Adding new loop_parse_if_digits function. [skip ci]

We can eliminate one of the subtractions from within is_made_of_eight_digits_fast and parse_eight_digits_unrolled by instead moving it out into when the value is collected. Additionally, we can also save some overhead by not collecting the value twice for each iteration of the loop within loop_parse_if_eight_digits.

Author: RealTimeChris

Diff for: include/fast_float/ascii_number.h

@@ -32,7 +32,7 @@ template <typename UC> fastfloat_really_inline constexpr bool has_simd_opt() {
 // able to optimize it well.
 template <typename UC>
 fastfloat_really_inline constexpr bool is_integer(UC c) noexcept {
-  return !(c > UC('9') || c < UC('0'));
+  return static_cast<uint8_t>(c - '0') < 10;
 }
 
 fastfloat_really_inline constexpr uint64_t byteswap(uint64_t val) {
@@ -232,6 +232,39 @@ loop_parse_if_eight_digits(char const *&p, char const *const pend,
   }
 }
 
+// credit @realtimechris
+fastfloat_really_inline constexpr bool
+parse_if_eight_digits_unrolled(const char *&string, size_t &value) {
+  constexpr size_t byte_mask = ~size_t(0) / 255ull;
+  constexpr size_t msb_mask = byte_mask * 128ull;
+  constexpr size_t threshold_byte_mask = byte_mask * (127ull - 9ull);
+  constexpr size_t mask = 0x000000FF000000FFull;
+  constexpr size_t mul1 = 0x000F424000000064ull;
+  constexpr size_t mul2 = 0x0000271000000001ull;
+  size_t value_new = read8_to_u64(string) - 0x3030303030303030;
+  if (!(((value_new + threshold_byte_mask) | value_new) & msb_mask)) {
+    value_new = (value_new * 10) + (value_new >> 8);
+    value =
+        value * 100000000 +
+        ((((value_new & mask) * mul1) + (((value_new >> 16) & mask) * mul2)) >>
+         32);
+    string += 8;
+    return true;
+  }
+  return false;
+}
+
+fastfloat_really_inline constexpr void
+loop_parse_if_digits(const char *&p, const char *const pend,
+                     size_t &i) noexcept {
+  while (pend - p >= 8 && parse_if_eight_digits_unrolled(p, i)) {
+  }
+  while (p < pend && is_integer(*p)) {
+    i = i * 10 + static_cast<uint8_t>(*p - '0');
+    ++p;
+  }
+}
+
 enum class parse_error {
   no_error,
   // [JSON-only] The minus sign must be followed by an integer.
@@ -347,13 +380,7 @@ parse_number_string(UC const *p, UC const *pend,
     UC const *before = p;
     // can occur at most twice without overflowing, but let it occur more, since
     // for integers with many digits, digit parsing is the primary bottleneck.
-    loop_parse_if_eight_digits(p, pend, i);
-
-    while ((p != pend) && is_integer(*p)) {
-      uint8_t digit = uint8_t(*p - UC('0'));
-      ++p;
-      i = i * 10 + digit; // in rare cases, this will overflow, but that's ok
-    }
+    loop_parse_if_digits(p, pend, i);
     exponent = before - p;
     answer.fraction = span<UC const>(before, size_t(p - before));
     digit_count -= exponent;