1 files changed, 101 insertions, 0 deletions
diff --git a/include/ftl/string.h b/include/ftl/string.h
new file mode 100644
index 0000000000..2d96b06a2f
--- /dev/null
+++ b/include/ftl/string.h
@@ -0,0 +1,101 @@
+/*
+ * Copyright 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <cassert>
+#include <charconv>
+#include <limits>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+namespace android::ftl {
+
+enum class Radix { kBin = 2, kDec = 10, kHex = 16 };
+
+template <typename T>
+struct to_chars_length {
+  static_assert(std::is_integral_v<T>);
+  // Maximum binary digits, plus minus sign and radix prefix.
+  static constexpr std::size_t value = std::numeric_limits<std::make_unsigned_t<T>>::digits + 3;
+};
+
+template <typename T>
+constexpr std::size_t to_chars_length_v = to_chars_length<T>::value;
+
+template <typename T = std::int64_t>
+using to_chars_buffer_t = char[to_chars_length_v<T>];
+
+// Lightweight (not allocating nor sprintf-based) alternative to std::to_string for integers, with
+// optional radix. See also ftl::to_string below.
+//
+//   ftl::to_chars_buffer_t<> buffer;
+//
+//   assert(ftl::to_chars(buffer, 123u) == "123");
+//   assert(ftl::to_chars(buffer, -42, ftl::Radix::kBin) == "-0b101010");
+//   assert(ftl::to_chars(buffer, 0xcafe, ftl::Radix::kHex) == "0xcafe");
+//   assert(ftl::to_chars(buffer, '*', ftl::Radix::kHex) == "0x2a");
+//
+template <typename T, std::size_t N>
+std::string_view to_chars(char (&buffer)[N], T v, Radix radix = Radix::kDec) {
+  static_assert(N >= to_chars_length_v<T>);
+
+  auto begin = buffer + 2;
+  const auto [end, err] = std::to_chars(begin, buffer + N, v, static_cast<int>(radix));
+  assert(err == std::errc());
+
+  if (radix == Radix::kDec) {
+    // TODO: Replace with {begin, end} in C++20.
+    return {begin, static_cast<std::size_t>(end - begin)};
+  }
+
+  const auto prefix = radix == Radix::kBin ? 'b' : 'x';
+  if constexpr (std::is_unsigned_v<T>) {
+    buffer[0] = '0';
+    buffer[1] = prefix;
+  } else {
+    if (*begin == '-') {
+      *buffer = '-';
+    } else {
+      --begin;
+    }
+
+    *begin-- = prefix;
+    *begin = '0';
+  }
+
+  // TODO: Replace with {buffer, end} in C++20.
+  return {buffer, static_cast<std::size_t>(end - buffer)};
+}
+
+// Lightweight (not sprintf-based) alternative to std::to_string for integers, with optional radix.
+//
+//   assert(ftl::to_string(123u) == "123");
+//   assert(ftl::to_string(-42, ftl::Radix::kBin) == "-0b101010");
+//   assert(ftl::to_string(0xcafe, ftl::Radix::kHex) == "0xcafe");
+//   assert(ftl::to_string('*', ftl::Radix::kHex) == "0x2a");
+//
+template <typename T>
+inline std::string to_string(T v, Radix radix = Radix::kDec) {
+  to_chars_buffer_t<T> buffer;
+  return std::string(to_chars(buffer, v, radix));
+}
+
+std::string to_string(bool) = delete;
+std::string to_string(bool, Radix) = delete;
+
+}  // namespace android::ftl