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Adopted lfs_ctz_index implementation using popcount

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This reduces the O(n^2logn) runtime to read a file to only O(nlog).
The extra O(n) did not touch the disk, so it isn't a problem until the
files become very large, but this solution comes with very little cost.

Long story short, you can find the block index + offset pair for a
CTZ linked-list with this series of formulas:

n' = floor(N / (B - 2w/8))
N' = (B - 2w/8)n' + (w/8)popcount(n')
off' = N - N'
n, off =
  n'-1, off'+B                if off' <  0
  n',   off'+(w/8)(ctz(n')+1) if off' >= 0

For the long story, you will need to see the updated DESIGN.md
This commit is contained in:
Christopher Haster
2017-10-16 19:08:47 -05:00
parent 4fdca15a0d
commit 46e22b2a38
3 changed files with 123 additions and 17 deletions
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21
lfs.c
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@@ -1004,15 +1004,22 @@ 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 i = 0;
while (*off >= lfs->cfg->block_size) {
i += 1;
*off -= lfs->cfg->block_size;
*off += 4*(lfs_ctz(i) + 1);
lfs_off_t size = *off;
lfs_off_t i = size / (lfs->cfg->block_size-2*4);
if (i == 0) {
return 0;
}
return i;
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;
}
}
static int lfs_ctz_find(lfs_t *lfs,