                // Decode L
                for (int cy = 0, j = 0; cy < ly; cy++) {
                    float fy2 = fy[cy] * 2.0f;
                    for (int cx = cy > 0 ? 0 : 1; cx * ly < lx * (ly - cy); cx++, j++)
                        l += l_ac[j] * fx[cx] * fy2;
                }

                // Decode P and Q
                for (int cy = 0, j = 0; cy < 3; cy++) {
                    float fy2 = fy[cy] * 2.0f;
                    for (int cx = cy > 0 ? 0 : 1; cx < 3 - cy; cx++, j++) {
                        float f = fx[cx] * fy2;
                        p += p_ac[j] * f;
                        q += q_ac[j] * f;
                    }
                }

                // Decode A
                if (hasAlpha)
                    for (int cy = 0, j = 0; cy < 5; cy++) {
                        float fy2 = fy[cy] * 2.0f;
                        for (int cx = cy > 0 ? 0 : 1; cx < 5 - cy; cx++, j++)
                            a += a_ac[j] * fx[cx] * fy2;
                    }

                // Convert to RGB
                float b = l - 2.0f / 3.0f * p;
                float r = (3.0f * l - b + q) / 2.0f;
                float g = r - q;
                rgba[i] = (byte) Math.max(0, Math.round(255.0f * Math.min(1, r)));
                rgba[i + 1] = (byte) Math.max(0, Math.round(255.0f * Math.min(1, g)));
                rgba[i + 2] = (byte) Math.max(0, Math.round(255.0f * Math.min(1, b)));
                rgba[i + 3] = (byte) Math.max(0, Math.round(255.0f * Math.min(1, a)));
            }
        }
        return new Image(w, h, rgba);
    }

    /**
     * Extracts the average color from a ThumbHash. RGB is not be premultiplied by A.
     *
     * @param hash The bytes of the ThumbHash.
     * @return The RGBA values for the average color. Each value ranges from 0 to 1.
     */
    public static RGBA thumbHashToAverageRGBA(byte[] hash) {
        int header = (hash[0] & 255) | ((hash[1] & 255) << 8) | ((hash[2] & 255) << 16);
        float l = (float) (header & 63) / 63.0f;
        float p = (float) ((header >> 6) & 63) / 31.5f - 1.0f;
        float q = (float) ((header >> 12) & 63) / 31.5f - 1.0f;
        boolean hasAlpha = (header >> 23) != 0;
        float a = hasAlpha ? (float) (hash[5] & 15) / 15.0f : 1.0f;
        float b = l - 2.0f / 3.0f * p;
        float r = (3.0f * l - b + q) / 2.0f;
        float g = r - q;
        return new RGBA(
                Math.max(0, Math.min(1, r)),
                Math.max(0, Math.min(1, g)),
                Math.max(0, Math.min(1, b)),
                a);
    }

    /**
     * Extracts the approximate aspect ratio of the original image.
     *
     * @param hash The bytes of the ThumbHash.
     * @return The approximate aspect ratio (i.e. width / height).
     */
    public static float thumbHashToApproximateAspectRatio(byte[] hash) {
        byte header = hash[3];
        boolean hasAlpha = (hash[2] & 0x80) != 0;
        boolean isLandscape = (hash[4] & 0x80) != 0;
        int lx = isLandscape ? hasAlpha ? 5 : 7 : header & 7;
        int ly = isLandscape ? header & 7 : hasAlpha ? 5 : 7;
        return (float) lx / (float) ly;
    }

    public static final class Image {
        public int width;
        public int height;
        public byte[] rgba;

        public Image(int width, int height, byte[] rgba) {
            this.width = width;
            this.height = height;
            this.rgba = rgba;
        }
    }

    public static final class RGBA {
        public float r;
        public float g;
        public float b;
        public float a;

        public RGBA(float r, float g, float b, float a) {
            this.r = r;
            this.g = g;
            this.b = b;
            this.a = a;
        }
    }

    private static final class Channel {
        int nx;
        int ny;
        float dc;
        float[] ac;
        float scale;

        Channel(int nx, int ny) {
            this.nx = nx;
            this.ny = ny;
            int n = 0;
            for (int cy = 0; cy < ny; cy++)
                for (int cx = cy > 0 ? 0 : 1; cx * ny < nx * (ny - cy); cx++)
                    n++;
            ac = new float[n];
        }

        Channel encode(int w, int h, float[] channel) {
            int n = 0;
            float[] fx = new float[w];
            for (int cy = 0; cy < ny; cy++) {
                for (int cx = 0; cx * ny < nx * (ny - cy); cx++) {
                    float f = 0;
                    for (int x = 0; x < w; x++)
                        fx[x] = (float) Math.cos(Math.PI / w * cx * (x + 0.5f));
                    for (int y = 0; y < h; y++) {
                        float fy = (float) Math.cos(Math.PI / h * cy * (y + 0.5f));
                        for (int x = 0; x < w; x++)
                            f += channel[x + y * w] * fx[x] * fy;
                    }
                    f /= w * h;
                    if (cx > 0 || cy > 0) {
                        ac[n++] = f;
                        scale = Math.max(scale, Math.abs(f));
                    } else {
                        dc = f;
                    }
                }
            }
            if (scale > 0)
                for (int i = 0; i < ac.length; i++)
                    ac[i] = 0.5f + 0.5f / scale * ac[i];
            return this;
        }

        int decode(byte[] hash, int start, int index, float scale) {
            for (int i = 0; i < ac.length; i++) {
                int data = hash[start + (index >> 1)] >> ((index & 1) << 2);
                ac[i] = ((float) (data & 15) / 7.5f - 1.0f) * scale;
                index++;
            }
            return index;
        }

        int writeTo(byte[] hash, int start, int index) {
            for (float v : ac) {
                hash[start + (index >> 1)] |= Math.round(15.0f * v) << ((index & 1) << 2);
                index++;
            }
            return index;
        }
    }
}