diff options
Diffstat (limited to 'absl/strings/cord.h')
| -rw-r--r-- | absl/strings/cord.h | 100 |
1 files changed, 91 insertions, 9 deletions
diff --git a/absl/strings/cord.h b/absl/strings/cord.h index 40566cbaa0..68a7e52feb 100644 --- a/absl/strings/cord.h +++ b/absl/strings/cord.h @@ -41,13 +41,13 @@ #include <iostream> #include <iterator> #include <string> +#include <type_traits> #include "absl/base/internal/endian.h" #include "absl/base/internal/invoke.h" #include "absl/base/internal/per_thread_tls.h" #include "absl/base/macros.h" #include "absl/base/port.h" -#include "absl/container/inlined_vector.h" #include "absl/functional/function_ref.h" #include "absl/meta/type_traits.h" #include "absl/strings/internal/cord_internal.h" @@ -66,6 +66,73 @@ template <typename H> H HashFragmentedCord(H, const Cord&); } +namespace cord_internal { + +// It's expensive to keep a tree perfectly balanced, so instead we keep trees +// approximately balanced. A tree node N of depth D(N) that contains a string +// of L(N) characters is considered balanced if L >= Fibonacci(D + 2). +// The "+ 2" is used to ensure that every leaf node contains at least one +// character. Here we presume that +// Fibonacci(0) = 0 +// Fibonacci(1) = 1 +// Fibonacci(2) = 1 +// Fibonacci(3) = 2 +// ... +// +// Fibonacci numbers are convenient because it means when two balanced trees of +// the same depth are made the children of a new node, the resulting tree is +// guaranteed to also be balanced: +// +// +// L(left) >= Fibonacci(D(left) + 2) +// L(right) >= Fibonacci(D(right) + 2) +// +// L(left) + L(right) >= Fibonacci(D(left) + 2) + Fibonacci(D(right) + 2) +// L(left) + L(right) == L(new_tree) +// +// L(new_tree) >= 2 * Fibonacci(D(child) + 2) +// D(child) == D(new_tree) - 1 +// +// L(new_tree) >= 2 * Fibonacci(D(new_tree) + 1) +// 2 * Fibonacci(N) >= Fibonacci(N + 1) +// +// L(new_tree) >= Fibonacci(D(new_tree) + 2) +// +// +// The 93rd Fibonacci number is the largest Fibonacci number that can be +// represented in 64 bits, so the size of a balanced Cord of depth 92 is too big +// for an unsigned 64 bit integer to hold. Therefore we can safely assume that +// the maximum depth of a Cord is 91. +constexpr size_t MaxCordDepth() { return 91; } + +// This class models fixed max size stack of CordRep pointers. +// The elements are being pushed back and popped from the back. +template <typename CordRepPtr, size_t N> +class CordTreePath { + public: + CordTreePath() {} + explicit CordTreePath(CordRepPtr root) { push_back(root); } + + bool empty() const { return size_ == 0; } + size_t size() const { return size_; } + void clear() { size_ = 0; } + + CordRepPtr back() { return data_[size_ - 1]; } + + void pop_back() { + --size_; + assert(size_ < N); + } + void push_back(CordRepPtr elem) { data_[size_++] = elem; } + + private: + CordRepPtr data_[N]; + size_t size_ = 0; +}; + +using CordTreeMutablePath = CordTreePath<CordRep*, MaxCordDepth()>; +} // namespace cord_internal + // A Cord is a sequence of characters. class Cord { private: @@ -114,7 +181,8 @@ class Cord { // finished with `data`. The data must remain live and unchanging until the // releaser is called. The requirements for the releaser are that it: // * is move constructible, - // * supports `void operator()(absl::string_view) const`, + // * supports `void operator()(absl::string_view) const` or + // `void operator()() const`, // * does not have alignment requirement greater than what is guaranteed by // ::operator new. This is dictated by alignof(std::max_align_t) before // C++17 and __STDCPP_DEFAULT_NEW_ALIGNMENT__ if compiling with C++17 or @@ -127,8 +195,8 @@ class Cord { // FillBlock(block); // return absl::MakeCordFromExternal( // block->ToStringView(), - // [pool, block](absl::string_view /*ignored*/) { - // pool->FreeBlock(block); + // [pool, block](absl::string_view v) { + // pool->FreeBlock(block, v); // }); // } // @@ -282,8 +350,7 @@ class Cord { absl::cord_internal::CordRep* current_leaf_ = nullptr; // The number of bytes left in the `Cord` over which we are iterating. size_t bytes_remaining_ = 0; - absl::InlinedVector<absl::cord_internal::CordRep*, 4> - stack_of_right_children_; + absl::cord_internal::CordTreeMutablePath stack_of_right_children_; }; // Returns an iterator to the first chunk of the `Cord`. @@ -667,6 +734,21 @@ ExternalRepReleaserPair NewExternalWithUninitializedReleaser( absl::string_view data, ExternalReleaserInvoker invoker, size_t releaser_size); +struct Rank1 {}; +struct Rank0 : Rank1 {}; + +template <typename Releaser, typename = ::absl::base_internal::InvokeT< + Releaser, absl::string_view>> +void InvokeReleaser(Rank0, Releaser&& releaser, absl::string_view data) { + ::absl::base_internal::Invoke(std::forward<Releaser>(releaser), data); +} + +template <typename Releaser, + typename = ::absl::base_internal::InvokeT<Releaser>> +void InvokeReleaser(Rank1, Releaser&& releaser, absl::string_view) { + ::absl::base_internal::Invoke(std::forward<Releaser>(releaser)); +} + // Creates a new `CordRep` that owns `data` and `releaser` and returns a pointer // to it, or `nullptr` if `data` was empty. template <typename Releaser> @@ -684,14 +766,14 @@ CordRep* NewExternalRep(absl::string_view data, Releaser&& releaser) { using ReleaserType = absl::decay_t<Releaser>; if (data.empty()) { // Never create empty external nodes. - ::absl::base_internal::Invoke( - ReleaserType(std::forward<Releaser>(releaser)), data); + InvokeReleaser(Rank0{}, ReleaserType(std::forward<Releaser>(releaser)), + data); return nullptr; } auto releaser_invoker = [](void* type_erased_releaser, absl::string_view d) { auto* my_releaser = static_cast<ReleaserType*>(type_erased_releaser); - ::absl::base_internal::Invoke(std::move(*my_releaser), d); + InvokeReleaser(Rank0{}, std::move(*my_releaser), d); my_releaser->~ReleaserType(); return sizeof(Releaser); }; |