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Adopted alternative implementation for lfs_ctz_index

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Same runtime cost, however reduces the logic and avoids one of
the two big branches. See the DESIGN.md for more info.

Now uses these equations instead of the messy guess and correct method:
n = (N - w/8(popcount(N/(B-2w/8)) + 2)) / (B-2w/8)
off = N - (B-w2/8)n - w/8popcount(n)
This commit is contained in:
Christopher Haster
2017-10-18 00:33:59 -05:00
parent 46e22b2a38
commit 0825d34f3d
2 changed files with 18 additions and 43 deletions
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16
lfs.c
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@@ -1005,21 +1005,15 @@ int lfs_dir_rewind(lfs_t *lfs, lfs_dir_t *dir) {
/// File index list operations ///
static int lfs_ctz_index(lfs_t *lfs, lfs_off_t *off) {
lfs_off_t size = *off;
lfs_off_t i = size / (lfs->cfg->block_size-2*4);
lfs_off_t b = lfs->cfg->block_size - 2*4;
lfs_off_t i = size / b;
if (i == 0) {
return 0;
}
lfs_off_t nsize = (lfs->cfg->block_size-2*4)*i + 4*lfs_popc(i-1) + 2*4;
lfs_soff_t noff = size - nsize;
if (noff < 0) {
*off = noff + lfs->cfg->block_size;
return i-1;
} else {
*off = noff + 4*(lfs_ctz(i) + 1);
return i;
}
i = (size - 4*(lfs_popc(i-1)+2)) / b;
*off = size - b*i - 4*lfs_popc(i);
return i;
}
static int lfs_ctz_find(lfs_t *lfs,