parser.c: Optimize json_parse_digits

We can use `ctz` builtin to get the number of consecutive digits.

```
== Parsing float parsing (2251051 bytes)
ruby 3.4.6 (2025-09-16 revision dbd83256b1) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
               after    25.000 i/100ms
Calculating -------------------------------------
               after    231.963 (± 0.0%) i/s    (4.31 ms/i) -      1.175k in   5.065467s

Comparison:
              before:      215.3 i/s
               after:      232.0 i/s - 1.08x  faster
```

Author: byroot

ext/json/ext/parser/parser.c

@@ -1094,20 +1094,27 @@ static inline int json_parse_digits(JSON_ParserState *state, uint64_t *accumulat
         // From: https://github.com/simdjson/simdjson/blob/32b301893c13d058095a07d9868edaaa42ee07aa/include/simdjson/generic/numberparsing.h#L333
         // Branchless version of: http://0x80.pl/articles/swar-digits-validate.html
         uint64_t match = (next_8bytes & 0xF0F0F0F0F0F0F0F0) | (((next_8bytes + 0x0606060606060606) & 0xF0F0F0F0F0F0F0F0) >> 4);
+        size_t consecutive_digits = trailing_zeros64(match ^ 0x3333333333333333) / CHAR_BIT;
 
-        if (match == 0x3333333333333333) { // 8 consecutive digits
+        if (consecutive_digits == 8) {
             *accumulator = (*accumulator * 100000000) + decode_8digits_unrolled(next_8bytes);
             state->cursor += 8;
             continue;
         }
 
-        if ((match & 0xFFFFFFFF) == 0x33333333) { // 4 consecutive digits
+        if (consecutive_digits >= 4) {
             *accumulator = (*accumulator * 10000) + decode_4digits_unrolled((uint32_t)next_8bytes);
             state->cursor += 4;
-            break;
+            consecutive_digits -= 4;
+        }
+
+        while (consecutive_digits) {
+            *accumulator = *accumulator * 10 + (*state->cursor - '0');
+            consecutive_digits--;
+            state->cursor++;
         }
 
-        break;
+        return (int)(state->cursor - start);
     }
 #endif
 

ext/json/ext/simd/simd.h

@@ -19,7 +19,7 @@ typedef enum {
 static inline uint32_t trailing_zeros64(uint64_t input)
 {
 #if HAVE_BUILTIN_CTZLL
-    return __builtin_ctzll(input);
+    return input ? __builtin_ctzll(input) : 0;
 #else
     uint32_t trailing_zeros = 0;
     uint64_t temp = input;
@@ -34,7 +34,7 @@ static inline uint32_t trailing_zeros64(uint64_t input)
 static inline int trailing_zeros(int input)
 {
 #if HAVE_BUILTIN_CTZLL
-    return __builtin_ctz(input);
+    return input ? __builtin_ctz(input) : 0;
 #else
     int trailing_zeros = 0;
     int temp = input;