8.2.10

Int overflow check with a faster approach

.github/workflows/emscripten.yml

@@ -5,8 +5,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@0c366fd6a839edf440554fa01a7085ccba70ac98 # v4.2.2
-      - uses: actions/setup-node@53b83947a5a98c8d113130e565377fae1a50d02f # v6.3.0
+      - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
-      - uses: mymindstorm/setup-emsdk@6ab9eb1bda2574c4ddb79809fc9247783eaf9021 # v14
+      - uses: mymindstorm/setup-emsdk@4528d102f7230f0e7b276855c01ea1159be0e984 # v16
       - name: Verify
         run: emcc -v
       - name: Checkout

.github/workflows/lint_and_format_check.yml

@@ -27,7 +27,7 @@ jobs:
       - uses: actions/checkout@0c366fd6a839edf440554fa01a7085ccba70ac98 # v4.1.7
 
       - name: Run clang-format
-        uses: jidicula/clang-format-action@3a18028048f01a66653b4e3bf8d968d2e7e2cb8b # v4.17.0
+        uses: jidicula/clang-format-action@654a770daa28443dd111d133e4083e21c1075674 # v4.18.0
         with:
           clang-format-version: '17'
 

.github/workflows/vs17-arm-ci.yml

@@ -10,8 +10,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - {gen: Visual Studio 17 2022, arch: ARM64, cfg: Release}
+          - {gen: Visual Studio 18 2026, arch: ARM64, cfg: Release}
-          - {gen: Visual Studio 17 2022, arch: ARM64, cfg: Debug}
+          - {gen: Visual Studio 18 2026, arch: ARM64, cfg: Debug}
     steps:
       - name: checkout
         uses: actions/checkout@v6.0.2

.github/workflows/vs17-ci.yml

@@ -10,10 +10,10 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - {gen: Visual Studio 17 2022, arch: Win32, cfg: Release}
+          - {gen: Visual Studio 18 2026, arch: Win32, cfg: Release}
-          #- {gen: Visual Studio 17 2022, arch: Win32, cfg: Debug}
+          #- {gen: Visual Studio 18 2026, arch: Win32, cfg: Debug}
-          - {gen: Visual Studio 17 2022, arch: x64, cfg: Release}
+          - {gen: Visual Studio 18 2026, arch: x64, cfg: Release}
-          - {gen: Visual Studio 17 2022, arch: x64, cfg: Debug}
+          - {gen: Visual Studio 18 2026, arch: x64, cfg: Debug}
     steps:
       - name: checkout
         uses: actions/checkout@v6.0.2

.github/workflows/vs17-clang-ci.yml

@@ -10,10 +10,10 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - {gen: Visual Studio 17 2022, arch: Win32, cfg: Release}
+          - {gen: Visual Studio 18 2026, arch: Win32, cfg: Release}
-          - {gen: Visual Studio 17 2022, arch: Win32, cfg: Debug}
+          - {gen: Visual Studio 18 2026, arch: Win32, cfg: Debug}
-          - {gen: Visual Studio 17 2022, arch: x64, cfg: Release}
+          - {gen: Visual Studio 18 2026, arch: x64, cfg: Release}
-          - {gen: Visual Studio 17 2022, arch: x64, cfg: Debug}
+          - {gen: Visual Studio 18 2026, arch: x64, cfg: Debug}
     steps:
       - name: checkout
         uses: actions/checkout@v6.0.2

.github/workflows/vs17-cxx20.yml

@@ -10,10 +10,10 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - {gen: Visual Studio 17 2022, arch: Win32, cfg: Release}
+          - {gen: Visual Studio 18 2026, arch: Win32, cfg: Release}
-          - {gen: Visual Studio 17 2022, arch: Win32, cfg: Debug}
+          - {gen: Visual Studio 18 2026, arch: Win32, cfg: Debug}
-          - {gen: Visual Studio 17 2022, arch: x64, cfg: Release}
+          - {gen: Visual Studio 18 2026, arch: x64, cfg: Release}
-          - {gen: Visual Studio 17 2022, arch: x64, cfg: Debug}
+          - {gen: Visual Studio 18 2026, arch: x64, cfg: Debug}
     steps:
       - name: checkout
         uses: actions/checkout@v6.0.2

CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.14)
 
 
-project(fast_float VERSION 8.2.6 LANGUAGES CXX)
+project(fast_float VERSION 8.2.10 LANGUAGES CXX)
 set(FASTFLOAT_CXX_STANDARD 11 CACHE STRING "the C++ standard to use for fastfloat")
 set(CMAKE_CXX_STANDARD ${FASTFLOAT_CXX_STANDARD})
 option(FASTFLOAT_TEST "Enable tests" OFF)

README.md

@@ -531,7 +531,7 @@ sufficiently recent version of CMake (3.11 or better at least):
 FetchContent_Declare(
   fast_float
   GIT_REPOSITORY https://github.com/fastfloat/fast_float.git
-  GIT_TAG tags/v8.2.6
+  GIT_TAG tags/v8.2.10
   GIT_SHALLOW TRUE)
 
 FetchContent_MakeAvailable(fast_float)
@@ -547,7 +547,7 @@ You may also use [CPM](https://github.com/cpm-cmake/CPM.cmake), like so:
 CPMAddPackage(
   NAME fast_float
   GITHUB_REPOSITORY "fastfloat/fast_float"
-  GIT_TAG v8.2.6)
+  GIT_TAG v8.2.10)
 ```
 
 ## Using as single header
@@ -559,7 +559,7 @@ if desired as described in the command line help.
 
 You may directly download automatically generated single-header files:
 
-<https://github.com/fastfloat/fast_float/releases/download/v8.2.6/fast_float.h>
+<https://github.com/fastfloat/fast_float/releases/download/v8.2.10/fast_float.h>
 
 ## Benchmarking
 

docs/index.html

@@ -221,7 +221,7 @@ constexpr double pi = parse("3.1415");  // computed at compile time</code></pre>
 <pre data-lang="cmake"><code>FetchContent_Declare(
   fast_float
   GIT_REPOSITORY https://github.com/fastfloat/fast_float.git
-  GIT_TAG tags/v{{VERSION}}
+  GIT_TAG tags/<span data-version>v{{VERSION}}</span>
   GIT_SHALLOW TRUE)
 FetchContent_MakeAvailable(fast_float)
 target_link_libraries(myprogram PUBLIC fast_float)</code></pre>
@@ -232,13 +232,13 @@ target_link_libraries(myprogram PUBLIC fast_float)</code></pre>
 <pre data-lang="cmake"><code>CPMAddPackage(
   NAME fast_float
   GITHUB_REPOSITORY "fastfloat/fast_float"
-  GIT_TAG v{{VERSION}})</code></pre>
+  GIT_TAG <span data-version>v{{VERSION}}</span>)</code></pre>
           </article>
 
           <article class="card">
             <h3>Single header</h3>
             <p>Download a pre-amalgamated header — no build system required.</p>
-<pre data-lang="bash"><code>curl -LO https://github.com/fastfloat/fast_float/releases/download/v{{VERSION}}/fast_float.h</code></pre>
+<pre data-lang="bash"><code>curl -LO https://github.com/fastfloat/fast_float/releases/download/<span data-version>v{{VERSION}}</span>/fast_float.h</code></pre>
           </article>
 
           <article class="card">

include/fast_float/ascii_number.h

@@ -330,10 +330,17 @@ report_parse_error(UC const *p, parse_error error) {
 
 // Assuming that you use no more than 19 digits, this will
 // parse an ASCII string.
+//
+// store_spans is a *runtime* flag (not a template parameter, deliberately: a
+// template would create a second instantiation of this whole function and the
+// extra icache pressure wipes out the gain). When false, the integer/fraction
+// spans (read only by the rare digit_comp slow path) are not materialized,
+// which keeps the fat parsed_number_string_t off the hot path. The caller
+// re-parses with store_spans=true if the slow path is actually reached.
 template <bool basic_json_fmt, typename UC>
 fastfloat_really_inline FASTFLOAT_CONSTEXPR20 parsed_number_string_t<UC>
-parse_number_string(UC const *p, UC const *pend,
+parse_number_string(UC const *p, UC const *pend, parse_options_t<UC> options,
-                    parse_options_t<UC> options) noexcept {
+                    bool store_spans = true) noexcept {
   chars_format const fmt = detail::adjust_for_feature_macros(options.format);
   UC const decimal_point = options.decimal_point;
 
@@ -402,7 +409,9 @@ parse_number_string(UC const *p, UC const *pend,
   }
   UC const *const end_of_integer_part = p;
   int64_t digit_count = int64_t(end_of_integer_part - start_digits);
+  if (store_spans) {
     answer.integer = span<UC const>(start_digits, size_t(digit_count));
+  }
   FASTFLOAT_IF_CONSTEXPR17(basic_json_fmt) {
     // at least 1 digit in integer part, without leading zeros
     if (digit_count == 0) {
@@ -429,7 +438,9 @@ parse_number_string(UC const *p, UC const *pend,
       i = i * 10 + digit; // in rare cases, this will overflow, but that's ok
     }
     exponent = before - p;
+    if (store_spans) {
       answer.fraction = span<UC const>(before, size_t(p - before));
+    }
     digit_count -= exponent;
   }
   FASTFLOAT_IF_CONSTEXPR17(basic_json_fmt) {
@@ -514,6 +525,11 @@ parse_number_string(UC const *p, UC const *pend,
 
     if (digit_count > 19) {
       answer.too_many_digits = true;
+      // The truncation recompute below reads the integer/fraction spans. When
+      // store_spans is false we didn't materialize them, so just flag
+      // too_many_digits; the caller re-parses with store_spans=true to obtain
+      // the corrected mantissa/exponent before taking the slow path.
+      if (store_spans) {
         // Let us start again, this time, avoiding overflows.
         // We don't need to call if is_integer, since we use the
         // pre-tokenized spans from above.
@@ -539,6 +555,7 @@ parse_number_string(UC const *p, UC const *pend,
         // We have now corrected both exponent and i, to a truncated value
       }
     }
+  }
   answer.exponent = exponent;
   answer.mantissa = i;
   return answer;
@@ -583,7 +600,8 @@ parse_int_string(UC const *p, UC const *pend, T &value,
 
   UC const *const start_digits = p;
 
-  FASTFLOAT_IF_CONSTEXPR17((std::is_same<T, std::uint8_t>::value)) {
+  FASTFLOAT_IF_CONSTEXPR17(
+      (std::is_same<T, std::uint8_t>::value && sizeof(UC) == 1)) {
     if (base == 10) {
       const size_t len = (size_t)(pend - p);
       if (len == 0) {
@@ -675,7 +693,8 @@ parse_int_string(UC const *p, UC const *pend, T &value,
     }
   }
 
-  FASTFLOAT_IF_CONSTEXPR17((std::is_same<T, std::uint16_t>::value)) {
+  FASTFLOAT_IF_CONSTEXPR17(
+      (std::is_same<T, std::uint16_t>::value && sizeof(UC) == 1)) {
     if (base == 10) {
       const size_t len = size_t(pend - p);
       if (len == 0) {
@@ -762,10 +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)) {
+  if (digit_count == max_digits) {
+    // At the max_digits boundary the accumulator `i` may have wrapped around
+    // 2^64. A plain `i < min_safe_u64(base)` test is not sufficient: for any
+    // base whose max_digits-length range exceeds 2^64 (base 10 reaches
+    // ~5.4 * 2^64 at 20 digits) the value can wrap a whole multiple of 2^64 and
+    // land back above min_safe, slipping through. Decide exactly in O(1) using
+    // the leading digit, following the approach used in simdjson:
+    //   ms   == min_safe_u64(base) == base^(max_digits-1), the smallest
+    //           max_digits-length value.
+    //   dmax == the largest leading digit whose number can still fit in u64.
+    // The leading-digit band [d*ms, (d+1)*ms) has width ms < 2^64, so within
+    // the single band where d == dmax the value straddles 2^64 at most once,
+    // and a single threshold separates wrapped from non-wrapped values. A
+    // leading digit above dmax always overflows; below dmax always fits.
+    uint64_t const ms = min_safe_u64(base);
+    uint64_t const dmax = (std::numeric_limits<uint64_t>::max)() / ms;
+    uint64_t const lead = ch_to_digit(*start_digits);
+    if (lead > dmax || (lead == dmax && i < dmax * ms)) {
       answer.ec = std::errc::result_out_of_range;
       return answer;
     }
+  }
 
   // check other types overflow
   if (!std::is_same<T, uint64_t>::value) {

include/fast_float/fast_float.h

@@ -7,12 +7,12 @@
 namespace fast_float {
 /**
  * This function parses the character sequence [first,last) for a number. It
- * parses floating-point numbers expecting a locale-indepent format equivalent
+ * parses floating-point numbers expecting a locale-independent format
- * to what is used by std::strtod in the default ("C") locale. The resulting
+ * equivalent to what is used by std::strtod in the default ("C") locale. The
- * floating-point value is the closest floating-point values (using either float
+ * resulting floating-point value is the closest floating-point values (using
- * or double), using the "round to even" convention for values that would
+ * either float or double), using the "round to even" convention for values that
- * otherwise fall right in-between two values. That is, we provide exact parsing
+ * would otherwise fall right in-between two values. That is, we provide exact
- * according to the IEEE standard.
+ * parsing according to the IEEE standard.
  *
  * Given a successful parse, the pointer (`ptr`) in the returned value is set to
  * point right after the parsed number, and the `value` referenced is set to the

include/fast_float/float_common.h

@@ -18,7 +18,7 @@
 
 #define FASTFLOAT_VERSION_MAJOR 8
 #define FASTFLOAT_VERSION_MINOR 2
-#define FASTFLOAT_VERSION_PATCH 6
+#define FASTFLOAT_VERSION_PATCH 10
 
 #define FASTFLOAT_STRINGIZE_IMPL(x) #x
 #define FASTFLOAT_STRINGIZE(x) FASTFLOAT_STRINGIZE_IMPL(x)
@@ -197,6 +197,32 @@ using parse_options = parse_options_t<char>;
 #define fastfloat_really_inline inline __attribute__((always_inline))
 #endif
 
+// Branch-probability hint marking the rare slow-path branches as cold, so the
+// optimizer keeps the out-of-line slow-path re-parse off the hot path (and does
+// not duplicate the force-inlined hot scanner into the caller, which bloated
+// the hot frame and hurt ILP on some targets). Used at the call site as
+//   if fastfloat_unlikely(cond) { ... }
+// (the macro supplies the parentheses). It expands to the standard [[unlikely]]
+// attribute when supported, otherwise to __builtin_expect on GCC/Clang, or
+// to a no-op elsewhere (e.g. pre-C++20 MSVC, which has no equivalent hint).
+#ifdef __has_cpp_attribute
+#if __has_cpp_attribute(unlikely) >= 201803L
+// g++-9 hits hits this branch, but then fails to compile
+// [[unlikely]]. This happens only with g++-9.
+#if !defined(__GNUC__) || (__GNUC__ != 9)
+#define FASTFLOAT_USE_UNLIKELY_ATTR
+#endif
+#endif
+#endif
+
+#ifdef FASTFLOAT_USE_UNLIKELY_ATTR
+#define fastfloat_unlikely(x) (x) [[unlikely]]
+#elif defined(__GNUC__) || defined(__clang__)
+#define fastfloat_unlikely(x) (__builtin_expect(!!(x), 0))
+#else
+#define fastfloat_unlikely(x) (x)
+#endif
+
 #ifndef FASTFLOAT_ASSERT
 #define FASTFLOAT_ASSERT(x)                                                    \
   { ((void)(x)); }

include/fast_float/parse_number.h

@@ -289,6 +289,23 @@ from_chars_advanced(parsed_number_string_t<UC> &pns, T &value) noexcept {
   return answer;
 }
 
+// Slow path: re-parse materializing the integer/fraction spans the hot no-span
+// parse skipped, then run the full algorithm. The two callers reach it only
+// through a fastfloat_unlikely branch, so the optimizer keeps this re-parse off
+// the hot path on its own (no function-level noinline needed).
+// from_chars_advanced already handles both the too_many_digits disambiguation
+// and the am.power2<0 digit_comp recompute, so both slow branches collapse to
+// one helper call.
+template <typename T, typename UC>
+FASTFLOAT_CONSTEXPR20 from_chars_result_t<UC>
+parse_number_slow_path(UC const *first, UC const *last, T &value,
+                       parse_options_t<UC> options, bool bjf) noexcept {
+  parsed_number_string_t<UC> pns =
+      bjf ? parse_number_string<true, UC>(first, last, options, true)
+          : parse_number_string<false, UC>(first, last, options, true);
+  return from_chars_advanced(pns, value);
+}
+
 template <typename T, typename UC>
 fastfloat_really_inline FASTFLOAT_CONSTEXPR20 from_chars_result_t<UC>
 from_chars_float_advanced(UC const *first, UC const *last, T &value,
@@ -312,10 +329,15 @@ from_chars_float_advanced(UC const *first, UC const *last, T &value,
     answer.ptr = first;
     return answer;
   }
+  bool const bjf = uint64_t(fmt & detail::basic_json_fmt) != 0;
+
+  // Fast path: parse WITHOUT materializing the integer/fraction spans (read
+  // only by the rare slow paths). Skipping their stores keeps the fat
+  // parsed_number_string_t off the hot path. store_spans is a runtime argument,
+  // so this reuses the single parse_number_string instantiation.
   parsed_number_string_t<UC> pns =
-      uint64_t(fmt & detail::basic_json_fmt)
+      bjf ? parse_number_string<true, UC>(first, last, options, false)
-          ? parse_number_string<true, UC>(first, last, options)
+          : parse_number_string<false, UC>(first, last, options, false);
-          : parse_number_string<false, UC>(first, last, options);
   if (!pns.valid) {
     if (uint64_t(fmt & chars_format::no_infnan)) {
       answer.ec = std::errc::invalid_argument;
@@ -326,8 +348,49 @@ from_chars_float_advanced(UC const *first, UC const *last, T &value,
     }
   }
 
-  // call overload that takes parsed_number_string_t directly.
+  // Slow path A (rare): > 19 significant digits. The no-span parse left the
-  return from_chars_advanced(pns, value);
+  // mantissa un-truncated and skipped the span-based recompute; the cold helper
+  // re-parses with spans and runs the full algorithm.
+  //
+// We have to disable -Wc++20-extensions for the [[unlikely]] attribute
+// See comment for @jwakely at
+// https://github.com/fastfloat/fast_float/pull/387#discussion_r3366943539
+// This is unfortunate.
+#ifdef __clang__
+#pragma clang diagnostic push
+#if (!defined(__APPLE_CC__) && __clang_major__ >= 10) || (__clang_major__ >= 13)
+#pragma clang diagnostic ignored "-Wc++20-extensions"
+#endif
+#endif
+  if fastfloat_unlikely (pns.too_many_digits) {
+    return parse_number_slow_path<T, UC>(first, last, value, options, bjf);
+  }
+  answer.ec = std::errc(); // be optimistic
+  answer.ptr = pns.lastmatch;
+
+  if (clinger_fast_path_impl(pns.mantissa, pns.exponent, pns.negative, value)) {
+    return answer;
+  }
+
+  adjusted_mantissa am =
+      compute_float<binary_format<T>>(pns.exponent, pns.mantissa);
+  // Slow path B (rare): Eisel-Lemire could not resolve; digit_comp needs the
+  // integer/fraction spans. Route to the cold helper (clinger there is a
+  // dead-effect since it already failed here; the cold re-parse + digit_comp
+  // via from_chars_advanced reproduces this branch).
+  if fastfloat_unlikely (am.power2 < 0) {
+    return parse_number_slow_path<T, UC>(first, last, value, options, bjf);
+  }
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif
+  to_float(pns.negative, am, value);
+  // Test for over/underflow.
+  if ((pns.mantissa != 0 && am.mantissa == 0 && am.power2 == 0) ||
+      am.power2 == binary_format<T>::infinite_power()) {
+    answer.ec = std::errc::result_out_of_range;
+  }
+  return answer;
 }
 
 template <typename T, typename UC, typename>

tests/fast_int.cpp

@@ -17,7 +17,10 @@
 #include <iostream>
 #include <vector>
 #include <string_view>
+#include <string>
 #include <cstring>
+#include <random>
+#include <algorithm>
 #include "fast_float/fast_float.h"
 #include <cstdint>
 
@@ -821,6 +824,61 @@ 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",
@@ -1295,6 +1353,330 @@ int main() {
       return EXIT_FAILURE;
     }
   }
+  // The uint8_t and uint16_t base-10 paths use a byte-oriented fast path. A
+  // wider code unit whose low byte is an ASCII digit (e.g. U+2131..U+2139) must
+  // not be mistaken for that digit. The generic path already rejects these for
+  // int; the fixed-width fast paths must agree. Lengths of 1..5 exercise both
+  // the uint8_t path and the 4-digit uint16_t SWAR path.
+  {
+    const std::u16string bad16[] = {
+        u"ℹ", u"ℱℲ", u"ℱℲℳ", u"ℱℲℳℴ", u"ℱℲℳℴℵ",
+    };
+    const std::u32string bad32[] = {
+        U"ℹ",
+        U"ℱℲℳ",
+        U"ℱℲℳℴ",
+        U"ℱℲℳℴℵ",
+    };
+    bool failed = false;
+    for (auto const &s : bad16) {
+      uint8_t r8 = 123;
+      auto a8 = fast_float::from_chars(s.data(), s.data() + s.size(), r8);
+      if (a8.ec == std::errc()) {
+        failed = true;
+        std::cerr << "Incorrectly parsed wide units as uint8_t " << unsigned(r8)
+                  << "." << std::endl;
+      }
+      uint16_t r16 = 123;
+      auto a16 = fast_float::from_chars(s.data(), s.data() + s.size(), r16);
+      if (a16.ec == std::errc()) {
+        failed = true;
+        std::cerr << "Incorrectly parsed wide units as uint16_t " << r16 << "."
+                  << std::endl;
+      }
+    }
+    for (auto const &s : bad32) {
+      uint8_t r8 = 123;
+      auto a8 = fast_float::from_chars(s.data(), s.data() + s.size(), r8);
+      if (a8.ec == std::errc()) {
+        failed = true;
+        std::cerr << "Incorrectly parsed wide units as uint8_t " << unsigned(r8)
+                  << "." << std::endl;
+      }
+      uint16_t r16 = 123;
+      auto a16 = fast_float::from_chars(s.data(), s.data() + s.size(), r16);
+      if (a16.ec == std::errc()) {
+        failed = true;
+        std::cerr << "Incorrectly parsed wide units as uint16_t " << r16 << "."
+                  << std::endl;
+      }
+    }
+
+    if (failed) {
+      return EXIT_FAILURE;
+    }
+  }
+
+  // Comprehensive, oracle-checked u64 overflow detection across every base.
+  //
+  // The accumulator in parse_int_string is allowed to overflow and the result
+  // is validated afterwards. At the max_digits boundary a value can wrap one or
+  // more whole multiples of 2^64 (a 20-digit base-10 number reaches ~5.4*2^64),
+  // so the boundary check must be exact. This section validates from_chars for
+  // bases 2..36 against an independent, trusted oracle: a plain 64-bit checked
+  // multiply-add. It hammers the single leading-digit band that straddles 2^64
+  // (where wrapped and non-wrapped values are hardest to tell apart) and also
+  // covers max_digits-1 (always in range) and max_digits+1 (always overflow).
+  {
+    auto digit_to_char = [](int d) -> char {
+      return d < 10 ? char('0' + d) : char('A' + (d - 10));
+    };
+    auto char_to_digit = [](char c) -> int {
+      if (c >= '0' && c <= '9') {
+        return c - '0';
+      }
+      if (c >= 'A' && c <= 'Z') {
+        return c - 'A' + 10;
+      }
+      return c - 'a' + 10;
+    };
+    // Trusted oracle: parse `s` in `base` with a checked 64-bit multiply-add.
+    // Returns true on u64 overflow; otherwise writes the value to `out`.
+    auto oracle = [&](std::string const &s, int base, uint64_t &out) -> bool {
+      uint64_t v = 0;
+      for (char c : s) {
+        uint64_t const d = uint64_t(char_to_digit(c));
+        if (v > (UINT64_MAX - d) / uint64_t(base)) {
+          return true;
+        }
+        v = uint64_t(base) * v + d;
+      }
+      out = v;
+      return false;
+    };
+    auto to_base = [&](uint64_t v, int base) -> std::string {
+      if (v == 0) {
+        return "0";
+      }
+      std::string s;
+      while (v != 0) {
+        s += digit_to_char(int(v % uint64_t(base)));
+        v /= uint64_t(base);
+      }
+      std::reverse(s.begin(), s.end());
+      return s;
+    };
+    // Add one (in base `base`) to the digit string `s`, carrying as needed.
+    auto increment = [&](std::string s, int base) -> std::string {
+      int carry = 1;
+      for (std::size_t k = s.size(); k-- > 0 && carry != 0;) {
+        int const d = char_to_digit(s[k]) + carry;
+        carry = d / base;
+        s[k] = digit_to_char(d % base);
+      }
+      if (carry != 0) {
+        s.insert(s.begin(), digit_to_char(carry));
+      }
+      return s;
+    };
+
+    // Subtract one (in base `base`) from a non-zero, non-negative string.
+    auto decrement = [&](std::string s, int base) -> std::string {
+      int borrow = 1;
+      for (std::size_t k = s.size(); k-- > 0 && borrow != 0;) {
+        int d = char_to_digit(s[k]) - borrow;
+        borrow = d < 0 ? 1 : 0;
+        if (d < 0) {
+          d += base;
+        }
+        s[k] = digit_to_char(d);
+      }
+      std::size_t lead = s.find_first_not_of('0'); // drop any leading zero
+      return lead == std::string::npos ? "0" : s.substr(lead);
+    };
+
+    std::mt19937_64 rng(0xC0FFEEULL);
+    long long checked = 0;
+    auto verify = [&](std::string const &s, int base) -> bool {
+      uint64_t expected = 0;
+      bool const ov = oracle(s, base, expected);
+      uint64_t result = 0xDEADBEEFULL;
+      auto answer =
+          fast_float::from_chars(s.data(), s.data() + s.size(), result, base);
+      ++checked;
+      if (ov) {
+        if (answer.ec != std::errc::result_out_of_range) {
+          std::cerr << "base " << base
+                    << ": expected result_out_of_range for \"" << s << "\""
+                    << std::endl;
+          return false;
+        }
+      } else {
+        if (answer.ec != std::errc()) {
+          std::cerr << "base " << base << ": unexpected error for \"" << s
+                    << "\"" << std::endl;
+          return false;
+        }
+        if (result != expected) {
+          std::cerr << "base " << base << ": \"" << s << "\" -> " << result
+                    << ", expected " << expected << std::endl;
+          return false;
+        }
+        if (answer.ptr != s.data() + s.size()) {
+          std::cerr << "base " << base << ": did not consume all of \"" << s
+                    << "\"" << std::endl;
+          return false;
+        }
+      }
+      return true;
+    };
+    // Leading zeros are stripped before the digit count, so the outcome must be
+    // unchanged. Checked only on hand-picked values (it exercises shared code).
+    auto verify_zeros = [&](std::string const &digits, int base) -> bool {
+      return verify(digits, base) && verify("0" + digits, base) &&
+             verify(std::string(40, '0') + digits, base);
+    };
+    auto random_tail = [&](std::string &s, int n, int base) {
+      for (int k = 0; k < n; ++k) {
+        // bias toward the extremes (0 and base-1) to hit boundaries often
+        std::uint64_t const r = rng();
+        int const mode = int(r % 4);
+        int const dig = mode == 0   ? 0
+                        : mode == 1 ? base - 1
+                                    : int((r >> 2) % std::uint64_t(base));
+        s += digit_to_char(dig);
+      }
+    };
+
+    for (int base = 2; base <= 36; ++base) {
+      // M = max number of base-`base` digits a u64 can hold.
+      std::string const maxstr = to_base(UINT64_MAX, base);
+      int const M = int(maxstr.size());
+      // b^(M-1): smallest M-digit value, and width of each leading-digit band.
+      uint64_t bM1 = 1;
+      for (int k = 0; k < M - 1; ++k) {
+        bM1 *= uint64_t(base);
+      }
+      int const dmax = int(UINT64_MAX / bM1); // largest leading digit that fits
+
+      // Exact-boundary sweep straddling 2^64 (the hardest transition): the
+      // 64 values UINT64_MAX-31 .. UINT64_MAX (in range) and 2^64 .. 2^64+31
+      // (overflow), built by walking the digit string up and down.
+      std::string below = maxstr, above = increment(maxstr, base);
+      for (int k = 0; k < 32; ++k) {
+        if (!verify(below, base) || !verify(above, base)) {
+          return EXIT_FAILURE;
+        }
+        below = decrement(below, base);
+        above = increment(above, base);
+      }
+      // Hand-picked values, also checked with leading zeros.
+      std::string const allmax(std::size_t(M), digit_to_char(base - 1));
+      if (!verify_zeros(maxstr, base) || // largest in-range value
+          !verify_zeros(increment(maxstr, base), base) || // smallest overflow
+          !verify_zeros(allmax, base)) { // largest M-digit (multi-wrap)
+        return EXIT_FAILURE;
+      }
+
+      // Randomized M-digit values across every leading digit. Bands with
+      // lead > dmax always overflow (this is where the naive min_safe check
+      // wrongly accepted multi-wrap values); lead < dmax always fits; lead ==
+      // dmax straddles 2^64 and gets the heaviest sampling.
+      for (int lead = 1; lead < base; ++lead) {
+        int const trials = lead == dmax ? 4000 : 300;
+        for (int trial = 0; trial < trials; ++trial) {
+          std::string s(1, digit_to_char(lead));
+          random_tail(s, M - 1, base);
+          if (!verify(s, base)) {
+            return EXIT_FAILURE;
+          }
+        }
+      }
+      // max_digits-1 digits never overflow; max_digits+1 digits always do.
+      for (int trial = 0; trial < 500; ++trial) {
+        std::string shorts(1,
+                           digit_to_char(1 + int(rng() % uint64_t(base - 1))));
+        random_tail(shorts, M - 2, base);
+        std::string longs(1,
+                          digit_to_char(1 + int(rng() % uint64_t(base - 1))));
+        random_tail(longs, M, base);
+        if (!verify(shorts, base) || !verify(longs, base)) {
+          return EXIT_FAILURE;
+        }
+      }
+    }
+    if (checked < 100000) {
+      std::cerr << "overflow sweep ran too few cases: " << checked << std::endl;
+      return EXIT_FAILURE;
+    }
+  }
+
+  // Signed (int64_t) boundary: every value that overflows u64 also overflows
+  // i64, and the exact i64 limits must parse. Reuses the oracle indirectly via
+  // hand-built extremes per base.
+  {
+    auto digit_to_char = [](int d) -> char {
+      return d < 10 ? char('0' + d) : char('A' + (d - 10));
+    };
+    auto to_base_signed = [&](int64_t value, int base) -> std::string {
+      // value may be INT64_MIN; accumulate magnitude in u64 to avoid UB.
+      bool const neg = value < 0;
+      uint64_t mag = neg ? (~uint64_t(value) + 1) : uint64_t(value);
+      std::string s;
+      if (mag == 0) {
+        s += '0';
+      }
+      while (mag != 0) {
+        s += digit_to_char(int(mag % uint64_t(base)));
+        mag /= uint64_t(base);
+      }
+      if (neg) {
+        s += '-';
+      }
+      std::reverse(s.begin(), s.end());
+      return s;
+    };
+    for (int base = 2; base <= 36; ++base) {
+      struct {
+        int64_t v;
+      } const limits[] = {{INT64_MAX}, {INT64_MIN}, {0}, {-1}, {1}};
+
+      for (auto const &lim : limits) {
+        std::string const s = to_base_signed(lim.v, base);
+        int64_t result = 123;
+        auto answer =
+            fast_float::from_chars(s.data(), s.data() + s.size(), result, base);
+        if (answer.ec != std::errc() || result != lim.v) {
+          std::cerr << "base " << base << ": signed limit \"" << s
+                    << "\" failed to round-trip (got " << result << ")"
+                    << std::endl;
+          return EXIT_FAILURE;
+        }
+      }
+      // Increment a non-negative magnitude string (in `base`) by one.
+      auto inc_mag = [&](std::string m) -> std::string {
+        int carry = 1;
+        for (std::size_t k = m.size(); k-- > 0 && carry != 0;) {
+          int d = (m[k] >= '0' && m[k] <= '9')   ? m[k] - '0'
+                  : (m[k] >= 'A' && m[k] <= 'Z') ? m[k] - 'A' + 10
+                                                 : m[k] - 'a' + 10;
+          d += carry;
+          carry = d / base;
+          m[k] = digit_to_char(d % base);
+        }
+        if (carry != 0) {
+          m.insert(m.begin(), digit_to_char(carry));
+        }
+        return m;
+      };
+      // INT64_MAX + 1 (= 2^63) overflows a positive int64_t.
+      // INT64_MIN - 1 (= -(2^63 + 1)) overflows a negative int64_t.
+      // Note that -(2^63) == INT64_MIN is in range and is covered above.
+      std::string const max_mag = to_base_signed(INT64_MAX, base); // 2^63 - 1
+      std::string const over = inc_mag(max_mag);                   // 2^63
+      std::string const under = "-" + inc_mag(over); // -(2^63 + 1)
+      for (std::string const &s : {over, under}) {
+        int64_t result = 123;
+        auto answer =
+            fast_float::from_chars(s.data(), s.data() + s.size(), result, base);
+        if (answer.ec != std::errc::result_out_of_range) {
+          std::cerr << "base " << base << ": expected result_out_of_range for "
+                    << "signed \"" << s << "\"" << std::endl;
+          return EXIT_FAILURE;
+        }
+      }
+    }
+  }
 
   return EXIT_SUCCESS;
 }

tests/fixedwidthtest.cpp

@@ -65,6 +65,4 @@ int main() {
 #endif
   std::cout << "All tests passed successfully." << std::endl;
   return EXIT_SUCCESS;
-
-  return 0;
 }

tests/long_exhaustive32_64.cpp

@@ -45,11 +45,6 @@ void all_32bit_values() {
         std::cerr << "I got " << std::hexfloat << result_value
                   << " but I was expecting " << v << std::endl;
         abort();
-      } else if (std::isnan(v)) {
-        if (!std::isnan(result_value)) {
-          std::cerr << "not nan" << buffer << std::endl;
-          abort();
-        }
       } else if (result_value != v) {
         std::cerr << "no match ? " << buffer << std::endl;
         std::cout << "started with " << std::hexfloat << v << std::endl;