X-Git-Url: https://git.ozlabs.org/?p=ccan;a=blobdiff_plain;f=ccan%2Fcrypto%2Fshachain%2Fshachain.c;h=6cfb7244f981472994c813eb8d91fb72df99fd75;hp=8c9fdcf50fe2de50f72ce64524e2e9c216f1e827;hb=d23fb57c8e276090325162966b094ebf71e73e68;hpb=bb88463014092a5550ea4c51a037e12e49446722

diff --git a/ccan/crypto/shachain/shachain.c b/ccan/crypto/shachain/shachain.c
index 8c9fdcf5..6cfb7244 100644
--- a/ccan/crypto/shachain/shachain.c
+++ b/ccan/crypto/shachain/shachain.c
@@ -5,15 +5,39 @@
 #include <string.h>
 #include <assert.h>
 
+#define INDEX_BITS ((sizeof(shachain_index_t)) * CHAR_BIT)
+
 static void change_bit(unsigned char *arr, size_t index)
 {
 	arr[index / CHAR_BIT] ^= (1 << (index % CHAR_BIT));
 }
 
-/* We can only ever *unset* bits, so to must only have bits in from. */
+static int count_trailing_zeroes(shachain_index_t index)
+{
+#if HAVE_BUILTIN_CTZLL
+	return index ? __builtin_ctzll(index) : INDEX_BITS;
+#else
+	int i;
+
+	for (i = 0; i < INDEX_BITS; i++) {
+		if (index & (1ULL << i))
+			break;
+	}
+	return i;
+#endif
+}
+
 static bool can_derive(shachain_index_t from, shachain_index_t to)
 {
-	return (~from & to) == 0;
+	shachain_index_t mask;
+
+	/* Corner case: can always derive from seed. */
+	if (from == 0)
+		return true;
+
+	/* Leading bits must be the same */
+	mask = ~((1ULL << count_trailing_zeroes(from))-1);
+	return ((from ^ to) & mask) == 0;
 }
 
 static void derive(shachain_index_t from, shachain_index_t to,
@@ -28,7 +52,7 @@ static void derive(shachain_index_t from, shachain_index_t to,
 	/* We start with the first hash. */
 	*hash = *from_hash;
 
-	/* This represents the bits set in from, and not to. */
+	/* This represents the bits set in to, and not from. */
 	branches = from ^ to;
 	for (i = ilog64(branches) - 1; i >= 0; i--) {
 		if (((branches >> i) & 1)) {
@@ -41,45 +65,42 @@ static void derive(shachain_index_t from, shachain_index_t to,
 void shachain_from_seed(const struct sha256 *seed, shachain_index_t index,
 			struct sha256 *hash)
 {
-	derive((shachain_index_t)-1ULL, index, seed, hash);
+	derive(0, index, seed, hash);
 }
 
 void shachain_init(struct shachain *chain)
 {
 	chain->num_valid = 0;
-	chain->max_index = 0;
+	chain->min_index = 0;
 }
 
 bool shachain_add_hash(struct shachain *chain,
 		       shachain_index_t index, const struct sha256 *hash)
 {
-	int i;
+	int i, pos;
 
 	/* You have to insert them in order! */
-	assert(index == chain->max_index + 1 ||
-	       (index == 0 && chain->num_valid == 0));
-	
-	for (i = 0; i < chain->num_valid; i++) {
-		/* If we could derive this value, we don't need it,
-		 * not any others (since they're in order). */
-		if (can_derive(index, chain->known[i].index)) {
-			struct sha256 expect;
-
-			/* Make sure the others derive as expected! */
-			derive(index, chain->known[i].index, hash, &expect);
-			if (memcmp(&expect, &chain->known[i].hash,
-				   sizeof(expect)) != 0)
-				return false;
-			break;
-		}
+	assert(index == chain->min_index - 1 ||
+	       (index == (shachain_index_t)(-1ULL) && chain->num_valid == 0));
+
+	pos = count_trailing_zeroes(index);
+
+	/* All derivable answers must be valid. */
+	/* FIXME: Is it sufficient to check just the next answer? */
+	for (i = 0; i < pos; i++) {
+		struct sha256 expect;
+
+		/* Make sure the others derive as expected! */
+		derive(index, chain->known[i].index, hash, &expect);
+		if (memcmp(&expect, &chain->known[i].hash, sizeof(expect)))
+			return false;
 	}
 
-	/* This can happen if you skip indices! */
-	assert(i < sizeof(chain->known) / sizeof(chain->known[0]));
-	chain->known[i].index = index;
-	chain->known[i].hash = *hash;
-	chain->num_valid = i+1;
-	chain->max_index = index;
+	chain->known[pos].index = index;
+	chain->known[pos].hash = *hash;
+	if (pos + 1 > chain->num_valid)
+		chain->num_valid = pos + 1;
+	chain->min_index = index;
 	return true;
 }