detect uint64 overflow that wraps past min_safe in parse_int_string

include/fast_float/ascii_number.h

@@ -781,9 +781,28 @@ parse_int_string(UC const *p, UC const *pend, T &value,
   }
   // this check can be eliminated for all other types, but they will all require
   // a max_digits(base) equivalent
-  if (digit_count == max_digits && i < min_safe_u64(base)) {
-    answer.ec = std::errc::result_out_of_range;
-    return answer;
+  if (digit_count == max_digits) {
+    // A value that wrapped below the smallest max_digits-length value has
+    // certainly overflowed.
+    if (i < min_safe_u64(base)) {
+      answer.ec = std::errc::result_out_of_range;
+      return answer;
+    }
+    // i >= min_safe_u64(base) is still not proof that it fits: for any base
+    // whose max_digits-length range exceeds 2^64 (base 10 reaches ~5.4 * 2^64
+    // at 20 digits) the accumulator can wrap a whole multiple of 2^64 and land
+    // back above min_safe, so the test above lets that overflow through. Re-run
+    // the parsed digits with a checked multiply-add to decide exactly.
+    uint64_t overflow_check = 0;
+    for (UC const *q = start_digits; q != p; ++q) {
+      uint8_t const digit = ch_to_digit(*q);
+      if (overflow_check >
+          (std::numeric_limits<uint64_t>::max() - digit) / uint64_t(base)) {
+        answer.ec = std::errc::result_out_of_range;
+        return answer;
+      }
+      overflow_check = uint64_t(base) * overflow_check + digit;
+    }
   }
 
   // check other types overflow

tests/fast_int.cpp

@@ -821,6 +821,61 @@ int main() {
     ++base_unsigned;
   }
 
+  // unsigned out of range error base test, multi-wrap (64 bit)
+  // These values overflow uint64_t, but the accumulator wraps a whole multiple
+  // of 2^64 and lands back at or above the smallest max_digits-length value, so
+  // a single comparison against that bound does not catch the overflow. Bases
+  // 2, 4 and 16 are excluded because their max_digits-length range fits within
+  // a single 2^64 span.
+  std::vector<int> const unsigned_multiwrap_base{
+      3,  5,  6,  7,  8,  9,  10, 11, 12, 13, 14, 15, 17, 18, 19, 20,
+      21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36};
+  std::vector<std::string_view> const unsigned_multiwrap_base_test{
+      "22222222222222222222222222222222222222222",
+      "4400000000000000000000000000",
+      "5555555555555555555555555",
+      "66666666666666666666666",
+      "7777777777777777777777",
+      "888888888888888888888",
+      "46893488147419103233",
+      "AAAAAAAAAAAAAAAAAAA",
+      "BBBBBBBBBBBBBBBBBB",
+      "427772311192C9BAAB",
+      "DDDDDDDDDDDDDDDDD",
+      "532C82996D3A44919",
+      "GGGGGGGGGGGGGGGG",
+      "HHHHHHHHHHHHHHHH",
+      "3835GEGDF36622EG",
+      "JJJJJJJJJJJJJJJ",
+      "KKKKKKKKKKKKKKK",
+      "LLLLLLLLLLLLLLL",
+      "444BGHB4EG5DA2D",
+      "NNNNNNNNNNNNNN",
+      "JE5H4MNDLJGNLO",
+      "PPPPPPPPPPPPPP",
+      "QQQQQQQQQQQQQQ",
+      "RRRRRRRRRRRRRR",
+      "4H7QS52310IHQK",
+      "TTTTTTTTTTTTTT",
+      "UUUUUUUUUUUUU",
+      "VVVVVVVVVVVVV",
+      "WWWWWWWWWWWWW",
+      "XXXXXXXXXXXXX",
+      "YYYYYYYYYYYYY",
+      "6U831JL976P6O"};
+
+  for (std::size_t i = 0; i < unsigned_multiwrap_base_test.size(); ++i) {
+    auto const &f = unsigned_multiwrap_base_test[i];
+    uint64_t result;
+    auto answer = fast_float::from_chars(f.data(), f.data() + f.size(), result,
+                                         unsigned_multiwrap_base[i]);
+    if (answer.ec != std::errc::result_out_of_range) {
+      std::cerr << "expected error for should be 'result_out_of_range': \"" << f
+                << "\"" << std::endl;
+      return EXIT_FAILURE;
+    }
+  }
+
   // just within range base test (64 bit)
   std::vector<std::string_view> const int_within_range_base_test{
       "111111111111111111111111111111111111111111111111111111111111111",