FTL: Add variadic concatenation

For now, the arguments can only be integers, string literals, and
truncated string views, as this will mainly be used for tracing.

Bug: 185536303
Test: ftl_test
Change-Id: I3141992c6c9438710f04b056e67a5e8811d39f78

4 files changed, 202 insertions, 0 deletions

diff --git a/include/ftl/concat.h b/include/ftl/concat.h
new file mode 100644
index 0000000000..ded48f7c8c
--- /dev/null
+++ b/include/ftl/concat.h
@@ -0,0 +1,84 @@
+/*
+ * 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 <ftl/details/concat.h>
+
+namespace android::ftl {
+
+// Lightweight (not allocating nor sprintf-based) concatenation.
+//
+//   std::string_view name = "Volume";
+//   ftl::Concat string(ftl::truncated<3>(name), ": ", -3, " dB");
+//
+//   assert(string.str() == "Vol: -3 dB");
+//   assert(string.c_str()[string.size()] == '\0');
+//
+template <std::size_t, typename... Ts>
+struct Concat;
+
+template <std::size_t N, typename T, typename... Ts>
+struct Concat<N, T, Ts...> : Concat<N + details::StaticString<T>::N, Ts...> {
+  explicit constexpr Concat(T v, Ts... args) { append(v, args...); }
+
+ protected:
+  constexpr Concat() = default;
+
+  constexpr void append(T v, Ts... args) {
+    using Str = details::StaticString<T>;
+    const Str str(v);
+
+    // TODO: Replace with constexpr std::copy in C++20.
+    for (auto it = str.view.begin(); it != str.view.end();) {
+      *this->end_++ = *it++;
+    }
+
+    using Base = Concat<N + Str::N, Ts...>;
+    this->Base::append(args...);
+  }
+};
+
+template <std::size_t N>
+struct Concat<N> {
+  static constexpr std::size_t max_size() { return N; }
+  constexpr std::size_t size() const { return end_ - buffer_; }
+
+  constexpr const char* c_str() const { return buffer_; }
+
+  constexpr std::string_view str() const {
+    // TODO: Replace with {buffer_, end_} in C++20.
+    return {buffer_, size()};
+  }
+
+ protected:
+  constexpr Concat() : end_(buffer_) {}
+  constexpr void append() { *end_ = '\0'; }
+
+  char buffer_[N + 1];
+  char* end_;
+};
+
+// Deduction guide.
+template <typename... Ts>
+Concat(Ts&&...) -> Concat<0, Ts...>;
+
+template <std::size_t N>
+constexpr auto truncated(std::string_view v) {
+  return details::Truncated<N>{v};
+}
+
+}  // namespace android::ftl
diff --git a/include/ftl/details/concat.h b/include/ftl/details/concat.h
new file mode 100644
index 0000000000..8ce949ef05
--- /dev/null
+++ b/include/ftl/details/concat.h
@@ -0,0 +1,62 @@
+/*
+ * 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 <functional>
+#include <string_view>
+
+#include <ftl/string.h>
+
+namespace android::ftl::details {
+
+template <typename T, typename = void>
+struct StaticString;
+
+template <typename T>
+struct StaticString<T, std::enable_if_t<std::is_integral_v<T>>> {
+  static constexpr std::size_t N = to_chars_length_v<T>;
+
+  explicit StaticString(T v) : view(to_chars(buffer, v)) {}
+
+  to_chars_buffer_t<T> buffer;
+  const std::string_view view;
+};
+
+template <std::size_t M>
+struct StaticString<const char (&)[M], void> {
+  static constexpr std::size_t N = M - 1;
+
+  explicit constexpr StaticString(const char (&str)[M]) : view(str, N) {}
+
+  const std::string_view view;
+};
+
+template <std::size_t N>
+struct Truncated {
+  std::string_view view;
+};
+
+template <std::size_t M>
+struct StaticString<Truncated<M>, void> {
+  static constexpr std::size_t N = M;
+
+  explicit constexpr StaticString(Truncated<M> str) : view(str.view.substr(0, N)) {}
+
+  const std::string_view view;
+};
+
+}  // namespace android::ftl::details
diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index 5a80ad067c..bc2eb23677 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -16,6 +16,7 @@ cc_test {
     srcs: [
         "Flags_test.cpp",
         "cast_test.cpp",
+        "concat_test.cpp",
         "enum_test.cpp",
         "future_test.cpp",
         "small_map_test.cpp",
diff --git a/libs/ftl/concat_test.cpp b/libs/ftl/concat_test.cpp
new file mode 100644
index 0000000000..8ecb1b252d
--- /dev/null
+++ b/libs/ftl/concat_test.cpp
@@ -0,0 +1,55 @@
+/*
+ * 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/concat.h>
+#include <gtest/gtest.h>
+
+namespace android::test {
+
+// Keep in sync with example usage in header file.
+TEST(Concat, Example) {
+  std::string_view name = "Volume";
+  ftl::Concat string(ftl::truncated<3>(name), ": ", -3, " dB");
+
+  EXPECT_EQ(string.str(), "Vol: -3 dB");
+  EXPECT_EQ(string.c_str()[string.size()], '\0');
+}
+
+namespace {
+
+static_assert(ftl::Concat{"foo"}.str() == "foo");
+static_assert(ftl::Concat{ftl::truncated<3>("foobar")}.str() == "foo");
+
+constexpr ftl::Concat kConcat{"po", "trz", "ebie"};
+
+static_assert(kConcat.size() == 9);
+static_assert(kConcat.max_size() == 9);
+static_assert(kConcat.str() == "potrzebie");
+static_assert(kConcat.str() == std::string_view(kConcat.c_str()));
+
+constexpr auto concat() {
+  return ftl::Concat{ftl::truncated<1>("v???"), ftl::truncated<2>("ee??"),
+                     ftl::truncated<3>("ble?"), ftl::truncated<4>("fetz"),
+                     ftl::truncated<90>("er")};
+}
+
+static_assert(concat().size() == 12);
+static_assert(concat().max_size() == 100);
+static_assert(concat().str() == "veeblefetzer");
+static_assert(concat().str() == std::string_view(concat().c_str()));
+
+}  // namespace
+}  // namespace android::test