refactor validate_ascii_fast function after CR comment

Signed-off-by: iko1 <me@remotecpp.dev>

src/core/detail/bitpacking.cc

@@ -113,30 +113,35 @@ bool validate_ascii_fast(const char* src, size_t len) {
   size_t i = 0;
 
   // Initialize a vector in which all the elements are set to zero.
-  vector signed char has_error = vec_splat_s8(0);
+  vector unsigned char has_error = vec_splat_s8(0);
   if (len >= 16) {
     for (; i <= len - 16; i += 16) {
       // Load 16 bytes from buffer into a vector.
-      vector signed char current_bytes = vec_load_len((signed char*)(src + i), 16);
+      vector unsigned char current_bytes = vec_load_len((signed char*)(src + i), 16);
       // Perform a bitwise OR operation between the current and the previously loaded contents.
       has_error = vec_orc(has_error, current_bytes);
     }
   }
 
-  int error_mask = 0;
-  for (int i = 0; i < 16; i++) {
-    if (has_error[i] & 0x80) {
-      return false;
-    }
+  // Initialize a vector in which all the elements are set to an invalid ASCII value.
+  vector unsigned char rep_invalid_values = vec_splat_s8(0x80);
+
+  // Perform bitwise AND-complement operation between two vectors.
+  vector unsigned char andc_result = vec_andc(rep_invalid_values, has_error);
+
+  // Tests whether any of corresponding elements of the given vectors are not equal.
+  // After the bitwise operation, both vectors should be equal if ASCII values.
+  if (!vec_all_eq(rep_invalid_values, andc_result)) {
+    return false;
   }
 
-  char tail_has_error = 0;
   for (; i < len; i++) {
-    tail_has_error |= src[i];
+    if (src[i] & 0x80) {
+      return false;
+    }
   }
-  error_mask |= (tail_has_error & 0x80);
 
-  return !error_mask;
+  return true;
 }
 #else
 bool validate_ascii_fast(const char* src, size_t len) {