/* NOTES
   - The encoder dumps uint16_t numbers as is into GIF files, which means it
     only works on little-endian machines, since this is what GIF requires.
*/

#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"

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

GIF *
new_gif(const char *fname, uint16_t w, uint16_t h, uint8_t *gct)
{
    GIF *gif = calloc(1, sizeof(*gif) + 2*w*h);
    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)
        return NULL;
    write(gif->fd, "GIF89a", 6);
    write(gif->fd, &w, 2);
    write(gif->fd, &h, 2);
    write(gif->fd, (uint8_t []) {0xF3, 0x00, 0x00}, 3);
    write(gif->fd, gct, 0x30);
    return gif;
}

/* 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;
    gif->buffer[byte_offset++] = gif->partial & 0xFF;
    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;

    root = malloc(sizeof(*root));
    write(gif->fd, ",", 1);
    write(gif->fd, &x, 2);
    write(gif->fd, &y, 2);
    write(gif->fd, &w, 2);
    write(gif->fd, &h, 2);
    write(gif->fd, (uint8_t []) {0x00, 0x04}, 2);
    /* Create nodes for single pixels. */
    for (nkeys = 0; nkeys < 0x10; nkeys++)
        root->children[nkeys] = new_node(nkeys);
    node = root;
    key_size = 5;
    nkeys += 2; /* skip clear code and stop code */
    put_key(gif, 0x10, key_size); /* clear code */
    for (i = y; i < y+h; i++) {
        for (j = x; j < x+w; j++) {
#ifdef CROP
            uint8_t pixel = 2;
#else
            uint8_t pixel = gif->cur[i*gif->w+j];
#endif
            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++);
                }
                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 void
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;
            }
        }
    }
    *x = left; *y = top;
    *w = right - left + 1;
    *h = bottom - top + 1;
}

static void
set_delay(GIF *gif, uint16_t d)
{
#ifdef CROP
    write(gif->fd, (uint8_t []) {'!', 0xF9, 0x04, 0x08}, 4);
#else
    write(gif->fd, (uint8_t []) {'!', 0xF9, 0x04, 0x04}, 4);
#endif
    write(gif->fd, &d, 2);
    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);
    get_bbox(gif, &w, &h, &x, &y);
    if (x == gif->w || y == gif->h) {
        /* image haven't changed; save one pixel just to add delay */
        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);
}