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// 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
//
//
// 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.
// # 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_