From 774ae676e432c22c1d63526ef6dcbdc99241fae4 Mon Sep 17 00:00:00 2001
From: Christopher Haster <geky@geky.net>
Date: Sun, 9 Apr 2023 02:30:13 -0500
Subject: [PATCH] Flipped layout of fcrcs to match B-tree branches

This may seem more complicated to decode, we can't assume crcs start
at the beginning of the data, but this layout of putting the crcs at the
end has the benefit of allowing the size of the crc to be unknown in
certain cases.

The is a bit of optimistic future proofing for the case where we may
support different crc widths.
---
 lfs.c | 41 +++++++++++++++++++++--------------------
 1 file changed, 21 insertions(+), 20 deletions(-)

diff --git a/lfs.c b/lfs.c
index 3d973f44..77198ff2 100644
--- a/lfs.c
+++ b/lfs.c
@@ -940,37 +940,38 @@ static void lfs_fcrc_tole32(struct lfs_fcrc *fcrc) {
 #endif
 
 // fcrc on-disk encoding
-struct lfsr_fcrc {
-    uint32_t crc;
+typedef struct lfsr_fcrc {
     lfs_size_t size;
-};
+    uint32_t crc;
+} lfsr_fcrc_t;
 
-#define LFSR_FCRC_DSIZE (4+5)
+// 1 leb128 + 1 crc32c => 9 bytes (worst case)
+#define LFSR_FCRC_DSIZE (5+4)
 
 static lfs_ssize_t lfsr_fcrc_todisk(
-        const struct lfsr_fcrc *fcrc,
+        const lfsr_fcrc_t *fcrc,
         uint8_t buf[static LFSR_FCRC_DSIZE]) {
-    lfs_tole32_(fcrc->crc, &buf[0]);
-
-    lfs_ssize_t delta = lfs_toleb128(fcrc->size, &buf[4], 5);
-    if (delta < 0) {
-        return delta;
+    lfs_ssize_t d = lfs_toleb128(fcrc->size, &buf[0], 5);
+    if (d < 0) {
+        return d;
     }
 
-    return sizeof(uint32_t) + delta;
+    lfs_tole32_(fcrc->crc, &buf[d]);
+
+    return d + sizeof(uint32_t);
 }
 
 static lfs_ssize_t lfsr_fcrc_fromdisk(
-        struct lfsr_fcrc *fcrc,
+        lfsr_fcrc_t *fcrc,
         const uint8_t buf[static LFSR_FCRC_DSIZE]) {
-    fcrc->crc = lfs_fromle32_(&buf[0]);
-
-    lfs_ssize_t delta = lfs_fromleb128(&fcrc->size, &buf[4], 5);
-    if (delta < 0) {
-        return delta;
+    lfs_ssize_t d = lfs_fromleb128(&fcrc->size, &buf[0], 5);
+    if (d < 0) {
+        return d;
     }
 
-    return sizeof(uint32_t) + delta;
+    fcrc->crc = lfs_fromle32_(&buf[d]);
+
+    return d + sizeof(uint32_t);
 }
 
 // other endianness operations
@@ -1321,7 +1322,7 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
     lfs_size_t weight = 0;
 
     // assume unerased until proven otherwise
-    struct lfsr_fcrc fcrc;
+    lfsr_fcrc_t fcrc;
     bool hasfcrc = false;
     bool maybeerased = false;
 
@@ -2478,7 +2479,7 @@ static int lfsr_rbyd_commit(lfs_t *lfs, lfsr_rbyd_t *rbyd,
 
         // find the expected fcrc, don't bother avoiding a reread of the
         // perturb byte, as it should still be in our cache
-        struct lfsr_fcrc fcrc = {.crc=0, .size=lfs->cfg->prog_size};
+        lfsr_fcrc_t fcrc = {.size=lfs->cfg->prog_size, .crc=0};
         err = lfs_bd_crc32c(lfs,
                 &lfs->pcache, &lfs->rcache, lfs->cfg->prog_size,
                 rbyd_.block, aligned, fcrc.size, &fcrc.crc);