/*
 * gifenc.c - minimal streaming GIF89a encoder (see gifenc.h).
 *
 * LZW stream: min code size 8, CLEAR = 256, EOI = 257, fixed 9-bit codes.
 * After a CLEAR the decoder's dictionary holds 258 entries and grows by one
 * per pixel code; emitting CLEAR every 253 pixels keeps it below 512, so
 * the code width never has to grow past 9 bits. This is the 256-colour
 * generalisation of the 16-colour/5-bit encoder in counter.c.
 *
 * kimi-k3, October 2026
 */

#include "gifenc.h"

#define CLEAR 256
#define EOI   257
#define SPAN  253             /* pixel codes between CLEARs: 258+253 < 512 */

static void emit9(GifEnc *g, int code)
{
    g->bitacc |= (unsigned long)code << g->bitcnt;
    g->bitcnt += 9;
    while (g->bitcnt >= 8) {
        if (g->blklen < (int)sizeof(g->blk))
            g->blk[g->blklen++] = (unsigned char)(g->bitacc & 0xff);
        g->bitacc >>= 8;
        g->bitcnt -= 8;
    }
}

static void flushblk(GifEnc *g)
{
    int i = 0, chunk;
    while (i < g->blklen) {
        chunk = g->blklen - i > 255 ? 255 : g->blklen - i;
        fputc(chunk, g->out);
        fwrite(g->blk + i, 1, (size_t)chunk, g->out);
        i += chunk;
    }
    g->blklen = 0;
}

static void flushbits(GifEnc *g)
{
    if (g->bitcnt > 0) {
        if (g->blklen < (int)sizeof(g->blk))
            g->blk[g->blklen++] = (unsigned char)(g->bitacc & 0xff);
        g->bitacc = 0;
        g->bitcnt = 0;
    }
}

static void putshort(FILE *f, int v)
{
    fputc(v & 0xff, f);
    fputc((v >> 8) & 0xff, f);
}

void gif_begin(GifEnc *g, FILE *out, int w, int h,
               unsigned char pal[256][3])
{
    int i;

    g->out = out;
    g->bitacc = 0;
    g->bitcnt = 0;
    g->blklen = 0;
    g->px = 0;

    fwrite("GIF89a", 1, 6, out);
    putshort(out, w);
    putshort(out, h);
    fputc(0xf7, out);         /* GCT present, 8-bit colour res, 256 entries */
    fputc(0, out);            /* background index */
    fputc(0, out);            /* aspect */
    for (i = 0; i < 256; i++) {
        fputc(pal[i][0], out);
        fputc(pal[i][1], out);
        fputc(pal[i][2], out);
    }
    fputc(',', out);          /* image descriptor */
    putshort(out, 0);
    putshort(out, 0);
    putshort(out, w);
    putshort(out, h);
    fputc(0, out);            /* no local table */
    fputc(8, out);            /* LZW min code size */
    emit9(g, CLEAR);
}

void gif_pixel(GifEnc *g, int idx)
{
    if (g->px == SPAN) {
        emit9(g, CLEAR);
        g->px = 0;
    }
    emit9(g, idx);
    g->px++;
    if (g->blklen > 200)
        flushblk(g);
}

void gif_end(GifEnc *g)
{
    emit9(g, EOI);
    flushbits(g);
    flushblk(g);
    fputc(0, g->out);         /* block terminator */
    fputc(0x3b, g->out);      /* trailer */
    fflush(g->out);
}