From f3578e3250d1027bbc9e3fe2fa535b8ed540c5d8 Mon Sep 17 00:00:00 2001
From: Christopher Haster <chaster@utexas.edu>
Date: Tue, 10 Oct 2017 18:48:24 -0500
Subject: [PATCH] Removed clamping to block size in ctz linked-list

Initially, I was concerned that the number of pointers in the ctz
linked-list could exceed the storage in a block. Long story short
this isn't really possible outside of extremely small block sizes.

Since clamping impacts the layout of files on disk, removing the
block size removed quite a bit of logic and corner cases. Replaced
with an assert on block size during initialization.

---

Long story long, the minimum block size needed to store all ctz
pointers in a filesystem can be found with this formula:

B = (w/8)*log2(2^w / (B - 2*(w/8)))

where:
B = block size in bytes
w = pointer width in bits

It's not a very pretty formula, but does give us some useful info
if we apply some math:

min block size:
32 bit ctz linked-list = 104 bytes
64 bit ctz linked-list = 448 bytes

For littlefs, 128 bytes is a perfectly reasonable minimum block size.
---
 lfs.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/lfs.c b/lfs.c
index 4733dcc..50760be 100644
--- a/lfs.c
+++ b/lfs.c
@@ -1005,12 +1005,11 @@ int lfs_dir_rewind(lfs_t *lfs, lfs_dir_t *dir) {
 /// File index list operations ///
 static int lfs_index(lfs_t *lfs, lfs_off_t *off) {
     lfs_off_t i = 0;
-    lfs_size_t words = lfs->cfg->block_size / 4;
 
     while (*off >= lfs->cfg->block_size) {
         i += 1;
         *off -= lfs->cfg->block_size;
-        *off += 4*lfs_min(lfs_ctz(i)+1, words-1);
+        *off += 4*(lfs_ctz(i) + 1);
     }
 
     return i;
@@ -1028,12 +1027,11 @@ static int lfs_index_find(lfs_t *lfs,
 
     lfs_off_t current = lfs_index(lfs, &(lfs_off_t){size-1});
     lfs_off_t target = lfs_index(lfs, &pos);
-    lfs_size_t words = lfs->cfg->block_size / 4;
 
     while (current > target) {
         lfs_size_t skip = lfs_min(
                 lfs_npw2(current-target+1) - 1,
-                lfs_min(lfs_ctz(current)+1, words-1) - 1);
+                lfs_ctz(current));
 
         int err = lfs_cache_read(lfs, rcache, pcache, head, 4*skip, &head, 4);
         if (err) {
@@ -1102,8 +1100,7 @@ static int lfs_index_extend(lfs_t *lfs,
 
         // append block
         index += 1;
-        lfs_size_t words = lfs->cfg->block_size / 4;
-        lfs_size_t skips = lfs_min(lfs_ctz(index)+1, words-1);
+        lfs_size_t skips = lfs_ctz(index) + 1;
 
         for (lfs_off_t i = 0; i < skips; i++) {
             int err = lfs_cache_prog(lfs, pcache, rcache,
@@ -1880,6 +1877,10 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
         }
     }
 
+    // check that the block size is large enough to fit ctz pointers
+    assert(4*lfs_npw2(0xffffffff / (lfs->cfg->block_size-2*4))
+            <= lfs->cfg->block_size);
+
     // setup default state
     lfs->root[0] = 0xffffffff;
     lfs->root[1] = 0xffffffff;