shachain: add shachain_next_index()

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

ccan/crypto/shachain/shachain.c

@@ -66,10 +66,16 @@ void shachain_from_seed(const struct sha256 *seed, shachain_index_t index,
 	derive(0, index, seed, hash);
 }
 
+shachain_index_t shachain_next_index(const struct shachain *chain)
+{
+	return chain->min_index - 1;
+}
+
 void shachain_init(struct shachain *chain)
 {
 	chain->num_valid = 0;
-	chain->min_index = 0;
+	/* This is 0 in the case where SHACHAIN_BITS is 64. */
+	chain->min_index = (shachain_index_t)((UINT64_MAX >> (64 - SHACHAIN_BITS)) + 1);
 }
 
 bool shachain_add_hash(struct shachain *chain,
@@ -78,9 +84,7 @@ bool shachain_add_hash(struct shachain *chain,
 	unsigned int i, pos;
 
 	/* You have to insert them in order! */
-	assert(index == chain->min_index - 1 ||
-	       (index == (shachain_index_t)(UINT64_MAX >> (64 - SHACHAIN_BITS))
-		&& chain->num_valid == 0));
+	assert(index == shachain_next_index(chain));
 
 	pos = count_trailing_zeroes(index);
 

ccan/crypto/shachain/shachain.h

@@ -71,20 +71,28 @@ struct shachain {
  */
 void shachain_init(struct shachain *chain);
 
+/**
+ * shachain_next_index - what's the next index I can add to the shachain?
+ * @chain: the chain
+ *
+ * This returns 0xFFFFFFFFFFFFFFFF (for a freshly
+ * initialized chain), or one less than the previously successfully
+ * added value.
+ */
+shachain_index_t shachain_next_index(const struct shachain *chain);
+
 /**
  * shachain_add_hash - record the hash for the next index.
  * @chain: the chain to add to
  * @index: the index of the hash
  * @hash: the hash value.
  *
- * You can only add index 0xFFFFFFFFFFFFFFFF (for a freshly
- * initialized chain), or one less than the previously successfully
- * added value.
+ * You can only add shachain_next_index(@chain).
  *
  * This can fail (return false without altering @chain) if the hash
  * for this index isn't consistent with previous hashes (ie. wasn't
  * generated from the same seed), though it can't always detect that.
- * If the hash is inconsistent yet undetected, the next addition will
+ * If the hash is inconsistent yet undetected, a future addition will
  * fail.
  *
  * Example:

ccan/crypto/shachain/test/run.c

@@ -13,7 +13,7 @@ int main(void)
 	uint64_t i, j;
 
 	/* This is how many tests you plan to run */
-	plan_tests(NUM_TESTS * 3 + NUM_TESTS * (NUM_TESTS + 1) - 1);
+	plan_tests(NUM_TESTS * 4 + NUM_TESTS * (NUM_TESTS + 1) - 1);
 
 	memset(&seed, 0, sizeof(seed));
 	/* Generate a whole heap. */
@@ -34,6 +34,7 @@ int main(void)
 	     i--) {
 		struct sha256 hash;
 		int expidx = 0xFFFFFFFFFFFFFFFFULL - i;
+		ok1(shachain_next_index(&chain) == i);
 		ok1(shachain_add_hash(&chain, i, &expect[expidx]));
 		for (j = i; j != 0; j++) {
 			ok1(shachain_get_hash(&chain, j, &hash));