FTL: Add to_chars

Wrap std::to_chars for a slightly faster alternative to std::to_string
with optional radix.

The upcoming ftl/{enum.h,flags.h} imported from IF will use the binary/
hex options, while SF will stringify display IDs for trace labels using
the non-allocating version.

Bug: 185536303
Test: ftl_test
Change-Id: I587622763303d2baeb844e52eff55ce8f0bcbf1e

3 files changed, 289 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
diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index 97626bec7d..4226e32ee7 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -18,6 +18,7 @@ cc_test {
         "small_map_test.cpp",
         "small_vector_test.cpp",
         "static_vector_test.cpp",
+        "string_test.cpp",
     ],
     cflags: [
         "-Wall",
diff --git a/libs/ftl/string_test.cpp b/libs/ftl/string_test.cpp
new file mode 100644
index 0000000000..f3d85c8319
--- /dev/null
+++ b/libs/ftl/string_test.cpp
@@ -0,0 +1,187 @@
+/*
+ * 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.
+ */
+
+#include <ftl/string.h>
+#include <gtest/gtest.h>
+
+#include <algorithm>
+#include <cstdint>
+#include <iterator>
+#include <limits>
+#include <sstream>
+#include <type_traits>
+
+namespace android::test {
+
+// Keep in sync with example usage in header file.
+TEST(String, ToChars) {
+  ftl::to_chars_buffer_t<> buffer;
+
+  EXPECT_EQ(ftl::to_chars(buffer, 123u), "123");
+  EXPECT_EQ(ftl::to_chars(buffer, -42, ftl::Radix::kBin), "-0b101010");
+  EXPECT_EQ(ftl::to_chars(buffer, 0xcafe, ftl::Radix::kHex), "0xcafe");
+  EXPECT_EQ(ftl::to_chars(buffer, '*', ftl::Radix::kHex), "0x2a");
+}
+
+namespace {
+
+template <typename F, typename T>
+void ToCharsTest() {
+  constexpr auto kRadix = F::kRadix;
+
+  using Limits = std::numeric_limits<T>;
+  constexpr auto kMin = Limits::min();
+  constexpr auto kMax = Limits::max();
+  constexpr auto kNeg = static_cast<T>(-42);
+  constexpr auto kPos = static_cast<T>(123);
+
+  ftl::to_chars_buffer_t<T> buffer;
+
+  EXPECT_EQ(ftl::to_chars(buffer, kMin, kRadix), F{}(kMin));
+  EXPECT_EQ(ftl::to_chars(buffer, kMax, kRadix), F{}(kMax));
+  EXPECT_EQ(ftl::to_chars(buffer, kNeg, kRadix), F{}(kNeg));
+  EXPECT_EQ(ftl::to_chars(buffer, kPos, kRadix), F{}(kPos));
+}
+
+template <typename...>
+struct Types {};
+
+template <typename F, typename Types>
+struct ToCharsTests;
+
+template <typename F, typename T, typename... Ts>
+struct ToCharsTests<F, Types<T, Ts...>> {
+  static void test() {
+    ToCharsTest<F, T>();
+    ToCharsTests<F, Types<Ts...>>::test();
+  }
+};
+
+template <typename F>
+struct ToCharsTests<F, Types<>> {
+  static void test() {}
+};
+
+template <typename T, typename U = std::make_unsigned_t<T>>
+U to_unsigned(std::ostream& stream, T v) {
+  if (std::is_same_v<T, U>) return v;
+
+  if (v < 0) {
+    stream << '-';
+    return std::numeric_limits<U>::max() - static_cast<U>(v) + 1;
+  } else {
+    return static_cast<U>(v);
+  }
+}
+
+struct Bin {
+  static constexpr auto kRadix = ftl::Radix::kBin;
+
+  template <typename T>
+  std::string operator()(T v) const {
+    std::ostringstream stream;
+    auto u = to_unsigned(stream, v);
+    stream << "0b";
+
+    if (u == 0) {
+      stream << 0;
+    } else {
+      std::ostringstream digits;
+      do {
+        digits << (u & 1);
+      } while (u >>= 1);
+
+      const auto str = digits.str();
+      std::copy(str.rbegin(), str.rend(), std::ostream_iterator<char>(stream));
+    }
+
+    return stream.str();
+  }
+};
+
+struct Dec {
+  static constexpr auto kRadix = ftl::Radix::kDec;
+
+  template <typename T>
+  std::string operator()(T v) const {
+    return std::to_string(v);
+  }
+};
+
+struct Hex {
+  static constexpr auto kRadix = ftl::Radix::kHex;
+
+  template <typename T>
+  std::string operator()(T v) const {
+    std::ostringstream stream;
+    const auto u = to_unsigned(stream, v);
+    stream << "0x" << std::hex << std::nouppercase;
+    stream << (sizeof(T) == 1 ? static_cast<unsigned>(u) : u);
+    return stream.str();
+  }
+};
+
+using IntegerTypes =
+    Types<char, unsigned char, signed char, std::uint8_t, std::uint16_t, std::uint32_t,
+          std::uint64_t, std::int8_t, std::int16_t, std::int32_t, std::int64_t>;
+
+}  // namespace
+
+TEST(String, ToCharsBin) {
+  ToCharsTests<Bin, IntegerTypes>::test();
+
+  {
+    const std::uint8_t x = 0b1111'1111;
+    ftl::to_chars_buffer_t<decltype(x)> buffer;
+    EXPECT_EQ(ftl::to_chars(buffer, x, ftl::Radix::kBin), "0b11111111");
+  }
+  {
+    const std::int16_t x = -0b1000'0000'0000'0000;
+    ftl::to_chars_buffer_t<decltype(x)> buffer;
+    EXPECT_EQ(ftl::to_chars(buffer, x, ftl::Radix::kBin), "-0b1000000000000000");
+  }
+}
+
+TEST(String, ToCharsDec) {
+  ToCharsTests<Dec, IntegerTypes>::test();
+
+  {
+    const std::uint32_t x = UINT32_MAX;
+    ftl::to_chars_buffer_t<decltype(x)> buffer;
+    EXPECT_EQ(ftl::to_chars(buffer, x), "4294967295");
+  }
+  {
+    const std::int32_t x = INT32_MIN;
+    ftl::to_chars_buffer_t<decltype(x)> buffer;
+    EXPECT_EQ(ftl::to_chars(buffer, x), "-2147483648");
+  }
+}
+
+TEST(String, ToCharsHex) {
+  ToCharsTests<Hex, IntegerTypes>::test();
+
+  {
+    const std::uint16_t x = 0xfade;
+    ftl::to_chars_buffer_t<decltype(x)> buffer;
+    EXPECT_EQ(ftl::to_chars(buffer, x, ftl::Radix::kHex), "0xfade");
+  }
+  {
+    ftl::to_chars_buffer_t<> buffer;
+    EXPECT_EQ(ftl::to_chars(buffer, INT64_MIN, ftl::Radix::kHex), "-0x8000000000000000");
+  }
+}
+
+}  // namespace android::test