crypto/shachain: a bit more common code.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --git a/ccan/crypto/shachain/shachain.c b/ccan/crypto/shachain/shachain.c
index 4b6a31db3f93f419282ead63467495999263fd54..c66196f71d881ce5e19b9bc088d66b9d2fc8b03b 100644 (file)
--- a/ccan/crypto/shachain/shachain.c
+++ b/ccan/crypto/shachain/shachain.c
@@ -10,12 +10,28 @@ static void change_bit(unsigned char *arr, size_t index)
        arr[index / CHAR_BIT] ^= (1 << (index % CHAR_BIT));
 }
 
-static void derive(shachain_index_t index, size_t bits, struct sha256 *hash)
+/* We can only ever *unset* bits, so to must only have bits in from. */
+static bool can_derive(shachain_index_t from, shachain_index_t to)
 {
+       return (~from & to) == 0;
+}
+
+static void derive(shachain_index_t from, shachain_index_t to,
+                  const struct sha256 *from_hash,
+                  struct sha256 *hash)
+{
+       shachain_index_t branches;
        int i;
 
-       for (i = bits - 1; i >= 0; i--) {
-               if (!((index >> i) & 1)) {
+       assert(can_derive(from, to));
+
+       /* We start with the first hash. */
+       *hash = *from_hash;
+
+       /* This represents the bits set in from, and not to. */
+       branches = from ^ to;
+       for (i = ilog64(branches) - 1; i >= 0; i--) {
+               if (((branches >> i) & 1)) {
                        change_bit(hash->u.u8, i);
                        sha256(hash, hash, 1);
                }
@@ -25,8 +41,7 @@ static void derive(shachain_index_t index, size_t bits, struct sha256 *hash)
 void shachain_from_seed(const struct sha256 *seed, shachain_index_t index,
                        struct sha256 *hash)
 {
-       *hash = *seed;
-       derive(index, sizeof(index) * CHAR_BIT, hash);
+       derive((shachain_index_t)-1ULL, index, seed, hash);
 }
 
 void shachain_init(struct shachain *shachain)
@@ -34,12 +49,6 @@ void shachain_init(struct shachain *shachain)
        shachain->num_valid = 0;
 }
 
-/* We can only ever *unset* bits, so to must only have bits in from. */
-static bool can_derive(shachain_index_t from, shachain_index_t to)
-{
-       return (~from & to) == 0;
-}
-
 void shachain_add_hash(struct shachain *chain,
                       shachain_index_t index, const struct sha256 *hash)
 {
@@ -65,22 +74,13 @@ bool shachain_get_hash(const struct shachain *chain,
        int i;
 
        for (i = 0; i < chain->num_valid; i++) {
-               shachain_index_t diff;
-
                /* If we can get from key to index only by resetting bits,
                 * we can derive from it => index has no bits key doesn't. */
                if (!can_derive(chain->known[i].index, index))
                        continue;
 
-               /* Start from this hash. */
-               *hash = chain->known[i].hash;
-
-               /* This indicates the bits which are in 'index' and
-                * not the key */
-               diff = index ^ chain->known[i].index;
-
-               /* Using ilog64 here is an optimization. */
-               derive(~diff, ilog64(diff), hash);
+               derive(chain->known[i].index, index, &chain->known[i].hash,
+                      hash);
                return true;
        }
        return false;