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
diff --git a/include/ftl/concat.h b/include/ftl/concat.h
new file mode 100644
index 0000000..ded48f7
--- /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