/* Copyright (c) 2018, Curtis McEnroe <programble@gmail.com>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <arpa/inet.h>
#include <assert.h>
#include <ctype.h>
#include <err.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sysexits.h>
#include <zlib.h>

#define PACKED __attribute__((packed))

#define CRC_INIT (crc32(0, Z_NULL, 0))

static void readExpect(
    const char *path, FILE *file,
    void *ptr, size_t size,
    const char *expect
) {
    fread(ptr, size, 1, file);
    if (ferror(file)) err(EX_IOERR, "%s", path);
    if (feof(file)) errx(EX_DATAERR, "%s: missing %s", path, expect);
}

static const uint8_t SIGNATURE[8] = {
    0x89, 'P', 'N', 'G', '\r', '\n', 0x1A, '\n'
};

static void readSignature(const char *path, FILE *file) {
    uint8_t signature[8];
    readExpect(path, file, signature, 8, "signature");
    if (memcmp(signature, SIGNATURE, 8)) {
        errx(EX_DATAERR, "%s: invalid signature", path);
    }
}

struct PACKED Chunk {
    uint32_t size;
    char type[4];
};

static const char *typeStr(const struct Chunk *chunk) {
    static char buf[5];
    memcpy(buf, chunk->type, 4);
    return buf;
}

static struct Chunk readChunk(const char *path, FILE *file) {
    struct Chunk chunk;
    readExpect(path, file, &chunk, sizeof(chunk), "chunk");
    chunk.size = ntohl(chunk.size);
    return chunk;
}

static void readCrc(const char *path, FILE *file, uint32_t expected) {
    uint32_t found;
    readExpect(path, file, &found, sizeof(found), "CRC32");
    if (ntohl(found) != expected) {
        errx(
            EX_DATAERR, "%s: expected CRC32 %08x, found %08x",
            path, expected, found
        );
    }
}

struct PACKED Header {
    uint32_t width;
    uint32_t height;
    uint8_t depth;
    enum PACKED {
        GRAYSCALE       = 0,
        TRUECOLOR       = 2,
        INDEXED         = 3,
        GRAYSCALE_ALPHA = 4,
        TRUECOLOR_ALPHA = 6,
    } color;
    uint8_t compression;
    uint8_t filter;
    uint8_t interlace;
};

static struct Header readHeader(const char *path, FILE *file) {
    struct Chunk ihdr = readChunk(path, file);
    if (memcmp(ihdr.type, "IHDR", 4)) {
        errx(EX_DATAERR, "%s: expected IHDR, found %s", path, typeStr(&ihdr));
    }
    if (ihdr.size != sizeof(struct Header)) {
        errx(
            EX_DATAERR, "%s: expected IHDR size %zu, found %u",
            path, sizeof(struct Header), ihdr.size
        );
    }
    uint32_t crc = crc32(CRC_INIT, (Bytef *)ihdr.type, sizeof(ihdr.type));

    struct Header header;
    readExpect(path, file, &header, sizeof(header), "IHDR data");
    readCrc(path, file, crc32(crc, (Bytef *)&header, sizeof(header)));

    header.width = ntohl(header.width);
    header.height = ntohl(header.height);

    if (!header.width) errx(EX_DATAERR, "%s: invalid width 0", path);
    if (!header.height) errx(EX_DATAERR, "%s: invalid height 0", path);
    if (
        header.depth != 1
        && header.depth != 2
        && header.depth != 4
        && header.depth != 8
        && header.depth != 16
    ) errx(EX_DATAERR, "%s: invalid bit depth %hhu", path, header.depth);
    if (
        header.color != GRAYSCALE
        && header.color != TRUECOLOR
        && header.color != INDEXED
        && header.color != GRAYSCALE_ALPHA
        && header.color != TRUECOLOR_ALPHA
    ) errx(EX_DATAERR, "%s: invalid color type %hhu", path, header.color);
    if (header.compression) {
        errx(
            EX_DATAERR, "%s: invalid compression method %hhu",
            path, header.compression
        );
    }
    if (header.filter) {
        errx(EX_DATAERR, "%s: invalid filter method %hhu", path, header.filter);
    }
    if (header.interlace > 1) {
        errx(EX_DATAERR, "%s: invalid interlace method %hhu", path, header.interlace);
    }

    return header;
}

struct Data {
    size_t size;
    uint8_t *ptr;
};

static struct Data readData(const char *path, FILE *file, struct Chunk idat) {
    uint32_t crc = crc32(CRC_INIT, (Bytef *)idat.type, sizeof(idat.type));

    uint8_t deflate[idat.size];
    readExpect(path, file, deflate, sizeof(deflate), "image data");
    readCrc(path, file, crc32(crc, deflate, sizeof(deflate)));

    z_stream stream = { .next_in = deflate, .avail_in = sizeof(deflate) };
    int error = inflateInit(&stream);
    if (error != Z_OK) errx(EX_SOFTWARE, "%s: inflateInit: %s", path, stream.msg);

    // "More typical zlib compression ratios are on the order of 2:1 to 5:1."
    // <https://www.zlib.net/zlib_tech.html>
    // FIXME: Compression ratio in PNG is usually much higher.
    struct Data data = { .size = idat.size * 5 };
    data.ptr = malloc(data.size);
    if (!data.ptr) err(EX_OSERR, "malloc(%zu)", data.size);

    for (;;) {
        stream.next_out = data.ptr + stream.total_out;
        stream.avail_out = data.size - stream.total_out;

        int status = inflate(&stream, Z_SYNC_FLUSH);
        if (status == Z_STREAM_END) break;
        if (status != Z_OK) errx(EX_DATAERR, "%s: inflate: %s", path, stream.msg);

        fprintf(stderr, "realloc %zu -> %zu\n", data.size, data.size * 2); // FIXME
        data.ptr = realloc(data.ptr, data.size *= 2);
        if (!data.ptr) err(EX_OSERR, "realloc(%zu)", data.size);
    }

    data.size = stream.total_out;
    fprintf(stderr, "total_out = %zu\n", data.size); // FIXME
    fprintf(stderr, "ratio = %zu\n", data.size / idat.size); // FIXME
    return data;
}

enum PACKED FilterType {
    FILT_NONE,
    FILT_SUB,
    FILT_UP,
    FILT_AVERAGE,
    FILT_PAETH,
};

static uint8_t paethPredictor(uint8_t a, uint8_t b, uint8_t c) {
    int32_t p = (int32_t)a + (int32_t)b - (int32_t)c;
    int32_t pa = labs(p - (int32_t)a);
    int32_t pb = labs(p - (int32_t)b);
    int32_t pc = labs(p - (int32_t)c);
    if (pa <= pb && pa <= pc) return a;
    if (pb < pc) return b;
    return c;
}

static uint8_t recon(
    enum FilterType type,
    uint8_t x, uint8_t a, uint8_t b, uint8_t c
) {
    switch (type) {
        case FILT_NONE:    return x;
        case FILT_SUB:     return x + a;
        case FILT_UP:      return x + b;
        case FILT_AVERAGE: return x + ((uint32_t)a + (uint32_t)b) / 2;
        case FILT_PAETH:   return x + paethPredictor(a, b, c);
    }
}

static uint8_t filt(
    enum FilterType type,
    uint8_t x, uint8_t a, uint8_t b, uint8_t c
) {
    switch (type) {
        case FILT_NONE:    return x;
        case FILT_SUB:     return x - a;
        case FILT_UP:      return x - b;
        case FILT_AVERAGE: return x - ((uint32_t)a + (uint32_t)b) / 2;
        case FILT_PAETH:   return x - paethPredictor(a, b, c);
    }
}

struct Scanline {
    enum FilterType *type;
    uint8_t *ptr;
};

static void reconData(const char *path, struct Header header, struct Data data) {
    uint8_t bytes = 3; // TODO

    size_t size = bytes * header.width * header.height;
    if (data.size < size) {
        errx(
            EX_DATAERR, "%s: expected data size %zu, found %zu",
            path, size, data.size
        );
    }

    struct Scanline lines[header.height];
    uint32_t stride = 1 + bytes * header.width;
    for (uint32_t y = 0; y < header.height; ++y) {
        lines[y].type = &data.ptr[y * stride];
        lines[y].ptr = &data.ptr[y * stride + 1];
        if (*lines[y].type > FILT_PAETH) {
            errx(EX_DATAERR, "%s: invalid filter type %hhu", path, *lines[y].type);
        }
    }

    for (uint32_t y = 0; y < header.height; ++y) {
        for (uint32_t x = 0; x < header.width; ++x) {
            for (uint8_t i = 0; i < bytes; ++i) {
                lines[y].ptr[x * bytes + i] = recon(
                    *lines[y].type,
                    lines[y].ptr[x * bytes + i],
                    x ? lines[y].ptr[(x - 1) * bytes + i] : 0,
                    y ? lines[y - 1].ptr[x * bytes + i] : 0,
                    (x && y) ? lines[y - 1].ptr[(x - 1) * bytes + i] : 0
                );
                *lines[y].type = FILT_NONE;
            }
        }
    }
}

static void filterData(struct Header header, struct Data data) {
    uint8_t bytes = 3; // TODO

    struct Scanline lines[header.height];
    uint32_t stride = 1 + bytes * header.width;
    for (uint32_t y = 0; y < header.height; ++y) {
        lines[y].type = &data.ptr[y * stride];
        lines[y].ptr = &data.ptr[y * stride + 1];
        assert(*lines[y].type == FILT_NONE);
    }

    for (uint32_t ry = 0; ry < header.height; ++ry) {
        uint32_t y = header.height - 1 - ry;
        for (uint32_t rx = 0; rx < header.width; ++rx) {
            uint32_t x = header.width - 1 - rx;
            for (uint8_t i = 0; i < bytes; ++i) {
                // TODO: Filter type heuristic.
                *lines[y].type = FILT_PAETH;
                lines[y].ptr[x * bytes + i] = filt(
                    FILT_PAETH,
                    lines[y].ptr[x * bytes + i],
                    x ? lines[y].ptr[(x - 1) * bytes + i] : 0,
                    y ? lines[y - 1].ptr[x * bytes + i] : 0,
                    (x && y) ? lines[y - 1].ptr[(x - 1) * bytes + i] : 0
                );
            }
        }
    }
}

static void writeExpect(const char *path, FILE *file, const void *ptr, size_t size) {
    fwrite(ptr, size, 1, file);
    if (ferror(file)) err(EX_IOERR, "%s", path);
}

static void writeSignature(const char *path, FILE *file) {
    writeExpect(path, file, SIGNATURE, sizeof(SIGNATURE));
}

static void writeChunk(const char *path, FILE *file, struct Chunk chunk) {
    chunk.size = htonl(chunk.size);
    writeExpect(path, file, &chunk, sizeof(chunk));
}

static void writeCrc(const char *path, FILE *file, uint32_t crc) {
    uint32_t net = htonl(crc);
    writeExpect(path, file, &net, sizeof(net));
}

static void writeHeader(const char *path, FILE *file, struct Header header) {
    struct Chunk ihdr = { .size = sizeof(header), .type = { 'I', 'H', 'D', 'R' } };
    writeChunk(path, file, ihdr);
    uint32_t crc = crc32(CRC_INIT, (Bytef *)ihdr.type, sizeof(ihdr.type));
    header.width = htonl(header.width);
    header.height = htonl(header.height);
    writeExpect(path, file, &header, sizeof(header));
    writeCrc(path, file, crc32(crc, (Bytef *)&header, sizeof(header)));
}

static void writeData(const char *path, FILE *file, struct Data data) {
    size_t bound = compressBound(data.size);
    uint8_t deflate[bound];
    int error = compress2(deflate, &bound, data.ptr, data.size, Z_BEST_COMPRESSION);
    if (error != Z_OK) errx(EX_SOFTWARE, "%s: compress2: %d", path, error);

    struct Chunk idat = { .size = bound, .type = { 'I', 'D', 'A', 'T' } };
    writeChunk(path, file, idat);
    uint32_t crc = crc32(CRC_INIT, (Bytef *)idat.type, sizeof(idat.type));
    writeExpect(path, file, deflate, bound);
    writeCrc(path, file, crc32(crc, deflate, bound));
}

static void writeEnd(const char *path, FILE *file) {
    struct Chunk iend = { .size = 0, .type = { 'I', 'E', 'N', 'D' } };
    writeChunk(path, file, iend);
    writeCrc(path, file, crc32(CRC_INIT, (Bytef *)iend.type, sizeof(iend.type)));
}

int main(int argc, char *argv[]) {
    const char *path = "stdin";
    FILE *file = stdin;
    if (argc > 1) {
        path = argv[1];
        file = fopen(path, "r");
        if (!file) err(EX_NOINPUT, "%s", path);
    }

    readSignature(path, file);
    struct Header header = readHeader(path, file);
    if (header.color != TRUECOLOR) {
        errx(EX_CONFIG, "%s: unsupported color type %u", path, header.color);
    }
    if (header.depth != 8) {
        errx(EX_CONFIG, "%s: unsupported bit depth %hhu", path, header.depth);
    }

    struct Data data = { .size = 0, .ptr = NULL };
    for (;;) {
        struct Chunk chunk = readChunk(path, file);
        if (!memcmp(chunk.type, "IDAT", 4)) {
            if (data.size) {
                errx(EX_CONFIG, "%s: unsupported multiple IDAT chunks", path);
            }
            data = readData(path, file, chunk);

        } else if (!memcmp(chunk.type, "IEND", 4)) {
            if (!data.size) errx(EX_DATAERR, "%s: missing IDAT chunk", path);
            int error = fclose(file);
            if (error) err(EX_IOERR, "%s", path);
            break;

        } else if (isupper(chunk.type[0])) {
            errx(
                EX_CONFIG, "%s: unsupported critical chunk %s",
                path, typeStr(&chunk)
            );

        } else {
            int error = fseek(file, chunk.size + 4, SEEK_CUR);
            if (error) err(EX_IOERR, "%s", path);
        }
    }

    reconData(path, header, data);

    // TODO: "Optimize".

    filterData(header, data);

    // TODO: -o
    path = "stdout";
    file = stdout;

    writeSignature(path, file);
    writeHeader(path, file, header);
    writeData(path, file, data);
    writeEnd(path, file);

    int error = fclose(file);
    if (error) err(EX_IOERR, "%s", path);
}