shachain: add shachain_next_index()

[ccan] / ccan / crypto / shachain / shachain.c

diff --git a/ccan/crypto/shachain/shachain.c b/ccan/crypto/shachain/shachain.c
index 4b6a31db3f93f419282ead63467495999263fd54..d4508a05a0f7addf2e33442ff1f13d3564bab9e7 100644 (file)
--- a/ccan/crypto/shachain/shachain.c
+++ b/ccan/crypto/shachain/shachain.c
@@ -10,14 +10,52 @@ 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)
+static unsigned int count_trailing_zeroes(shachain_index_t index)
 {
+#if HAVE_BUILTIN_CTZLL
+       return index ? (unsigned int)__builtin_ctzll(index) : SHACHAIN_BITS;
+#else
+       unsigned int i;
+
+       for (i = 0; i < SHACHAIN_BITS; i++) {
+               if (index & (1ULL << i))
+                       break;
+       }
+       return i;
+#endif
+}
+
+static bool can_derive(shachain_index_t from, shachain_index_t to)
+{
+       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,
+                  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 to, and not from. */
+       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);
+                       sha256(hash, hash, sizeof(*hash));
                }
        }
 }
@@ -25,62 +63,63 @@ 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(0, index, seed, hash);
 }
 
-void shachain_init(struct shachain *shachain)
+shachain_index_t shachain_next_index(const struct shachain *chain)
 {
-       shachain->num_valid = 0;
+       return chain->min_index - 1;
 }
 
-/* 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)
+void shachain_init(struct shachain *chain)
 {
-       return (~from & to) == 0;
+       chain->num_valid = 0;
+       /* This is 0 in the case where SHACHAIN_BITS is 64. */
+       chain->min_index = (shachain_index_t)((UINT64_MAX >> (64 - SHACHAIN_BITS)) + 1);
 }
 
-void shachain_add_hash(struct shachain *chain,
+bool shachain_add_hash(struct shachain *chain,
                       shachain_index_t index, const struct sha256 *hash)
 {
-       int i;
+       unsigned int i, pos;
 
-       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))
-                       break;
+       /* You have to insert them in order! */
+       assert(index == shachain_next_index(chain));
+
+       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->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;
 }
 
 bool shachain_get_hash(const struct shachain *chain,
                       shachain_index_t index, struct sha256 *hash)
 {
-       int i;
+       unsigned 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;