Tiny PCX reader

pcx.h

#ifndef PCX_H
#define PCX_H

#include <assert.h>

#include "common.h"

typedef struct PCXHeader
{
    u8  id;
    u8  version;
    u8  encoding;
    u8  bits_per_px;
    s16 x_min;
    s16 y_min;
    s16 x_max;
    s16 y_max;
    s16 x_res;
    s16 y_res;
    u8  pal[48];
    u8  __reserved_1;
    u8  planes;
    s16 bytes_per_line;
    s16 palette_type;
    s16 x_screen_size;
    s16 y_screen_size;
    u8  __reserved_2[54];     /* Reserved (Always 0) */
} PCXHeader;

typedef struct PCXColor
{
    u8 r;
    u8 g;
    u8 b;
} PCXColor;

typedef PCXColor PCXPalette[256];

typedef struct PCX
{
    PCXHeader *header;
    u8 *data_begin;
    PCXPalette *palette;
    u32 size;
    u16 w;
    u16 pitch;
    u16 h;
    u16 bpp;
} PCX;

PCX
pcx_open(u8 *buffer, u32 size)
{
    assert(size >= sizeof(PCXHeader));
    PCX pcx = {0};
    pcx.header = (PCXHeader*)buffer;
    // PCX header control byte 0x0a.
    assert(pcx.header->id == 0x0a);
    // Encoding must be RLE.
    assert(pcx.header->encoding == 1);
    // Only 8-bit format is supported.
    assert(pcx.header->bits_per_px == 8);

    assert(pcx.header->x_max > pcx.header->x_min);
    assert(pcx.header->y_max > pcx.header->y_min);

    pcx.w = pcx.header->x_max - pcx.header->x_min + 1;
    pcx.h = pcx.header->y_max - pcx.header->y_min + 1;
    pcx.bpp = 8 / pcx.header->bits_per_px;
    pcx.pitch = pcx.header->bytes_per_line * pcx.header->planes * pcx.bpp;

    // No padding allowed.
    assert((pcx.pitch - pcx.w) == 0);

    pcx.size = pcx.w * pcx.h * pcx.bpp;
    pcx.data_begin = (u8*)(pcx.header + 1);

    u8 *palette = buffer + size - sizeof(PCXPalette) - 1;
    // Control byte 0xC before the palette.
    assert(*palette == 0xC);
    pcx.palette = (PCXPalette*)(palette + 1);

    return pcx;
}

void
pcx_decode(PCX *pcx, u8 *buffer)
{
    u8 *wit = buffer;
    u8 *rit = pcx->data_begin;

    u8 rc;
    while ((wit - buffer) < (s32)pcx->size)
    {
        if ((*rit & 0xC0) == 0xC0)
        {
            rc = *rit & 0x3F;
            rit++;
            assert(rit != (u8*)pcx->palette);
            do {
                *wit++ = *rit;
            } while (--rc);
            rit++;
        }
        else
        {
            *wit++ = *rit++;
        }
    }
    assert((wit - buffer) == (s32)pcx->size);
}

#endif // PCX_H