Small, no-dependency 8-bit indexed PNG writer

png.c

// 8-bit indexed PNG writer, no dependencies
// $ cc -o png png.c
// $ ./png >image.png
// This is free and unencumbered software released into the public domain.
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>

static uint32_t adler32(uint32_t sum, uint8_t *buf, ptrdiff_t len)
{
    uint32_t a = sum & 0xffff;
    uint32_t b = sum >> 16;
    for (ptrdiff_t i = 0; i < len; i++) {
        a = (a + buf[i]) % 0xfff1UL;
        b = (b + a     ) % 0xfff1UL;
    }
    return b<<16 | a;
}

static uint32_t crc32(uint32_t crc, uint8_t *buf, ptrdiff_t len)
{
    static uint32_t t[256] = {
        0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
        0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
        0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
        0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
        0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
        0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
        0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
        0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
        0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
        0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
        0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
        0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
        0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
        0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
        0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
        0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
        0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
        0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
        0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
        0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
        0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
        0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
        0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
        0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
        0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
        0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
        0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
        0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
        0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
        0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
        0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
        0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
        0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
        0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
        0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
        0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
        0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
        0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
        0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
        0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
        0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
        0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
        0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
        0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
        0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
        0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
        0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
        0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
        0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
        0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
        0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
        0x2d02ef8d
    };
    crc ^= -1;
    for (ptrdiff_t n = 0; n < len; n++) {
        crc = t[(crc ^ buf[n]) & 255] ^ (crc >> 8);
    }
    return crc ^ -1;
}

static uint32_t writebuf(uint32_t crc, uint8_t *buf, ptrdiff_t len)
{
    fwrite(buf, len, 1, stdout);
    return crc32(crc, buf, len);
}

static uint32_t writestr(uint32_t crc, char *buf, ptrdiff_t len)
{
    return writebuf(crc, (uint8_t *)buf, len);
}

static uint32_t writeu8(uint32_t crc, uint8_t b)
{
    return writebuf(crc, &b, 1);
}

static uint32_t writeu16(uint32_t crc, uint16_t x)
{
    uint8_t buf[2] = {(uint8_t)x, (uint8_t)(x>>8)};  // little endian
    return writebuf(crc, buf, 2);
}

static uint32_t writeu32(uint32_t crc, uint32_t x)
{
    uint8_t buf[4] = {
        (uint8_t)(x>>24), (uint8_t)(x>>16), (uint8_t)(x>>8), (uint8_t)x
    };
    return writebuf(crc, buf, 4);
}

// Write an 8-bit indexed PNG to standard output. RGB palette is 3*256
// bytes, and the image can be no wider than 65,534 pixels.
static _Bool writepng(uint8_t *pal, uint8_t *data, int32_t w, int32_t h)
{
    if (w>0xfffe || w<=0 || h<=0 || h>(0x7fffffff-6)/(6+w)) return 0;

    writestr(0, "\x89PNG\r\n\x1a\n", 8);

    uint32_t crc = 0;
          writeu32(crc, 4+4+1+1+1+1+1);
    crc = writestr(crc, "IHDR", 4);
    crc = writeu32(crc, w);
    crc = writeu32(crc, h);
    crc = writeu8 (crc, 8); // depth
    crc = writeu8 (crc, 3); // indexed
    crc = writeu8 (crc, 0); // compression
    crc = writeu8 (crc, 0); // filter
    crc = writeu8 (crc, 0); // interlace
          writeu32(crc, crc);

    crc = 0;
          writeu32(crc, 3*256);
    crc = writestr(crc, "PLTE", 4);
    crc = writebuf(crc, pal, 3*256);
          writeu32(crc, crc);

    crc = 0;
    int32_t len = (1+2+2+1+w) * h;
          writeu32(crc, 2+len+4);
    crc = writestr(crc, "IDAT", 4);
    crc = writestr(crc, "\x78\x01", 2);  // zlib header
    uint32_t sum  = 1;
    uint8_t  zero = 0;
    for (int32_t y = 0; y < h; y++) {
        uint8_t bhdr = y == h-1;
        crc = writebuf(crc, &bhdr, 1);  // block header
        crc = writeu16(crc, (uint16_t) (w+1));
        crc = writeu16(crc, (uint16_t)~(w+1));
        sum = adler32 (sum, &zero, 1);
        crc = writeu8 (crc, 0);  // filter (none)
        sum = adler32 (sum, data + y*w, w);
        crc = writebuf(crc, data + y*w, w);
    }
    crc = writeu32(crc, sum);
          writeu32(crc, crc);

    crc = 0;
          writeu32(crc, 0);
    crc = writestr(crc, "IEND", 4);
          writeu32(crc, crc);
    return 1;
}

int main(void)
{
    #if _WIN32
    int _setmode(int, int);
    _setmode(0, 0x8000);
    _setmode(1, 0x8000);
    #endif

    uint8_t pal[3*256];
    for (int i = 0; i < 256; i++) {
        pal[3*i+0] = pal[3*i+1] = pal[3*i+2] = (uint8_t)i;
    }

    enum { W=640, H=480 };
    uint8_t data[W*H];
    uint64_t rng = 1;
    for (int i = 0; i < W*H; i++) {
        data[i] = (uint8_t)((rng = rng*0x3243f6a8885a308d + 1) >> 56);
    }

    _Bool err = !writepng(pal, data, W, H);
    fflush(stdout);
    return ferror(stdout) || err;
}