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stb_vorbis: Fix handling of negative numbers in ilog. 

For negative n, the original code went down the "n < (1<<4)"
path and performed an out-of-bounds array access. Fix the code
to agree with section 9.2.1 of the Vorbis spec. (Verified by
exhaustive testing of all 32-bit ints.)

Fixes issue #355.
This commit is contained in:
Fabian Giesen 2016-09-06 11:58:00 -07:00 committed by Fabian Giesen
parent 2f4166e91d
commit f32854c809
1 changed files with 7 additions and 6 deletions
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13
stb_vorbis.c
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@ -986,17 +986,18 @@ static int ilog(int32 n)
{
static signed char log2_4[16] = { 0,1,2,2,3,3,3,3,4,4,4,4,4,4,4,4 };
if (n < 0) return 0; // signed n returns 0
// 2 compares if n < 16, 3 compares otherwise (4 if signed or n > 1<<29)
if (n < (1 << 14))
if (n < (1 << 4)) return 0 + log2_4[n ];
else if (n < (1 << 9)) return 5 + log2_4[n >> 5];
if (n < (1 << 4)) return 0 + log2_4[n ];
else if (n < (1 << 9)) return 5 + log2_4[n >> 5];
else return 10 + log2_4[n >> 10];
else if (n < (1 << 24))
if (n < (1 << 19)) return 15 + log2_4[n >> 15];
if (n < (1 << 19)) return 15 + log2_4[n >> 15];
else return 20 + log2_4[n >> 20];
else if (n < (1 << 29)) return 25 + log2_4[n >> 25];
else if (n < (1 << 31)) return 30 + log2_4[n >> 30];
else return 0; // signed n returns 0
else if (n < (1 << 29)) return 25 + log2_4[n >> 25];
else return 30 + log2_4[n >> 30];
}
#ifndef M_PI