The issue is that state->cur[] is 8-bits, but a+b+1 can overflow before being clipped to 0xF in the following line, causing an incorrect state to be saved for the next symbol. This solves numerous bitstream desyncs, particularly when coefficients with magnitude greater than 127 are sent.
Unified · Split
| @@ -91,10 +91,7 @@ uint32_t ff_aac_ac_get_pk(uint32_t c) · 90 unmodified lines | ||
| 91 | 91 | void ff_aac_ac_update_context(AACArithState *state, int idx, |
| 92 | 92 | uint16_t a, uint16_t b) |
| 93 | 93 | { |
| 94 | − state->cur[0] = a + b + 1; | |
| 95 | − if (state->cur[0] > 0xF) | |
| 96 | − state->cur[0] = 0xF; | |
| 97 | − | |
| 94 | + state->cur[0] = FFMIN(a + b + 1, 0xF); | |
| 98 | 95 | state->cur[3] = state->cur[2]; |
| 99 | 96 | state->cur[2] = state->cur[1]; |
| 100 | 97 | state->cur[1] = state->cur[0]; |