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diff --git a/gitstatus/src/arena.h b/gitstatus/src/arena.h
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+// Copyright 2019 Roman Perepelitsa.
+//
+// This file is part of GitStatus.
+//
+// GitStatus is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// GitStatus is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with GitStatus. If not, see <https://www.gnu.org/licenses/>.
+
+#ifndef ROMKATV_GITSTATUS_ARENA_H_
+#define ROMKATV_GITSTATUS_ARENA_H_
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <new>
+#include <type_traits>
+#include <vector>
+
+#include "string_view.h"
+
+namespace gitstatus {
+
+// Thread-compatible. Very fast and very flexible w.r.t. allocation size and alignment.
+//
+// Natural API extensions:
+//
+//   // Donates a block to the arena. When the time comes, it'll be freed with
+//   // free(p, size, userdata).
+//   void Donate(void* p, size_t size, void* userdata, void(*free)(void*, void*));
+class Arena {
+ public:
+  struct Options {
+    // The first call to Allocate() will allocate a block of this size. There is one exception when
+    // the first requested allocation size is larger than this limit. Subsequent blocks will be
+    // twice as large as the last until they saturate at max_block_size.
+    size_t min_block_size = 64;
+
+    // Allocate blocks at most this large. There is one exception when the requested allocation
+    // size is larger than this limit.
+    size_t max_block_size = 8 << 10;
+
+    // When the size of the first allocation in a block is larger than this threshold, the block
+    // size will be equal to the allocation size. This is meant to reduce memory waste when making
+    // many allocations with sizes slightly over max_block_size / 2. With max_alloc_threshold equal
+    // to max_block_size / N, the upper bound on wasted memory when making many equally-sized
+    // allocations is 100.0 / (N + 1) percent. When making allocations of different sizes, the upper
+    // bound on wasted memory is 50%.
+    size_t max_alloc_threshold = 1 << 10;
+
+    // Natural extensions:
+    //
+    //   void* userdata;
+    //   void (*alloc)(size_t size, size_t alignment, void* userdata);
+    //   void (*free)(size_t size, void* userdata);
+  };
+
+  // Requires: opt.min_block_size <= opt.max_block_size.
+  //
+  // Doesn't allocate any memory.
+  Arena(Options opt);
+  Arena() : Arena(Options()) {}
+  Arena(Arena&&);
+  ~Arena();
+
+  Arena& operator=(Arena&& other);
+
+  // Requires: alignment is a power of 2.
+  //
+  // Result is never null and always aligned. If size is zero, the result may be equal to the last.
+  // Alignment above alignof(std::max_align_t) is supported. There is no requirement for alignment
+  // to be less than size or to divide it.
+  inline void* Allocate(size_t size, size_t alignment) {
+    assert(alignment && !(alignment & (alignment - 1)));
+    uintptr_t p = Align(top_->tip, alignment);
+    uintptr_t e = p + size;
+    if (e <= top_->end) {
+      top_->tip = e;
+      return reinterpret_cast<void*>(p);
+    }
+    return AllocateSlow(size, alignment);
+  }
+
+  template <class T>
+  inline T* Allocate(size_t n) {
+    static_assert(!std::is_reference<T>(), "");
+    return static_cast<T*>(Allocate(n * sizeof(T), alignof(T)));
+  }
+
+  template <class T>
+  inline T* Allocate() {
+    return Allocate<T>(1);
+  }
+
+  inline char* MemDup(const char* p, size_t len) {
+    char* res = Allocate<char>(len);
+    std::memcpy(res, p, len);
+    return res;
+  }
+
+  // Copies the null-terminated string (including the trailing null character) to the arena and
+  // returns a pointer to the copy.
+  inline char* StrDup(const char* s) {
+    size_t len = std::strlen(s);
+    return MemDup(s, len + 1);
+  }
+
+  // Guarantees: !StrDup(p, len)[len].
+  inline char* StrDup(const char* p, size_t len) {
+    char* res = Allocate<char>(len + 1);
+    std::memcpy(res, p, len);
+    res[len] = 0;
+    return res;
+  }
+
+  // Guarantees: !StrDup(s)[s.len].
+  inline char* StrDup(StringView s) {
+    return StrDup(s.ptr, s.len);
+  }
+
+  template <class... Ts>
+  inline char* StrCat(const Ts&... ts) {
+    return [&](std::initializer_list<StringView> ss) {
+      size_t len = 0;
+      for (StringView s : ss) len += s.len;
+      char* p = Allocate<char>(len + 1);
+      for (StringView s : ss) {
+        std::memcpy(p, s.ptr, s.len);
+        p += s.len;
+      }
+      *p = 0;
+      return p - len;
+    }({ts...});
+  }
+
+  // Copies/moves `val` to the arena and returns a pointer to it.
+  template <class T>
+  inline std::remove_const_t<std::remove_reference_t<T>>* Dup(T&& val) {
+    return DirectInit<std::remove_const_t<std::remove_reference_t<T>>>(std::forward<T>(val));
+  }
+
+  // The same as `new T{args...}` but on the arena.
+  template <class T, class... Args>
+  inline T* DirectInit(Args&&... args) {
+    T* res = Allocate<T>();
+    ::new (const_cast<void*>(static_cast<const void*>(res))) T(std::forward<Args>(args)...);
+    return res;
+  }
+
+  // The same as `new T(args...)` but on the arena.
+  template <class T, class... Args>
+  inline T* BraceInit(Args&&... args) {
+    T* res = Allocate<T>();
+    ::new (const_cast<void*>(static_cast<const void*>(res))) T{std::forward<Args>(args)...};
+    return res;
+  }
+
+  // Tip() and TipSize() allow you to allocate the remainder of the current block. They can be
+  // useful if you are flexible w.r.t. the allocation size.
+  //
+  // Invariant:
+  //
+  //   const void* tip = Tip();
+  //   void* p = Allocate(TipSize(), 1);  // grab the remainder of the current block
+  //   assert(p == tip);
+  const void* Tip() const { return reinterpret_cast<const void*>(top_->tip); }
+  size_t TipSize() const { return top_->end - top_->tip; }
+
+  // Invalidates all allocations (without running destructors of allocated objects) and frees all
+  // blocks except at most the specified number of blocks. The retained blocks will be used to
+  // fulfil future allocation requests.
+  void Reuse(size_t num_blocks = std::numeric_limits<size_t>::max());
+
+ private:
+  struct Block {
+    size_t size() const { return end - start; }
+    uintptr_t start;
+    uintptr_t tip;
+    uintptr_t end;
+  };
+
+  inline static size_t Align(size_t n, size_t m) { return (n + m - 1) & ~(m - 1); };
+
+  void AddBlock(size_t size, size_t alignment);
+  bool ReuseBlock(size_t size, size_t alignment);
+
+  __attribute__((noinline)) void* AllocateSlow(size_t size, size_t alignment);
+
+  Options opt_;
+  std::vector<Block> blocks_;
+  // Invariant: !blocks_.empty() <= reusable_ && reusable_ <= blocks_.size().
+  size_t reusable_ = 0;
+  // Invariant: (top_ == &g_empty_block) == blocks_.empty().
+  // Invariant: blocks_.empty() || top_ == &blocks_.back() || top_ < blocks_.data() + reusable_.
+  Block* top_;
+
+  static Block g_empty_block;
+};
+
+// Copies of ArenaAllocator use the same thread-compatible Arena without synchronization.
+template <class T>
+class ArenaAllocator {
+ public:
+  using value_type = T;
+  using pointer = T*;
+  using const_pointer = const T*;
+  using reference = T&;
+  using const_reference = const T&;
+  using size_type = size_t;
+  using difference_type = ptrdiff_t;
+  using propagate_on_container_move_assignment = std::true_type;
+  template <class U>
+  struct rebind {
+    using other = ArenaAllocator<U>;
+  };
+  using is_always_equal = std::false_type;
+
+  ArenaAllocator(Arena* arena = nullptr) : arena_(*arena) {}
+
+  Arena& arena() const { return arena_; }
+
+  pointer address(reference x) const { return &x; }
+  const_pointer address(const_reference x) const { return &x; }
+  pointer allocate(size_type n, const void* hint = nullptr) { return arena_.Allocate<T>(n); }
+  void deallocate(T* p, std::size_t n) {}
+  size_type max_size() const { return std::numeric_limits<size_type>::max() / sizeof(value_type); }
+
+  template <class U, class... Args>
+  void construct(U* p, Args&&... args) {
+    ::new (const_cast<void*>(static_cast<const void*>(p))) U(std::forward<Args>(args)...);
+  }
+
+  template <class U>
+  void destroy(U* p) {
+    p->~U();
+  }
+
+  bool operator==(const ArenaAllocator& other) const { return &arena_ == &other.arena_; }
+  bool operator!=(const ArenaAllocator& other) const { return &arena_ != &other.arena_; }
+
+ private:
+  Arena& arena_;
+};
+
+template <class C>
+struct LazyWithArena;
+
+template <template <class, class> class C, class T1, class A>
+struct LazyWithArena<C<T1, A>> {
+  using type = C<T1, ArenaAllocator<typename C<T1, A>::value_type>>;
+};
+
+template <template <class, class, class> class C, class T1, class T2, class A>
+struct LazyWithArena<C<T1, T2, A>> {
+  using type = C<T1, T2, ArenaAllocator<typename C<T1, T2, A>::value_type>>;
+};
+
+template <class C>
+using WithArena = typename LazyWithArena<C>::type;
+
+}  // namespace gitstatus
+
+#endif  // ROMKATV_GITSTATUS_DIR_H_