Revert changes to fr_batch_inv & bytes_from_uint64

src/common/bytes.c

@@ -27,8 +27,8 @@
  * @remark The output format is big-endian.
  */
 void bytes_from_uint64(uint8_t out[8], uint64_t n) {
-    for (size_t i = 0; i < 8; i++) {
-        out[7 - i] = n & 0xFF;
+    for (int i = 7; i >= 0; i--) {
+        out[i] = n & 0xFF;
         n >>= 8;
     }
 }

src/eip4844/eip4844.c

@@ -81,17 +81,20 @@ static bool fr_is_zero(const fr_t *p) {
  * @param[in]   a   A vector of field elements, length `len`
  * @param[in]   len The number of field elements
  *
+ * @remark This function only supports len > 0.
  * @remark This function does NOT support in-place computation.
  * @remark Return C_KZG_BADARGS if a zero is found in the input. In this case,
  *         the `out` output array has already been mutated.
  */
-static C_KZG_RET fr_batch_inv(fr_t *out, const fr_t *a, size_t len) {
+static C_KZG_RET fr_batch_inv(fr_t *out, const fr_t *a, int len) {
+    int i;
+
     assert(len > 0);
     assert(a != out);
 
     fr_t accumulator = FR_ONE;
 
-    for (size_t i = 0; i < len; i++) {
+    for (i = 0; i < len; i++) {
         out[i] = accumulator;
         blst_fr_mul(&accumulator, &accumulator, &a[i]);
     }
@@ -103,10 +106,9 @@ static C_KZG_RET fr_batch_inv(fr_t *out, const fr_t *a, size_t len) {
 
     blst_fr_eucl_inverse(&accumulator, &accumulator);
 
-    for (size_t i = len; i > 0; i--) {
-        size_t index = i - 1;
-        blst_fr_mul(&out[index], &out[index], &accumulator);
-        blst_fr_mul(&accumulator, &accumulator, &a[index]);
+    for (i = len - 1; i >= 0; i--) {
+        blst_fr_mul(&out[i], &out[i], &accumulator);
+        blst_fr_mul(&accumulator, &accumulator, &a[i]);
     }
 
     return C_KZG_OK;