Add utilities for log2

src/modules/bulletproofs/bulletproofs_util.h

@@ -39,4 +39,17 @@ static void secp256k1_bulletproofs_le64(unsigned char *output, const uint64_t n)
     output[7] = n >> 56;
 }
 
+/* Check if n is power of two*/
+static int secp256k1_is_power_of_two(size_t n) {
+    return n > 0 && (n & (n - 1)) == 0;
+}
+
+/* Compute the log2 of n. If n is not a power of two, it returns the largest
+ * `k` such that 2^k <= n. Assumes n < 2^64. In Bulletproofs, this is bounded
+ * by len of input vectors which can be safely assumed to be less than 2^64.
+*/
+static size_t secp256k1_bulletproofs_pp_log2(size_t n) {
+    return 64 - 1 - secp256k1_clz64_var((uint64_t)n);
+}
+
 #endif

src/modules/bulletproofs/tests_impl.h

@@ -7,6 +7,10 @@
 #ifndef _SECP256K1_MODULE_BULLETPROOFS_TEST_
 #define _SECP256K1_MODULE_BULLETPROOFS_TEST_
 
+#include <stdint.h>
+
+#include "include/secp256k1_bulletproofs.h"
+#include "bulletproofs_util.h"
 #include "bulletproofs_pp_transcript_impl.h"
 
 static void test_bulletproofs_generators_api(void) {
@@ -130,7 +134,23 @@ static void test_bulletproofs_pp_tagged_hash(void) {
     CHECK(secp256k1_memcmp_var(output, output_cached, 32) == 0);
 }
 
+void test_log_exp(void) {
+    CHECK(secp256k1_is_power_of_two(0) == 0);
+    CHECK(secp256k1_is_power_of_two(1) == 1);
+    CHECK(secp256k1_is_power_of_two(2) == 1);
+    CHECK(secp256k1_is_power_of_two(64) == 1);
+    CHECK(secp256k1_is_power_of_two(63) == 0);
+    CHECK(secp256k1_is_power_of_two(256) == 1);
+
+    CHECK(secp256k1_bulletproofs_pp_log2(1) == 0);
+    CHECK(secp256k1_bulletproofs_pp_log2(2) == 1);
+    CHECK(secp256k1_bulletproofs_pp_log2(255) == 7);
+    CHECK(secp256k1_bulletproofs_pp_log2(256) == 8);
+    CHECK(secp256k1_bulletproofs_pp_log2(257) == 8);
+}
+
 void run_bulletproofs_tests(void) {
+    test_log_exp();
     test_bulletproofs_generators_api();
     test_bulletproofs_generators_fixed();
     test_bulletproofs_pp_tagged_hash();