Make constexpr ceil

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
fmtlib · May 23, 2023 · a09d23d · a09d23d
1 parent 06c5e32
commit a09d23d
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Showing 2 changed files with 43 additions and 2 deletions.
diff --git a/include/fmt/format.h b/include/fmt/format.h
@@ -3317,6 +3317,32 @@ FMT_CONSTEXPR20 void format_hexfloat(Float value, int precision,
   format_hexfloat(static_cast<double>(value), precision, specs, buf);
 }
 
+template <typename Integral, typename Float>
+FMT_CONSTEXPR auto iceil(Float value) -> Integral {
+  static_assert(
+      std::is_integral<Integral>::value && !std::is_same<Integral, bool>::value,
+      "Return type is not integer");
+  auto min = (std::numeric_limits<Integral>::min)();
+  auto max = (std::numeric_limits<Integral>::max)();
+  ignore_unused(min, max);
+  FMT_ASSERT(min <= value && value <= max, "value not in Integral range");
+  if (is_constant_evaluated()) {
+    using U = typename std::make_unsigned<Integral>::type;
+    do {
+      auto mid =
+          min +
+          static_cast<Integral>(
+              static_cast<U>(static_cast<U>(max) - static_cast<U>(min)) / 2);
+      if (mid < value)
+        min = mid;
+      else
+        max = mid;
+    } while (min + 1 != max);
+    return max;
+  }
+  return static_cast<Integral>(std::ceil(value));
+}
+
 template <typename Float>
 FMT_CONSTEXPR20 auto format_float(Float value, int precision, float_specs specs,
                                   buffer<char>& buf) -> int {
@@ -3347,8 +3373,7 @@ FMT_CONSTEXPR20 auto format_float(Float value, int precision, float_specs specs,
     //   10^(exp - 1) <= value < 10^exp or 10^exp <= value < 10^(exp + 1).
     // This is based on log10(value) == log2(value) / log2(10) and approximation
     // of log2(value) by e + num_fraction_bits idea from double-conversion.
-    exp = static_cast<int>(
-        std::ceil((f.e + count_digits<1>(f.f) - 1) * inv_log2_10 - 1e-10));
+    exp = iceil<int>((f.e + count_digits<1>(f.f) - 1) * inv_log2_10 - 1e-10);
     dragon_flags = dragon::fixup;
   } else if (precision < 0) {
     // Use Dragonbox for the shortest format.

diff --git a/test/format-impl-test.cc b/test/format-impl-test.cc
@@ -523,3 +523,19 @@ TEST(format_impl_test, to_utf8) {
   EXPECT_EQ(s, u.str());
   EXPECT_EQ(s.size(), u.size());
 }
+
+TEST(format_impl_test, iceil) {
+  using T = int;
+  for (double v : std::initializer_list<double>{
+           ((std::numeric_limits<T>::min)() + 0.5),
+           -1.2,
+           -0.2,
+           0.0,
+           0.2,
+           1.2,
+           4.0,
+           ((std::numeric_limits<T>::max)() - 0.5),
+       }) {
+    EXPECT_EQ(fmt::detail::iceil<T>(v), static_cast<T>(std::ceil(v)));
+  }
+}