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avcodec/mss4: Simplify creating VLC tables

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The lengths of the codes used by the mss4 decoder are ascending from
left to right and therefore the lengths can be run-length encoded and
the codes can be easily derived from them. And this is how it is indeed
done. Yet some things can nevertheless be improved:

a) The number of entries of the current VLC is implicitly contained in
the run-length table and needn't be externally prescribed.
b) The maximum length of a code is just the length of the last code
(given that the lengths are ascending), so there is no point in setting
max_bits in the loop itself.
c) One can offload the actual calculation of the codes to
ff_init_vlc_from_lengths().

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
This commit is contained in:
Andreas Rheinhardt 2020-11-01 00:26:32 +01:00
parent e7f15f777d
commit 70bbb4cce8
1 changed files with 7 additions and 11 deletions
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18
libavcodec/mss4.c
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@ -143,25 +143,21 @@ typedef struct MSS4Context {
} MSS4Context;
static av_cold int mss4_init_vlc(VLC *vlc, const uint8_t *lens,
const uint8_t *syms, int num_syms)
const uint8_t *syms)
{
uint8_t bits[MAX_ENTRIES];
uint16_t codes[MAX_ENTRIES];
int i, j;
int prefix = 0, max_bits = 0, idx = 0;
int idx = 0;
for (i = 0; i < 16; i++) {
for (j = 0; j < lens[i]; j++) {
bits[idx] = i + 1;
codes[idx] = prefix++;
max_bits = i + 1;
idx++;
}
prefix <<= 1;
}
return ff_init_vlc_sparse(vlc, FFMIN(max_bits, 9), num_syms, bits, 1, 1,
codes, 2, 2, syms, 1, 1, 0);
return ff_init_vlc_from_lengths(vlc, FFMIN(bits[idx - 1], 9), idx,
bits, 1, syms, 1, 1, 0, 0, NULL);
}
static av_cold int mss4_init_vlcs(MSS4Context *ctx)
@ -169,15 +165,15 @@ static av_cold int mss4_init_vlcs(MSS4Context *ctx)
int ret, i;
for (i = 0; i < 2; i++) {
ret = mss4_init_vlc(&ctx->dc_vlc[i], mss4_dc_vlc_lens[i], NULL, 12);
ret = mss4_init_vlc(&ctx->dc_vlc[i], mss4_dc_vlc_lens[i], NULL);
if (ret)
return ret;
ret = mss4_init_vlc(&ctx->ac_vlc[i], mss4_ac_vlc_lens[i],
mss4_ac_vlc_syms[i], 162);
mss4_ac_vlc_syms[i]);
if (ret)
return ret;
ret = mss4_init_vlc(&ctx->vec_entry_vlc[i], mss4_vec_entry_vlc_lens[i],
mss4_vec_entry_vlc_syms[i], 9);
mss4_vec_entry_vlc_syms[i]);
if (ret)
return ret;
}