Simplify log_2_byte & add test (#109)

src/c_kzg_4844.c

@@ -220,20 +220,18 @@ static C_KZG_RET new_fr_array(fr_t **x, size_t n) {
 /**
  * Fast log base 2 of a byte.
  *
- * Corresponds to the index of the highest bit set in the byte. Adapted from
- * https://graphics.stanford.edu/~seander/bithacks.html#IntegerLog.
- *
  * @param[in] b A non-zero byte
  * @return The index of the highest set bit
  */
 STATIC int log_2_byte(byte b) {
-    int r, shift;
-    r = (b > 0xF) << 2;
-    b >>= r;
-    shift = (b > 0x3) << 1;
-    b >>= (shift + 1);
-    r |= shift | b;
-    return r;
+    if (b < 2) return 0;
+    if (b < 4) return 1;
+    if (b < 8) return 2;
+    if (b < 16) return 3;
+    if (b < 32) return 4;
+    if (b < 64) return 5;
+    if (b < 128) return 6;
+    return 7;
 }
 
 /**
@@ -242,8 +240,6 @@ STATIC int log_2_byte(byte b) {
  * @param p The field element to be checked
  * @retval true  The element is one
  * @retval false The element is not one
- *
- * @todo See if there is a more efficient way to check for one in the finite field.
  */
 static bool fr_is_one(const fr_t *p) {
     uint64_t a[4];
@@ -535,8 +531,6 @@ static bool is_power_of_two(uint64_t n) {
  *
  * @param a A byte
  * @return A byte that is bit-reversed with respect to @p a
- *
- * @todo Benchmark some of the other bit-reversal options in the list. Maybe.
  */
 #define rev_byte(a) ((((a)&0xff) * 0x0202020202ULL & 0x010884422010ULL) % 1023)
 

src/c_kzg_4844.h

@@ -136,6 +136,7 @@ C_KZG_RET evaluate_polynomial_in_evaluation_form(fr_t *out, const Polynomial *p,
 C_KZG_RET blob_to_polynomial(Polynomial *p, const Blob *blob);
 C_KZG_RET bytes_to_bls_field(fr_t *out, const Bytes32 *b);
 uint32_t reverse_bits(uint32_t a);
+int log_2_byte(byte b);
 
 #endif
 

src/test_c_kzg_4844.c

@@ -412,6 +412,32 @@ static void test_reverse_bits__all_bits_are_one(void) {
     ASSERT_EQUALS(reverse_bits(original), reversed);
 }
 
+///////////////////////////////////////////////////////////////////////////////
+// Tests for log_2_byte
+///////////////////////////////////////////////////////////////////////////////
+
+static void test_log_2_byte__expected_values(void) {
+    byte i = 0;
+    while (true) {
+        /*
+         * Corresponds to the index of the highest bit set in the byte.
+         * Adapted from https://graphics.stanford.edu/~seander/bithacks.html#IntegerLog.
+         */
+        byte b = i;
+        int r, shift;
+        r = (b > 0xF) << 2;
+        b >>= r;
+        shift = (b > 0x3) << 1;
+        b >>= (shift + 1);
+        r |= shift | b;
+
+        ASSERT_EQUALS(r, log_2_byte(i));
+
+        if (i == 255) break;
+        i += 1;
+    }
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Tests for compute_kzg_proof
 ///////////////////////////////////////////////////////////////////////////////
@@ -505,6 +531,7 @@ int main(void) {
     RUN(test_reverse_bits__all_bits_are_zero);
     RUN(test_reverse_bits__some_bits_are_one);
     RUN(test_reverse_bits__all_bits_are_one);
+    RUN(test_log_2_byte__expected_values);
     RUN(test_compute_and_verify_kzg_proof);
     teardown();