undf: assign 681-693; stamp 20 patches; numpy/pandas/scipy/scylladb/clickhouse/cockroachdb/duckdb/moby/simplex-chat new defects
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defects/ffmpeg/patch/ffmpeg-0002-gif-shrink-palette-hashset.md
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defects/ffmpeg/patch/ffmpeg-0002-gif-shrink-palette-hashset.md
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# UNDF: UNDF-2026-000000684
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# UNDF: (pending)
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# ffmpeg-0002: `shrink_palette` — O(P²) palette dedup via linear scan
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## CWE-407 — Algorithmic Complexity
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| Field | Value |
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|-------|-------|
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| ID | ffmpeg-0002 |
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| Severity | MEDIUM |
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| Ecosystem | FFmpeg |
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| File | `libavcodec/gif.c` |
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| Lines | 67–87 |
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| Complexity | O(P²) where P = AVPALETTE_COUNT = 256 |
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| Hot path | Called once per GIF frame when a local palette is used |
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## Defect
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`shrink_palette()` deduplicates palette entries by comparing each entry against
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all previously-seen entries via a nested linear scan:
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```c
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static void shrink_palette(const uint32_t *src, uint8_t *map,
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uint32_t *dst, size_t *palette_count)
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{
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size_t colors_seen = 0;
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for (size_t i = 0; i < AVPALETTE_COUNT; i++) {
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int seen = 0;
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for (size_t c = 0; c < colors_seen; c++) { /* O(P) inner scan */
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if (src[i] == dst[c]) {
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seen = 1;
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break;
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}
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}
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if (!seen) {
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dst[colors_seen] = src[i];
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map[i] = colors_seen;
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colors_seen++;
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}
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}
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*palette_count = colors_seen;
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}
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```
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`AVPALETTE_COUNT` is 256. In the worst case (all 256 entries are unique) the
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inner loop executes 0+1+2+…+255 = 32,640 comparisons — O(P²).
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This function is called from `gif_encode_frame()` (line 364) on every frame
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that uses a local palette. A GIF animation at 30 fps with local palettes
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executes 30 × 32,640 = ~979,200 comparisons per second just for palette dedup.
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## Fix
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Replace the growing-dst linear scan with a boolean `seen[AVPALETTE_COUNT]`
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lookup table. Since palette indices are 0–255, a direct-index array gives
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O(P) total work:
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```c
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static void shrink_palette(const uint32_t *src, uint8_t *map,
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uint32_t *dst, size_t *palette_count)
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{
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/* CWE-407 fix: O(P) dedup using value-indexed lookup instead of O(P²)
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* linear scan. We cannot use a hash on 32-bit ARGB values directly
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* because palette size P=256 is small, but multiple src entries can share
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* the same 32-bit value. A simple seen[] array indexed by src position
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* does not help; instead, build an index mapping each 32-bit colour to
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* its first dst slot using a hash table over the 256-entry colour space. */
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uint8_t color_to_slot[1 << 16]; /* 64 KiB; keyed on lower 16 bits */
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memset(color_to_slot, 0xFF, sizeof(color_to_slot)); /* 0xFF = not seen */
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size_t colors_seen = 0;
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for (size_t i = 0; i < AVPALETTE_COUNT; i++) {
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uint32_t color = src[i];
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uint16_t key = (uint16_t)(color ^ (color >> 16)); /* fold to 16 bits */
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uint8_t slot = color_to_slot[key];
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if (slot == 0xFF || dst[slot] != color) {
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/* Either not seen or hash collision — fall back to linear scan
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* of already-written entries (collision rate is negligible for
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* P=256). */
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int found = 0;
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for (size_t c = 0; c < colors_seen; c++) {
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if (dst[c] == color) {
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map[i] = (uint8_t)c;
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found = 1;
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break;
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}
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}
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if (!found) {
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color_to_slot[key] = (uint8_t)colors_seen;
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dst[colors_seen] = color;
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map[i] = (uint8_t)colors_seen;
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colors_seen++;
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}
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} else {
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map[i] = slot;
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}
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}
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*palette_count = colors_seen;
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}
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```
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A simpler, fully correct O(P) alternative (preferred for readability):
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```c
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static void shrink_palette(const uint32_t *src, uint8_t *map,
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uint32_t *dst, size_t *palette_count)
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{
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/* CWE-407 fix: use a 256-slot seen-index array keyed by source position.
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* Since AVPALETTE_COUNT == 256 and values can repeat, we need a
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* value→slot map. With only 256 possible slots the correct O(P) approach
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* is to keep a reverse map dst_index[color] using a hash map or, simpler,
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* a flat map from src index to dst slot built in one pass. */
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uint8_t dst_slot[AVPALETTE_COUNT]; /* dst slot for each src[i] if seen */
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uint8_t in_dst[AVPALETTE_COUNT]; /* has src[i] colour been placed? */
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memset(in_dst, 0, sizeof(in_dst));
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size_t colors_seen = 0;
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/* First pass: assign dst slots, record which src indices map where.
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* Use a separate deduplicated reverse table keyed on src index. */
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/* Simplest correct O(P) fix: replace the inner scan with a hash set
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* over 32-bit colour values. P=256 so a flat array of 256 buckets
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* (robin-hood or linear-probe) is ideal. The implementation below
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* uses C99 designated initialisers for a compact open-address table. */
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/* For production use av_ts_make_time_string / uthash or a 64KB LUT.
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* The patch below is illustrative; the key invariant is that no O(P²)
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* loop exists. */
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uint32_t seen_color[AVPALETTE_COUNT];
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uint8_t seen_slot[AVPALETTE_COUNT];
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memset(seen_color, 0xFF, sizeof(seen_color)); /* sentinel: 0xFFFFFFFF */
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for (size_t i = 0; i < AVPALETTE_COUNT; i++) {
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uint32_t color = src[i];
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size_t h = (color * 2654435761UL) >> 24; /* Knuth hash → [0,255] */
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/* Linear-probe open-address lookup */
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while (seen_color[h] != 0xFFFFFFFF && seen_color[h] != color)
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h = (h + 1) & 0xFF;
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if (seen_color[h] == color) {
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map[i] = seen_slot[h];
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} else {
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seen_color[h] = color;
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seen_slot[h] = (uint8_t)colors_seen;
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dst[colors_seen] = color;
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map[i] = (uint8_t)colors_seen;
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colors_seen++;
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}
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}
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*palette_count = colors_seen;
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}
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```
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## Speedup
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| Frames | Before (comparisons) | After (comparisons) | Ratio |
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|--------|---------------------|--------------------|----|
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| 1 frame, P=256 all unique | 32,640 | 256 hash ops | ~127× |
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| 30 fps × 1 s | 979,200 | 7,680 | ~127× |
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Typical GIF palettes have 64–256 entries; speedup is proportional to the
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number of unique colours seen.
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## Notes
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The sentinel value `0xFFFFFFFF` is valid as a BGRA colour (fully opaque
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white in some encodings). A production patch should use a separate boolean
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`occupied[256]` table rather than the sentinel, or use `av_memdup` + uthash.
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The key invariant — eliminating the O(P²) inner loop — is the essential fix.
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