Adopted alternative implementation for lfs_ctz_index

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)

1 files changed, 5 insertions, 11 deletions

diff --git a/lfs.c b/lfs.c
index 927e7c8..2859783 100644
--- a/lfs.c
+++ b/lfs.c
@@ -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,