gif.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#include "gif.h"

/* helper to write a little-endian 16-bit number portably */
#define write_num(fd, n) write((fd), (uint8_t []) {(n) & 0xFF, (n) >> 8}, 2)

struct Node {
    uint16_t key;
    struct Node *children[0x10];
};
typedef struct Node Node;

static Node *
new_node(uint16_t key)
{
    Node *node = calloc(1, sizeof(*node));
    if (node)
        node->key = key;
    return node;
}

static Node *
new_trie(int *nkeys)
{
    Node *root = new_node(0);
    /* Create nodes for single pixels. */
    for (*nkeys = 0; *nkeys < 0x10; (*nkeys)++)
        root->children[*nkeys] = new_node(*nkeys);
    *nkeys += 2; /* skip clear code and stop code */
    return root;
}

static void
del_trie(Node *root)
{
    int i;

    if (!root)
        return;
    for (i = 0; i < 0x10; i++)
        del_trie(root->children[i]);
    free(root);
}

static void put_loop(GIF *gif, uint16_t loop);

GIF *
new_gif(const char *fname, uint16_t w, uint16_t h, uint8_t *gct, int loop)
{
    GIF *gif = calloc(1, sizeof(*gif) + 2*w*h);
    if (!gif)
        goto no_gif;
    gif->w = w; gif->h = h;
    gif->cur = (uint8_t *) &gif[1];
    gif->old = &gif->cur[w*h];
    /* fill back-buffer with invalid pixels to force overwrite */
    memset(gif->old, 0x10, w*h);
    gif->fd = creat(fname, 0666);
    if (gif->fd == -1)
        goto no_fd;
    write(gif->fd, "GIF89a", 6);
    write_num(gif->fd, w);
    write_num(gif->fd, h);
    write(gif->fd, (uint8_t []) {0xF3, 0x00, 0x00}, 3);
    write(gif->fd, gct, 0x30);
    if (loop >= 0 && loop <= 0xFFFF)
        put_loop(gif, (uint16_t) loop);
    return gif;
no_fd:
    free(gif);
no_gif:
    return NULL;
}

static void
put_loop(GIF *gif, uint16_t loop)
{
    write(gif->fd, (uint8_t []) {'!', 0xFF, 0x0B}, 3);
    write(gif->fd, "NETSCAPE2.0", 11);
    write(gif->fd, (uint8_t []) {0x03, 0x01}, 2);
    write_num(gif->fd, loop);
    write(gif->fd, "\0", 1);
}

/* Add packed key to buffer, updating offset and partial.
 *   gif->offset holds position to put next *bit*
 *   gif->partial holds bits to include in next byte */
static void
put_key(GIF *gif, uint16_t key, int key_size)
{
    int byte_offset, bit_offset, bits_to_write;
    byte_offset = gif->offset / 8;
    bit_offset = gif->offset % 8;
    gif->partial |= ((uint32_t) key) << bit_offset;
    bits_to_write = bit_offset + key_size;
    while (bits_to_write >= 8) {
        gif->buffer[byte_offset++] = gif->partial & 0xFF;
        if (byte_offset == 0xFF) {
            write(gif->fd, "\xFF", 1);
            write(gif->fd, gif->buffer, 0xFF);
            byte_offset = 0;
        }
        gif->partial >>= 8;
        bits_to_write -= 8;
    }
    gif->offset = (gif->offset + key_size) % (0xFF * 8);
}

static void
end_key(GIF *gif)
{
    int byte_offset;
    byte_offset = gif->offset / 8;
    if (gif->offset % 8)
        gif->buffer[byte_offset++] = gif->partial & 0xFF;
    if (byte_offset) {
	write(gif->fd, (uint8_t []) {byte_offset}, 1);
	write(gif->fd, gif->buffer, byte_offset);
    }
    write(gif->fd, "\0", 1);
    gif->offset = gif->partial = 0;
}

static void
put_image(GIF *gif, uint16_t w, uint16_t h, uint16_t x, uint16_t y)
{
    int nkeys, key_size, i, j;
    Node *node, *child, *root;
    uint8_t id_packed = 0x00;

    if (gif->plt) {
        id_packed &= ~0x7;
        id_packed |= 0x83; /* Local clut, 4 bits. */
    }

    root = malloc(sizeof(*root));
    write(gif->fd, ",", 1);
    write_num(gif->fd, x);
    write_num(gif->fd, y);
    write_num(gif->fd, w);
    write_num(gif->fd, h);
    write(gif->fd, &id_packed, 1);
    if (id_packed & 0x80)
        write(gif->fd, gif->plt, 3<<((id_packed & 0x7)+1));

    write(gif->fd, "\x04", 1); /* Min code size */
    root = node = new_trie(&nkeys);
    key_size = 5;
    put_key(gif, 0x10, key_size); /* clear code */
    for (i = y; i < y+h; i++) {
        for (j = x; j < x+w; j++) {
            uint8_t pixel = gif->cur[i*gif->w+j];
            child = node->children[pixel];
            if (child) {
                node = child;
            } else {
                put_key(gif, node->key, key_size);
                if (nkeys < 0x1000) {
                    if (nkeys == (1 << key_size))
                        key_size++;
                    node->children[pixel] = new_node(nkeys++);
                } else {
                    put_key(gif, 0x10, key_size); /* clear code */
                    del_trie(root);
                    root = node = new_trie(&nkeys);
                    key_size = 5;
                }
                node = root->children[pixel];
            }
        }
    }
    put_key(gif, node->key, key_size);
    put_key(gif, 0x11, key_size); /* stop code */
    end_key(gif);
    del_trie(root);
}

static int
get_bbox(GIF *gif, uint16_t *w, uint16_t *h, uint16_t *x, uint16_t *y)
{
    int i, j, k;
    int left, right, top, bottom;
    left = gif->w; right = 0;
    top = gif->h; bottom = 0;
    k = 0;
    for (i = 0; i < gif->h; i++) {
        for (j = 0; j < gif->w; j++, k++) {
            if (gif->cur[k] != gif->old[k]) {
                if (j < left)   left    = j;
                if (j > right)  right   = j;
                if (i < top)    top     = i;
                if (i > bottom) bottom  = i;
            }
        }
    }
    if (left != gif->w && top != gif->h) {
        *x = left; *y = top;
        *w = right - left + 1;
        *h = bottom - top + 1;
        return 1;
    } else {
        return 0;
    }
}

static void
set_delay(GIF *gif, uint16_t d)
{
    write(gif->fd, (uint8_t []) {'!', 0xF9, 0x04, 0x04}, 4);
    write_num(gif->fd, d);
    write(gif->fd, "\0\0", 2);
}

void
add_frame(GIF *gif, uint16_t d)
{
    uint16_t w, h, x, y;
    uint8_t *tmp;

    if (d)
        set_delay(gif, d);
    if (gif->plt_dirty) {
        w = gif->w; h = gif->h; x = y = 0;
        gif->plt_dirty = 0;
    } else
    if (!get_bbox(gif, &w, &h, &x, &y)) {
        /* image's not changed; save one pixel just to add delay */
        if (!d) return;
        w = h = 1;
        x = y = 0;
    }
    put_image(gif, w, h, x, y);
    tmp = gif->old;
    gif->old = gif->cur;
    gif->cur = tmp;
}

void
close_gif(GIF* gif)
{
    write(gif->fd, ";", 1);
    close(gif->fd);
    free(gif);
}