[flac-dev] [PATCH] stream_encoder : Improve selection of residual accumulator width

[lvqcl]

Miroslav Lichvar цкщеу:

> +/*
> + * This is used to avoid overflow with unusual signals in 32-bit
> + * accumulator in the *precompute_partition_info_sums_* functions.
> + */
> +#define FLAC__MAX_EXTRA_RESIDUAL_BPS 4

> +		/* WATCHOUT: "+ bps + FLAC__MAX_EXTRA_RESIDUAL_BPS" is the maximum
> +		 * assumed size of the average residual magnitude */
> +		if(FLAC__bitmath_ilog2(default_partition_samples) + bps + FLAC__MAX_EXTRA_RESIDUAL_BPS < 32) {

 From FLAC__fixed_compute_residual:
     residual[i] = data[i] - 4*data[i-1] + 6*data[i-2] - 4*data[i-3] + data[i-4];

so max(residual[i]) == 16 * max(data[j]), or:  max_bps(residual[]) == 4 + max_bps(data[]).

Am I right that it's the reason why FLAC__MAX_EXTRA_RESIDUAL_BPS is equal to 4?