EIC code displayed by LXR master/​include/​absl/​strings/​internal/​cord_rep_btree.h	Source navigation Diff markup Identifier search General search
	Version: master test Revert   Change

File indexing completed on 2024-11-15 09:01:13

 // Copyright 2021 The Abseil Authors
 //
 // 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
 //
 //     https://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.
 
 #ifndef ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_H_
 #define ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_H_
 
 #include <cassert>
 #include <cstdint>
 #include <iosfwd>
 
 #include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/optimization.h"
 #include "absl/strings/internal/cord_data_edge.h"
 #include "absl/strings/internal/cord_internal.h"
 #include "absl/strings/internal/cord_rep_flat.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/span.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
 // `SetCordBtreeExhaustiveValidation()` can be set to force exhaustive
 // validation in debug assertions, and code that calls `IsValid()`
 // explicitly. By default, assertions should be relatively cheap and
 // AssertValid() can easily lead to O(n^2) complexity as recursive / full tree
 // validation is O(n).
 void SetCordBtreeExhaustiveValidation(bool do_exaustive_validation);
 bool IsCordBtreeExhaustiveValidationEnabled();
 
 class CordRepBtreeNavigator;
 
 // CordRepBtree is as the name implies a btree implementation of a Cordrep tree.
 // Data is stored at the leaf level only, non leaf nodes contain down pointers
 // only. Allowed types of data edges are FLAT, EXTERNAL and SUBSTRINGs of FLAT
 // or EXTERNAL nodes. The implementation allows for data to be added to either
 // end of the tree only, it does not provide any 'insert' logic. This has the
 // benefit that we can expect good fill ratios: all nodes except the outer
 // 'legs' will have 100% fill ratios for trees built using Append/Prepend
 // methods. Merged trees will typically have a fill ratio well above 50% as in a
 // similar fashion, one side of the merged tree will typically have a 100% fill
 // ratio, and the 'open' end will average 50%. All operations are O(log(n)) or
 // better, and the tree never needs balancing.
 //
 // All methods accepting a CordRep* or CordRepBtree* adopt a reference on that
 // input unless explicitly stated otherwise. All functions returning a CordRep*
 // or CordRepBtree* instance transfer a reference back to the caller.
 // Simplified, callers both 'donate' and 'consume' a reference count on each
 // call, simplifying the API. An example of building a tree:
 //
 //   CordRepBtree* tree = CordRepBtree::Create(MakeFlat("Hello"));
 //   tree = CordRepBtree::Append(tree, MakeFlat("world"));
 //
 // In the above example, all inputs are consumed, making each call affecting
 // `tree` reference count neutral. The returned `tree` value can be different
 // from the input if the input is shared with other threads, or if the tree
 // grows in height, but callers typically never have to concern themselves with
 // that and trust that all methods DTRT at all times.
 class CordRepBtree : public CordRep {
  public:
   // EdgeType identifies `front` and `back` enum values.
   // Various implementations in CordRepBtree such as `Add` and `Edge` are
   // generic and templated on operating on either of the boundary edges.
   // For more information on the possible edges contained in a CordRepBtree
   // instance see the documentation for `edges_`.
   enum class EdgeType { kFront, kBack };
 
   // Convenience constants into `EdgeType`
   static constexpr EdgeType kFront = EdgeType::kFront;
   static constexpr EdgeType kBack = EdgeType::kBack;
 
   // Maximum number of edges: based on experiments and performance data, we can
   // pick suitable values resulting in optimum cacheline aligned values. The
   // preferred values are based on 64-bit systems where we aim to align this
   // class onto 64 bytes, i.e.:  6 = 64 bytes, 14 = 128 bytes, etc.
   // TODO(b/192061034): experiment with alternative sizes.
   static constexpr size_t kMaxCapacity = 6;
 
   // Reasonable maximum height of the btree. We can expect a fill ratio of at
   // least 50%: trees are always expanded at the front or back. Concatenating
   // trees will then typically fold at the top most node, where the lower nodes
   // are at least at capacity on one side of joined inputs. At a lower fill
   // rate of 4 edges per node, we have capacity for ~16 million leaf nodes.
   // We will fail / abort if an application ever exceeds this height, which
   // should be extremely rare (near impossible) and be an indication of an
   // application error: we do not assume it reasonable for any application to
   // operate correctly with such monster trees.
   // Another compelling reason for the number `12` is that any contextual stack
   // required for navigation or insertion requires 12 words and 12 bytes, which
   // fits inside 2 cache lines with some room to spare, and is reasonable as a
   // local stack variable compared to Cord's current near 400 bytes stack use.
   // The maximum `height` value of a node is then `kMaxDepth - 1` as node height
   // values start with a value of 0 for leaf nodes.
   static constexpr size_t kMaxDepth = 12;
   // See comments on height() for why this is an int and not a size_t.
   static constexpr int kMaxHeight = static_cast<int>(kMaxDepth - 1);
 
   // `Action` defines the action for unwinding changes done at the btree's leaf
   // level that need to be propagated up to the parent node(s). Each operation
   // on a node has an effect / action defined as follows:
   // - kSelf
   //   The operation (add / update, etc) was performed directly on the node as
   //   the node is private to the current thread (i.e.: not shared directly or
   //   indirectly through a refcount > 1). Changes can be propagated directly to
   //   all parent nodes as all parent nodes are also then private to the current
   //   thread.
   // - kCopied
   //   The operation (add / update, etc) was performed on a copy of the original
   //   node, as the node is (potentially) directly or indirectly shared with
   //   other threads. Changes need to be propagated into the parent nodes where
   //   the old down pointer must be unreffed and replaced with this new copy.
   //   Such changes to parent nodes may themselves require a copy if the parent
   //   node is also shared. A kCopied action can propagate all the way to the
   //   top node where we then must unref the `tree` input provided by the
   //   caller, and return the new copy.
   // - kPopped
   //   The operation (typically add) could not be satisfied due to insufficient
   //   capacity in the targeted node, and a new 'leg' was created that needs to
   //   be added into the parent node. For example, adding a FLAT inside a leaf
   //   node that is at capacity will create a new leaf node containing that
   //   FLAT, that needs to be 'popped' up the btree. Such 'pop' actions can
   //   cascade up the tree if parent nodes are also at capacity. A 'Popped'
   //   action propagating all the way to the top of the tree will result in
   //   the tree becoming one level higher than the current tree through a final
   //   `CordRepBtree::New(tree, popped)` call, resulting in a new top node
   //   referencing the old tree and the new (fully popped upwards) 'leg'.
   enum Action { kSelf, kCopied, kPopped };
 
   // Result of an operation on a node. See the `Action` enum for details.
   struct OpResult {
     CordRepBtree* tree;
     Action action;
   };
 
   // Return value of the CopyPrefix and CopySuffix methods which can
   // return a node or data edge at any height inside the tree.
   // A height of 0 defines the lowest (leaf) node, a height of -1 identifies
   // `edge` as being a plain data node: EXTERNAL / FLAT or SUBSTRING thereof.
   struct CopyResult {
     CordRep* edge;
     int height;
   };
 
   // Logical position inside a node:
   // - index: index of the edge.
   // - n: size or offset value depending on context.
   struct Position {
     size_t index;
     size_t n;
   };
 
   // Creates a btree from the given input. Adopts a ref of `rep`.
   // If the input `rep` is itself a btree, i.e., `IsBtree()`, then this
   // function immediately returns `rep->btree()`. If the input is a valid data
   // edge (see IsDataEdge()), then a new leaf node is returned containing `rep`
   // as the sole data edge. Else, the input is assumed to be a (legacy) concat
   // tree, and the input is consumed and transformed into a btree().
   static CordRepBtree* Create(CordRep* rep);
 
   // Destroys the provided tree. Should only be called by cord internal API's,
   // typically after a ref_count.Decrement() on the last reference count.
   static void Destroy(CordRepBtree* tree);
 
   // Destruction
   static void Delete(CordRepBtree* tree) { delete tree; }
 
   // Use CordRep::Unref() as we overload for absl::Span<CordRep* const>.
   using CordRep::Unref;
 
   // Unrefs all edges in `edges` which are assumed to be 'likely one'.
   static void Unref(absl::Span<CordRep* const> edges);
 
   // Appends / Prepends an existing CordRep instance to this tree.
   // The below methods accept three types of input:
   // 1) `rep` is a data node (See `IsDataNode` for valid data edges).
   // `rep` is appended or prepended to this tree 'as is'.
   // 2) `rep` is a BTREE.
   // `rep` is merged into `tree` respecting the Append/Prepend order.
   // 3) `rep` is some other (legacy) type.
   // `rep` is converted in place and added to `tree`
   // Requires `tree` and `rep` to be not null.
   static CordRepBtree* Append(CordRepBtree* tree, CordRep* rep);
   static CordRepBtree* Prepend(CordRepBtree* tree, CordRep* rep);
 
   // Append/Prepend the data in `data` to this tree.
   // The `extra` parameter defines how much extra capacity should be allocated
   // for any additional FLAT being allocated. This is an optimization hint from
   // the caller. For example, a caller may need to add 2 string_views of data
   // "abc" and "defghi" which are not consecutive. The caller can in this case
   // invoke `AddData(tree, "abc", 6)`, and any newly added flat is allocated
   // where possible with at least 6 bytes of extra capacity beyond `length`.
   // This helps avoiding data getting fragmented over multiple flats.
   // There is no limit on the size of `data`. If `data` can not be stored inside
   // a single flat, then the function will iteratively add flats until all data
   // has been consumed and appended or prepended to the tree.
   static CordRepBtree* Append(CordRepBtree* tree, string_view data,
                               size_t extra = 0);
   static CordRepBtree* Prepend(CordRepBtree* tree, string_view data,
                                size_t extra = 0);
 
   // Returns a new tree, containing `n` bytes of data from this instance
   // starting at offset `offset`. Where possible, the returned tree shares
   // (re-uses) data edges and nodes with this instance to minimize the
   // combined memory footprint of both trees.
   // Requires `offset + n <= length`. Returns `nullptr` if `n` is zero.
   CordRep* SubTree(size_t offset, size_t n);
 
   // Removes `n` trailing bytes from `tree`, and returns the resulting tree
   // or data edge. Returns `tree` if n is zero, and nullptr if n == length.
   // This function is logically identical to:
   //   result = tree->SubTree(0, tree->length - n);
   //   Unref(tree);
   //   return result;
   // However, the actual implementation will as much as possible perform 'in
   // place' modifications on the tree on all nodes and edges that are mutable.
   // For example, in a fully privately owned tree with the last edge being a
   // flat of length 12, RemoveSuffix(1) will simply set the length of that data
   // edge to 11, and reduce the length of all nodes on the edge path by 1.
   static CordRep* RemoveSuffix(CordRepBtree* tree, size_t n);
 
   // Returns the character at the given offset.
   char GetCharacter(size_t offset) const;
 
   // Returns true if this node holds a single data edge, and if so, sets
   // `fragment` to reference the contained data. `fragment` is an optional
   // output parameter and allowed to be null.
   bool IsFlat(absl::string_view* fragment) const;
 
   // Returns true if the data of `n` bytes starting at offset `offset`
   // is contained in a single data edge, and if so, sets fragment to reference
   // the contained data. `fragment` is an optional output parameter and allowed
   // to be null.
   bool IsFlat(size_t offset, size_t n, absl::string_view* fragment) const;
 
   // Returns a span (mutable range of bytes) of up to `size` bytes into the
   // last FLAT data edge inside this tree under the following conditions:
   // - none of the nodes down into the FLAT node are shared.
   // - the last data edge in this tree is a non-shared FLAT.
   // - the referenced FLAT has additional capacity available.
   // If all these conditions are met, a non-empty span is returned, and the
   // length of the flat node and involved tree nodes have been increased by
   // `span.length()`. The caller is responsible for immediately assigning values
   // to all uninitialized data reference by the returned span.
   // Requires `this->refcount.IsOne()`: this function forces the caller to do
   // this fast path check on the top level node, as this is the most commonly
   // shared node of a cord tree.
   Span<char> GetAppendBuffer(size_t size);
 
   // Extracts the right-most data edge from this tree iff:
   // - the tree and all internal edges to the right-most node are not shared.
   // - the right-most node is a FLAT node and not shared.
   // - the right-most node has at least the desired extra capacity.
   //
   // Returns {tree, nullptr} if any of the above conditions are not met.
   // This method effectively removes data from the tree. The intent of this
   // method is to allow applications appending small string data to use
   // pre-existing capacity, and add the modified rep back to the tree.
   //
   // Simplified such code would look similar to this:
   //   void MyTreeBuilder::Append(string_view data) {
   //     ExtractResult result = CordRepBtree::ExtractAppendBuffer(tree_, 1);
   //     if (CordRep* rep = result.extracted) {
   //       size_t available = rep->Capacity() - rep->length;
   //       size_t n = std::min(data.size(), n);
   //       memcpy(rep->Data(), data.data(), n);
   //       rep->length += n;
   //       data.remove_prefix(n);
   //       if (!result.tree->IsBtree()) {
   //         tree_ = CordRepBtree::Create(result.tree);
   //       }
   //       tree_ = CordRepBtree::Append(tree_, rep);
   //     }
   //     ...
   //     // Remaining edge in `result.tree`.
   //   }
   static ExtractResult ExtractAppendBuffer(CordRepBtree* tree,
                                            size_t extra_capacity = 1);
 
   // Returns the `height` of the tree. The height of a tree is limited to
   // kMaxHeight. `height` is implemented as an `int` as in some places we
   // use negative (-1) values for 'data edges'.
   int height() const { return static_cast<int>(storage[0]); }
 
   // Properties: begin, back, end, front/back boundary indexes.
   size_t begin() const { return static_cast<size_t>(storage[1]); }
   size_t back() const { return static_cast<size_t>(storage[2]) - 1; }
   size_t end() const { return static_cast<size_t>(storage[2]); }
   size_t index(EdgeType edge) const {
     return edge == kFront ? begin() : back();
   }
 
   // Properties: size and capacity.
   // `capacity` contains the current capacity of this instance, where
   // `kMaxCapacity` contains the maximum capacity of a btree node.
   // For now, `capacity` and `kMaxCapacity` return the same value, but this may
   // change in the future if we see benefit in dynamically sizing 'small' nodes
   // to 'large' nodes for large data trees.
   size_t size() const { return end() - begin(); }
   size_t capacity() const { return kMaxCapacity; }
 
   // Edge access
   inline CordRep* Edge(size_t index) const;
   inline CordRep* Edge(EdgeType edge_type) const;
   inline absl::Span<CordRep* const> Edges() const;
   inline absl::Span<CordRep* const> Edges(size_t begin, size_t end) const;
 
   // Returns reference to the data edge at `index`.
   // Requires this instance to be a leaf node, and `index` to be valid index.
   inline absl::string_view Data(size_t index) const;
 
   // Diagnostics: returns true if `tree` is valid and internally consistent.
   // If `shallow` is false, then the provided top level node and all child nodes
   // below it are recursively checked. If `shallow` is true, only the provided
   // node in `tree` and the cumulative length, type and height of the direct
   // child nodes of `tree` are checked. The value of `shallow` is ignored if the
   // internal `cord_btree_exhaustive_validation` diagnostics variable is true,
   // in which case the performed validations works as if `shallow` were false.
   // This function is intended for debugging and testing purposes only.
   static bool IsValid(const CordRepBtree* tree, bool shallow = false);
 
   // Diagnostics: asserts that the provided tree is valid.
   // `AssertValid()` performs a shallow validation by default. `shallow` can be
   // set to false in which case an exhaustive validation is performed. This
   // function is implemented in terms of calling `IsValid()` and asserting the
   // return value to be true. See `IsValid()` for more information.
   // This function is intended for debugging and testing purposes only.
   static CordRepBtree* AssertValid(CordRepBtree* tree, bool shallow = true);
   static const CordRepBtree* AssertValid(const CordRepBtree* tree,
                                          bool shallow = true);
 
   // Diagnostics: dump the contents of this tree to `stream`.
   // This function is intended for debugging and testing purposes only.
   static void Dump(const CordRep* rep, std::ostream& stream);
   static void Dump(const CordRep* rep, absl::string_view label,
                    std::ostream& stream);
   static void Dump(const CordRep* rep, absl::string_view label,
                    bool include_contents, std::ostream& stream);
 
   // Adds the edge `edge` to this node if possible. `owned` indicates if the
   // current node is potentially shared or not with other threads. Returns:
   // - {kSelf, <this>}
   //   The edge was directly added to this node.
   // - {kCopied, <node>}
   //   The edge was added to a copy of this node.
   // - {kPopped, New(edge, height())}
   //   A new leg with the edge was created as this node has no extra capacity.
   template <EdgeType edge_type>
   inline OpResult AddEdge(bool owned, CordRep* edge, size_t delta);
 
   // Replaces the front or back edge with the provided new edge. Returns:
   // - {kSelf, <this>}
   //   The edge was directly set in this node. The old edge is unreffed.
   // - {kCopied, <node>}
   //   A copy of this node was created with the new edge value.
   // In both cases, the function adopts a reference on `edge`.
   template <EdgeType edge_type>
   OpResult SetEdge(bool owned, CordRep* edge, size_t delta);
 
   // Creates a new empty node at the specified height.
   static CordRepBtree* New(int height = 0);
 
   // Creates a new node containing `rep`, with the height being computed
   // automatically based on the type of `rep`.
   static CordRepBtree* New(CordRep* rep);
 
   // Creates a new node containing both `front` and `back` at height
   // `front.height() + 1`. Requires `back.height() == front.height()`.
   static CordRepBtree* New(CordRepBtree* front, CordRepBtree* back);
 
   // Creates a fully balanced tree from the provided tree by rebuilding a new
   // tree from all data edges in the input. This function is automatically
   // invoked internally when the tree exceeds the maximum height.
   static CordRepBtree* Rebuild(CordRepBtree* tree);
 
  private:
   CordRepBtree() = default;
   ~CordRepBtree() = default;
 
   // Initializes the main properties `tag`, `begin`, `end`, `height`.
   inline void InitInstance(int height, size_t begin = 0, size_t end = 0);
 
   // Direct property access begin / end
   void set_begin(size_t begin) { storage[1] = static_cast<uint8_t>(begin); }
   void set_end(size_t end) { storage[2] = static_cast<uint8_t>(end); }
 
   // Decreases the value of `begin` by `n`, and returns the new value. Notice
   // how this returns the new value unlike atomic::fetch_add which returns the
   // old value. This is because this is used to prepend edges at 'begin - 1'.
   size_t sub_fetch_begin(size_t n) {
     storage[1] -= static_cast<uint8_t>(n);
     return storage[1];
   }
 
   // Increases the value of `end` by `n`, and returns the previous value. This
   // function is typically used to append edges at 'end'.
   size_t fetch_add_end(size_t n) {
     const uint8_t current = storage[2];
     storage[2] = static_cast<uint8_t>(current + n);
     return current;
   }
 
   // Returns the index of the last edge starting on, or before `offset`, with
   // `n` containing the relative offset of `offset` inside that edge.
   // Requires `offset` < length.
   Position IndexOf(size_t offset) const;
 
   // Returns the index of the last edge starting before `offset`, with `n`
   // containing the relative offset of `offset` inside that edge.
   // This function is useful to find the edges for some span of bytes ending at
   // `offset` (i.e., `n` bytes). For example:
   //
   //   Position pos = IndexBefore(n)
   //   edges = Edges(begin(), pos.index)     // All full edges (may be empty)
   //   last = Sub(Edge(pos.index), 0, pos.n) // Last partial edge (may be empty)
   //
   // Requires 0 < `offset` <= length.
   Position IndexBefore(size_t offset) const;
 
   // Returns the index of the edge ending at (or on) length `length`, and the
   // number of bytes inside that edge up to `length`. For example, if we have a
   // Node with 2 edges, one of 10 and one of 20 long, then IndexOfLength(27)
   // will return {1, 17}, and IndexOfLength(10) will return {0, 10}.
   Position IndexOfLength(size_t n) const;
 
   // Identical to the above function except starting from the position `front`.
   // This function is equivalent to `IndexBefore(front.n + offset)`, with
   // the difference that this function is optimized to start at `front.index`.
   Position IndexBefore(Position front, size_t offset) const;
 
   // Returns the index of the edge directly beyond the edge containing offset
   // `offset`, with `n` containing the distance of that edge from `offset`.
   // This function is useful for iteratively finding suffix nodes and remaining
   // partial bytes in left-most suffix nodes as for example in CopySuffix.
   // Requires `offset` < length.
   Position IndexBeyond(size_t offset) const;
 
   // Creates a new leaf node containing as much data as possible from `data`.
   // The data is added either forwards or reversed depending on `edge_type`.
   // Callers must check the length of the returned node to determine if all data
   // was copied or not.
   // See the `Append/Prepend` function for the meaning and purpose of `extra`.
   template <EdgeType edge_type>
   static CordRepBtree* NewLeaf(absl::string_view data, size_t extra);
 
   // Creates a raw copy of this Btree node with the specified length, copying
   // all properties, but without adding any references to existing edges.
   CordRepBtree* CopyRaw(size_t new_length) const;
 
   // Creates a full copy of this Btree node, adding a reference on all edges.
   CordRepBtree* Copy() const;
 
   // Creates a partial copy of this Btree node, copying all edges up to `end`,
   // adding a reference on each copied edge, and sets the length of the newly
   // created copy to `new_length`.
   CordRepBtree* CopyBeginTo(size_t end, size_t new_length) const;
 
   // Returns a tree containing the edges [tree->begin(), end) and length
   // of `new_length`. This method consumes a reference on the provided
   // tree, and logically performs the following operation:
   //   result = tree->CopyBeginTo(end, new_length);
   //   CordRep::Unref(tree);
   //   return result;
   static CordRepBtree* ConsumeBeginTo(CordRepBtree* tree, size_t end,
                                       size_t new_length);
 
   // Creates a partial copy of this Btree node, copying all edges starting at
   // `begin`, adding a reference on each copied edge, and sets the length of
   // the newly created copy to `new_length`.
   CordRepBtree* CopyToEndFrom(size_t begin, size_t new_length) const;
 
   // Extracts and returns the front edge from the provided tree.
   // This method consumes a reference on the provided tree, and logically
   // performs the following operation:
   //   edge = CordRep::Ref(tree->Edge(kFront));
   //   CordRep::Unref(tree);
   //   return edge;
   static CordRep* ExtractFront(CordRepBtree* tree);
 
   // Returns a tree containing the result of appending `right` to `left`.
   static CordRepBtree* MergeTrees(CordRepBtree* left, CordRepBtree* right);
 
   // Fallback functions for `Create()`, `Append()` and `Prepend()` which
   // deal with legacy / non conforming input, i.e.: CONCAT trees.
   static CordRepBtree* CreateSlow(CordRep* rep);
   static CordRepBtree* AppendSlow(CordRepBtree*, CordRep* rep);
   static CordRepBtree* PrependSlow(CordRepBtree*, CordRep* rep);
 
   // Recursively rebuilds `tree` into `stack`. If 'consume` is set to true, the
   // function will consume a reference on `tree`. `stack` is a null terminated
   // array containing the new tree's state, with the current leaf node at
   // stack[0], and parent nodes above that, or null for 'top of tree'.
   static void Rebuild(CordRepBtree** stack, CordRepBtree* tree, bool consume);
 
   // Aligns existing edges to start at index 0, to allow for a new edge to be
   // added to the back of the current edges.
   inline void AlignBegin();
 
   // Aligns existing edges to end at `capacity`, to allow for a new edge to be
   // added in front of the current edges.
   inline void AlignEnd();
 
   // Adds the provided edge to this node.
   // Requires this node to have capacity for the edge. Realigns / moves
   // existing edges as needed to prepend or append the new edge.
   template <EdgeType edge_type>
   inline void Add(CordRep* rep);
 
   // Adds the provided edges to this node.
   // Requires this node to have capacity for the edges. Realigns / moves
   // existing edges as needed to prepend or append the new edges.
   template <EdgeType edge_type>
   inline void Add(absl::Span<CordRep* const>);
 
   // Adds data from `data` to this node until either all data has been consumed,
   // or there is no more capacity for additional flat nodes inside this node.
   // Requires the current node to be a leaf node, data to be non empty, and the
   // current node to have capacity for at least one more data edge.
   // Returns any remaining data from `data` that was not added, which is
   // depending on the edge type (front / back) either the remaining prefix of
   // suffix of the input.
   // See the `Append/Prepend` function for the meaning and purpose of `extra`.
   template <EdgeType edge_type>
   absl::string_view AddData(absl::string_view data, size_t extra);
 
   // Replace the front or back edge with the provided value.
   // Adopts a reference on `edge` and unrefs the old edge.
   template <EdgeType edge_type>
   inline void SetEdge(CordRep* edge);
 
   // Returns a partial copy of the current tree containing the first `n` bytes
   // of data. `CopyResult` contains both the resulting edge and its height. The
   // resulting tree may be less high than the current tree, or even be a single
   // matching data edge if `allow_folding` is set to true.
   // For example, if `n == 1`, then the result will be the single data edge, and
   // height will be set to -1 (one below the owning leaf node). If n == 0, this
   // function returns null. Requires `n <= length`
   CopyResult CopyPrefix(size_t n, bool allow_folding = true);
 
   // Returns a partial copy of the current tree containing all data starting
   // after `offset`. `CopyResult` contains both the resulting edge and its
   // height. The resulting tree may be less high than the current tree, or even
   // be a single matching data edge. For example, if `n == length - 1`, then the
   // result will be a single data edge, and height will be set to -1 (one below
   // the owning leaf node).
   // Requires `offset < length`
   CopyResult CopySuffix(size_t offset);
 
   // Returns a OpResult value of {this, kSelf} or {Copy(), kCopied}
   // depending on the value of `owned`.
   inline OpResult ToOpResult(bool owned);
 
   // Adds `rep` to the specified tree, returning the modified tree.
   template <EdgeType edge_type>
   static CordRepBtree* AddCordRep(CordRepBtree* tree, CordRep* rep);
 
   // Adds `data` to the specified tree, returning the modified tree.
   // See the `Append/Prepend` function for the meaning and purpose of `extra`.
   template <EdgeType edge_type>
   static CordRepBtree* AddData(CordRepBtree* tree, absl::string_view data,
                                size_t extra = 0);
 
   // Merges `src` into `dst` with `src` being added either before (kFront) or
   // after (kBack) `dst`. Requires the height of `dst` to be greater than or
   // equal to the height of `src`.
   template <EdgeType edge_type>
   static CordRepBtree* Merge(CordRepBtree* dst, CordRepBtree* src);
 
   // Fallback version of GetAppendBuffer for large trees: GetAppendBuffer()
   // implements an inlined version for trees of limited height (3 levels),
   // GetAppendBufferSlow implements the logic for large trees.
   Span<char> GetAppendBufferSlow(size_t size);
 
   // `edges_` contains all edges starting from this instance.
   // These are explicitly `child` edges only, a cord btree (or any cord tree in
   // that respect) does not store `parent` pointers anywhere: multiple trees /
   // parents can reference the same shared child edge. The type of these edges
   // depends on the height of the node. `Leaf nodes` (height == 0) contain `data
   // edges` (external or flat nodes, or sub-strings thereof). All other nodes
   // (height > 0) contain pointers to BTREE nodes with a height of `height - 1`.
   CordRep* edges_[kMaxCapacity];
 
   friend class CordRepBtreeTestPeer;
   friend class CordRepBtreeNavigator;
 };
 
 inline CordRepBtree* CordRep::btree() {
   assert(IsBtree());
   return static_cast<CordRepBtree*>(this);
 }
 
 inline const CordRepBtree* CordRep::btree() const {
   assert(IsBtree());
   return static_cast<const CordRepBtree*>(this);
 }
 
 inline void CordRepBtree::InitInstance(int height, size_t begin, size_t end) {
   tag = BTREE;
   storage[0] = static_cast<uint8_t>(height);
   storage[1] = static_cast<uint8_t>(begin);
   storage[2] = static_cast<uint8_t>(end);
 }
 
 inline CordRep* CordRepBtree::Edge(size_t index) const {
   assert(index >= begin());
   assert(index < end());
   return edges_[index];
 }
 
 inline CordRep* CordRepBtree::Edge(EdgeType edge_type) const {
   return edges_[edge_type == kFront ? begin() : back()];
 }
 
 inline absl::Span<CordRep* const> CordRepBtree::Edges() const {
   return {edges_ + begin(), size()};
 }
 
 inline absl::Span<CordRep* const> CordRepBtree::Edges(size_t begin,
                                                       size_t end) const {
   assert(begin <= end);
   assert(begin >= this->begin());
   assert(end <= this->end());
   return {edges_ + begin, static_cast<size_t>(end - begin)};
 }
 
 inline absl::string_view CordRepBtree::Data(size_t index) const {
   assert(height() == 0);
   return EdgeData(Edge(index));
 }
 
 inline CordRepBtree* CordRepBtree::New(int height) {
   CordRepBtree* tree = new CordRepBtree;
   tree->length = 0;
   tree->InitInstance(height);
   return tree;
 }
 
 inline CordRepBtree* CordRepBtree::New(CordRep* rep) {
   CordRepBtree* tree = new CordRepBtree;
   int height = rep->IsBtree() ? rep->btree()->height() + 1 : 0;
   tree->length = rep->length;
   tree->InitInstance(height, /*begin=*/0, /*end=*/1);
   tree->edges_[0] = rep;
   return tree;
 }
 
 inline CordRepBtree* CordRepBtree::New(CordRepBtree* front,
                                        CordRepBtree* back) {
   assert(front->height() == back->height());
   CordRepBtree* tree = new CordRepBtree;
   tree->length = front->length + back->length;
   tree->InitInstance(front->height() + 1, /*begin=*/0, /*end=*/2);
   tree->edges_[0] = front;
   tree->edges_[1] = back;
   return tree;
 }
 
 inline void CordRepBtree::Unref(absl::Span<CordRep* const> edges) {
   for (CordRep* edge : edges) {
     if (ABSL_PREDICT_FALSE(!edge->refcount.Decrement())) {
       CordRep::Destroy(edge);
     }
   }
 }
 
 inline CordRepBtree* CordRepBtree::CopyRaw(size_t new_length) const {
   CordRepBtree* tree = new CordRepBtree;
 
   // `length` and `refcount` are the first members of `CordRepBtree`.
   // We initialize `length` using the given length, have `refcount` be set to
   // ref = 1 through its default constructor, and copy all data beyond
   // 'refcount' which starts with `tag` using a single memcpy: all contents
   // except `refcount` is trivially copyable, and the compiler does not
   // efficiently coalesce member-wise copy of these members.
   // See https://gcc.godbolt.org/z/qY8zsca6z
   // # LINT.IfChange(copy_raw)
   tree->length = new_length;
   uint8_t* dst = &tree->tag;
   const uint8_t* src = &tag;
   const ptrdiff_t offset = src - reinterpret_cast<const uint8_t*>(this);
   memcpy(dst, src, sizeof(CordRepBtree) - static_cast<size_t>(offset));
   return tree;
   // # LINT.ThenChange()
 }
 
 inline CordRepBtree* CordRepBtree::Copy() const {
   CordRepBtree* tree = CopyRaw(length);
   for (CordRep* rep : Edges()) CordRep::Ref(rep);
   return tree;
 }
 
 inline CordRepBtree* CordRepBtree::CopyToEndFrom(size_t begin,
                                                  size_t new_length) const {
   assert(begin >= this->begin());
   assert(begin <= this->end());
   CordRepBtree* tree = CopyRaw(new_length);
   tree->set_begin(begin);
   for (CordRep* edge : tree->Edges()) CordRep::Ref(edge);
   return tree;
 }
 
 inline CordRepBtree* CordRepBtree::CopyBeginTo(size_t end,
                                                size_t new_length) const {
   assert(end <= capacity());
   assert(end >= this->begin());
   CordRepBtree* tree = CopyRaw(new_length);
   tree->set_end(end);
   for (CordRep* edge : tree->Edges()) CordRep::Ref(edge);
   return tree;
 }
 
 inline void CordRepBtree::AlignBegin() {
   // The below code itself does not need to be fast as typically we have
   // mono-directional append/prepend calls, and `begin` / `end` are typically
   // adjusted no more than once. But we want to avoid potential register clobber
   // effects, making the compiler emit register save/store/spills, and minimize
   // the size of code.
   const size_t delta = begin();
   if (ABSL_PREDICT_FALSE(delta != 0)) {
     const size_t new_end = end() - delta;
     set_begin(0);
     set_end(new_end);
     // TODO(mvels): we can write this using 2 loads / 2 stores depending on
     // total size for the kMaxCapacity = 6 case. I.e., we can branch (switch) on
     // size, and then do overlapping load/store of up to 4 pointers (inlined as
     // XMM, YMM or ZMM load/store) and up to 2 pointers (XMM / YMM), which is a)
     // compact and b) not clobbering any registers.
     ABSL_ASSUME(new_end <= kMaxCapacity);
 #ifdef __clang__
 #pragma unroll 1
 #endif
     for (size_t i = 0; i < new_end; ++i) {
       edges_[i] = edges_[i + delta];
     }
   }
 }
 
 inline void CordRepBtree::AlignEnd() {
   // See comments in `AlignBegin` for motivation on the hand-rolled for loops.
   const size_t delta = capacity() - end();
   if (delta != 0) {
     const size_t new_begin = begin() + delta;
     const size_t new_end = end() + delta;
     set_begin(new_begin);
     set_end(new_end);
     ABSL_ASSUME(new_end <= kMaxCapacity);
 #ifdef __clang__
 #pragma unroll 1
 #endif
     for (size_t i = new_end - 1; i >= new_begin; --i) {
       edges_[i] = edges_[i - delta];
     }
   }
 }
 
 template <>
 inline void CordRepBtree::Add<CordRepBtree::kBack>(CordRep* rep) {
   AlignBegin();
   edges_[fetch_add_end(1)] = rep;
 }
 
 template <>
 inline void CordRepBtree::Add<CordRepBtree::kBack>(
     absl::Span<CordRep* const> edges) {
   AlignBegin();
   size_t new_end = end();
   for (CordRep* edge : edges) edges_[new_end++] = edge;
   set_end(new_end);
 }
 
 template <>
 inline void CordRepBtree::Add<CordRepBtree::kFront>(CordRep* rep) {
   AlignEnd();
   edges_[sub_fetch_begin(1)] = rep;
 }
 
 template <>
 inline void CordRepBtree::Add<CordRepBtree::kFront>(
     absl::Span<CordRep* const> edges) {
   AlignEnd();
   size_t new_begin = begin() - edges.size();
   set_begin(new_begin);
   for (CordRep* edge : edges) edges_[new_begin++] = edge;
 }
 
 template <CordRepBtree::EdgeType edge_type>
 inline void CordRepBtree::SetEdge(CordRep* edge) {
   const int idx = edge_type == kFront ? begin() : back();
   CordRep::Unref(edges_[idx]);
   edges_[idx] = edge;
 }
 
 inline CordRepBtree::OpResult CordRepBtree::ToOpResult(bool owned) {
   return owned ? OpResult{this, kSelf} : OpResult{Copy(), kCopied};
 }
 
 inline CordRepBtree::Position CordRepBtree::IndexOf(size_t offset) const {
   assert(offset < length);
   size_t index = begin();
   while (offset >= edges_[index]->length) offset -= edges_[index++]->length;
   return {index, offset};
 }
 
 inline CordRepBtree::Position CordRepBtree::IndexBefore(size_t offset) const {
   assert(offset > 0);
   assert(offset <= length);
   size_t index = begin();
   while (offset > edges_[index]->length) offset -= edges_[index++]->length;
   return {index, offset};
 }
 
 inline CordRepBtree::Position CordRepBtree::IndexBefore(Position front,
                                                         size_t offset) const {
   size_t index = front.index;
   offset = offset + front.n;
   while (offset > edges_[index]->length) offset -= edges_[index++]->length;
   return {index, offset};
 }
 
 inline CordRepBtree::Position CordRepBtree::IndexOfLength(size_t n) const {
   assert(n <= length);
   size_t index = back();
   size_t strip = length - n;
   while (strip >= edges_[index]->length) strip -= edges_[index--]->length;
   return {index, edges_[index]->length - strip};
 }
 
 inline CordRepBtree::Position CordRepBtree::IndexBeyond(
     const size_t offset) const {
   // We need to find the edge which `starting offset` is beyond (>=)`offset`.
   // For this we can't use the `offset -= length` logic of IndexOf. Instead, we
   // track the offset of the `current edge` in `off`, which we increase as we
   // iterate over the edges until we find the matching edge.
   size_t off = 0;
   size_t index = begin();
   while (offset > off) off += edges_[index++]->length;
   return {index, off - offset};
 }
 
 inline CordRepBtree* CordRepBtree::Create(CordRep* rep) {
   if (IsDataEdge(rep)) return New(rep);
   return CreateSlow(rep);
 }
 
 inline Span<char> CordRepBtree::GetAppendBuffer(size_t size) {
   assert(refcount.IsOne());
   CordRepBtree* tree = this;
   const int height = this->height();
   CordRepBtree* n1 = tree;
   CordRepBtree* n2 = tree;
   CordRepBtree* n3 = tree;
   switch (height) {
     case 3:
       tree = tree->Edge(kBack)->btree();
       if (!tree->refcount.IsOne()) return {};
       n2 = tree;
       ABSL_FALLTHROUGH_INTENDED;
     case 2:
       tree = tree->Edge(kBack)->btree();
       if (!tree->refcount.IsOne()) return {};
       n1 = tree;
       ABSL_FALLTHROUGH_INTENDED;
     case 1:
       tree = tree->Edge(kBack)->btree();
       if (!tree->refcount.IsOne()) return {};
       ABSL_FALLTHROUGH_INTENDED;
     case 0:
       CordRep* edge = tree->Edge(kBack);
       if (!edge->refcount.IsOne()) return {};
       if (edge->tag < FLAT) return {};
       size_t avail = edge->flat()->Capacity() - edge->length;
       if (avail == 0) return {};
       size_t delta = (std::min)(size, avail);
       Span<char> span = {edge->flat()->Data() + edge->length, delta};
       edge->length += delta;
       switch (height) {
         case 3:
           n3->length += delta;
           ABSL_FALLTHROUGH_INTENDED;
         case 2:
           n2->length += delta;
           ABSL_FALLTHROUGH_INTENDED;
         case 1:
           n1->length += delta;
           ABSL_FALLTHROUGH_INTENDED;
         case 0:
           tree->length += delta;
           return span;
       }
       break;
   }
   return GetAppendBufferSlow(size);
 }
 
 extern template CordRepBtree* CordRepBtree::AddCordRep<CordRepBtree::kBack>(
     CordRepBtree* tree, CordRep* rep);
 
 extern template CordRepBtree* CordRepBtree::AddCordRep<CordRepBtree::kFront>(
     CordRepBtree* tree, CordRep* rep);
 
 inline CordRepBtree* CordRepBtree::Append(CordRepBtree* tree, CordRep* rep) {
   if (ABSL_PREDICT_TRUE(IsDataEdge(rep))) {
     return CordRepBtree::AddCordRep<kBack>(tree, rep);
   }
   return AppendSlow(tree, rep);
 }
 
 inline CordRepBtree* CordRepBtree::Prepend(CordRepBtree* tree, CordRep* rep) {
   if (ABSL_PREDICT_TRUE(IsDataEdge(rep))) {
     return CordRepBtree::AddCordRep<kFront>(tree, rep);
   }
   return PrependSlow(tree, rep);
 }
 
 #ifdef NDEBUG
 
 inline CordRepBtree* CordRepBtree::AssertValid(CordRepBtree* tree,
                                                bool /* shallow */) {
   return tree;
 }
 
 inline const CordRepBtree* CordRepBtree::AssertValid(const CordRepBtree* tree,
                                                      bool /* shallow */) {
   return tree;
 }
 
 #endif
 
 }  // namespace cord_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_CORD_REP_BTREE_H_

This page was automatically generated by the 2.3.7 LXR engine. The LXR team