mini/gif.h

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <stdint.h>

#define READ(dst, size) memcpy(dst, buffer, size); buffer += size

struct rgb { uint8_t r, g, b; };

struct image_descriptor_t
{
    uint16_t left, top, width, height;
    uint8_t  fields;
};

struct dictionary_entry_t
{
    unsigned char byte;
    int prev, len;
};

void uncompress( int code_length, const uint8_t *input, int input_length, uint8_t *out )
{
    dictionary_entry_t *dictionary;
    int dictionary_ind;

    const int clear_code = 1 << ( code_length );
    const int stop_code = clear_code + 1;
    const int reset_code_length = code_length;

    dictionary = ( dictionary_entry_t * )
    malloc( sizeof( dictionary_entry_t ) * ( 1 << ( code_length + 1 ) ) );

    for ( dictionary_ind = 0; dictionary_ind < ( 1 << code_length ); dictionary_ind++ )
    {
        dictionary[ dictionary_ind ].byte = dictionary_ind;
        dictionary[ dictionary_ind ].prev = -1;
        dictionary[ dictionary_ind ].len = 1;
    }
    dictionary_ind+=2;

    int prev = -1;
    uint32_t mask = 0x01;
    while ( input_length )
    {
        int code = 0x0;
        
        for (int i=0; i<(code_length + 1); i++)
        {
            const int bit = ( *input & mask ) ? 1 : 0;
            mask <<= 1;

            if ( mask == 0x100 )
            {
                mask = 0x01;
                input++;
                input_length--;
            }

            code = code | ( bit << i );
        }

        if ( code == clear_code )
        {
            code_length = reset_code_length;
            dictionary = ( dictionary_entry_t * ) realloc( dictionary, sizeof( dictionary_entry_t ) * ( 1 << ( code_length + 1 ) ) );

            for ( dictionary_ind = 0; dictionary_ind < ( 1 << code_length ); dictionary_ind++ )
            {
                dictionary[ dictionary_ind ].byte = dictionary_ind;
                dictionary[ dictionary_ind ].prev = -1;
                dictionary[ dictionary_ind ].len = 1;
            }
            dictionary_ind+=2;
            prev = -1;
            continue;
        }
        else if ( code == stop_code ) break;

        if ( (prev > -1) && (code_length < 12) )
        {
            if (code > dictionary_ind)
            {
                fprintf( stderr, "code = %.02x, but dictionary_ind = %.02x\n", code, dictionary_ind );
                exit( 0 );
            }

            int ptr = (code == dictionary_ind) ? prev : code;
            while ( dictionary[ ptr ].prev != -1 ) ptr = dictionary[ ptr ].prev;
            dictionary[ dictionary_ind ].byte = dictionary[ ptr ].byte;

            dictionary[ dictionary_ind ].prev = prev;
            dictionary[ dictionary_ind ].len = dictionary[ prev ].len + 1;
            dictionary_ind++;

            if ( (dictionary_ind == (1 << (code_length + 1))) && (code_length < 11) )
            {
                code_length++;
                dictionary = ( dictionary_entry_t * ) realloc( dictionary, sizeof( dictionary_entry_t ) * ( 1 << ( code_length + 1 ) ) );
            }
        }

        prev = code;

        const int match_len = dictionary[ code ].len;
        while ( code != -1 )
        {
            out[ dictionary[ code ].len - 1 ] = dictionary[ code ].byte;
            if ( dictionary[ code ].prev == code )
            {
                fprintf( stderr, "Internal error; self-reference." );
                exit( 0 );
            }
            code = dictionary[ code ].prev;
        }
        out += match_len;
    }
}

static int read_sub_blocks( uint8_t* buffer, uint8_t **data )
{
    int data_length = 0;
    *data = NULL;
    int index = 0;

    while (1) {
        uint8_t block_size = *(buffer++);
        if ( block_size == 0 ) break;

        data_length += block_size;
        *data = (uint8_t*)realloc( *data, data_length );
		READ(*data + index, block_size);
        index += block_size;
    }

    return data_length;
}

uint8_t* process_image_descriptor( uint8_t* buffer, rgb *gct, int gct_size, int resolution_bits )
{
    image_descriptor_t image_descriptor;
	READ(&image_descriptor, 9);

    uint8_t lzw_code_size;
	READ(&lzw_code_size, 1);

    uint8_t *compressed_data = NULL;
    const int compressed_data_length = read_sub_blocks( buffer, &compressed_data );

    const int uncompressed_data_length = image_descriptor.width * image_descriptor.height;
    uint8_t *uncompressed_data = (uint8_t*)malloc( uncompressed_data_length );

    uncompress( lzw_code_size, compressed_data, compressed_data_length, uncompressed_data );

    if ( compressed_data ) free( compressed_data );

    return uncompressed_data;
}

uint32_t* LoadPalette(uint8_t *buffer, uint16_t *size = NULL )
{
    buffer += 10;
    const uint8_t fields = *buffer;

	uint32_t *global_color_table = (uint32_t *)calloc(1, 1024);;

    if (size) *size = 0;
	if (fields & 0x80) {
		int global_color_table_size = 1 << (((fields & 0x07) + 1));
        if (size) *size = global_color_table_size;

        for (int i=0; i<global_color_table_size;++i) {
            buffer+=3;
            global_color_table[i] = (buffer[0]<<16) + (buffer[1]<<8) + buffer[2];
        }
	}
	return global_color_table;
}

static uint8_t* LoadGif(uint8_t *buffer, uint16_t* w, uint16_t* h)
{
    int global_color_table_size = 0;  // number of entries in global_color_table
    rgb *global_color_table = NULL;

    buffer += 6; // Ignore header
    *w = *((uint16_t*)buffer); buffer+=2;
    *h = *((uint16_t*)buffer); buffer+=2;
    const uint8_t fields = *buffer; buffer+=3;

    const int color_resolution_bits = ( ( fields & 0x70 ) >> 4 ) + 1;

    if ( fields & 0x80 )
    {
        global_color_table_size = 1 << ( ( ( fields & 0x07 ) + 1 ) );
        global_color_table = (rgb*)malloc( 3 * global_color_table_size );
		READ(global_color_table, 3 * global_color_table_size);
    }
    
    uint8_t block_type = 0x0;
    while ( block_type != 0x3B )
    {
		READ(&block_type, 1);
        
        uint8_t size;
        switch ( block_type )
        {
            case 0x2C:
				return process_image_descriptor(buffer, global_color_table, global_color_table_size, color_resolution_bits);
            case 0x21:
                buffer++; //size = *(buffer++); buffer += size;
                do { size = *(buffer++); buffer += size; } while (size != 0);
                break;
            case 0x3B:
                break;
            default:
                fprintf( stderr, "Bailing on unrecognized block type %.02x\n", block_type );
                return NULL;
        }
    }
	return NULL;
}


/*unsigned char* LoadGif(unsigned char *buffer, unsigned short* w, unsigned short* h) {
	return process_gif_stream(buffer, w, h);
}*/