diff options
Diffstat (limited to 'absl/strings/internal/cord_rep_btree_navigator.cc')
-rw-r--r-- | absl/strings/internal/cord_rep_btree_navigator.cc | 185 |
1 files changed, 185 insertions, 0 deletions
diff --git a/absl/strings/internal/cord_rep_btree_navigator.cc b/absl/strings/internal/cord_rep_btree_navigator.cc new file mode 100644 index 00000000..d1f9995d --- /dev/null +++ b/absl/strings/internal/cord_rep_btree_navigator.cc @@ -0,0 +1,185 @@ +// 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. + +#include "absl/strings/internal/cord_rep_btree_navigator.h" + +#include <cassert> + +#include "absl/strings/internal/cord_internal.h" +#include "absl/strings/internal/cord_rep_btree.h" + +namespace absl { +ABSL_NAMESPACE_BEGIN +namespace cord_internal { + +using ReadResult = CordRepBtreeNavigator::ReadResult; + +namespace { + +// Returns a `CordRepSubstring` from `rep` starting at `offset` of size `n`. +// If `rep` is already a `CordRepSubstring` instance, an adjusted instance is +// created based on the old offset and new offset. +// Adopts a reference on `rep`. Rep must be a valid data edge. Returns +// nullptr if `n == 0`, `rep` if `n == rep->length`. +// Requires `offset < rep->length` and `offset + n <= rep->length`. +// TODO(192061034): move to utility library in internal and optimize for small +// substrings of larger reps. +inline CordRep* Substring(CordRep* rep, size_t offset, size_t n) { + assert(n <= rep->length); + assert(offset < rep->length); + assert(offset <= rep->length - n); + assert(CordRepBtree::IsDataEdge(rep)); + + if (n == 0) return nullptr; + if (n == rep->length) return CordRep::Ref(rep); + + if (rep->tag == SUBSTRING) { + offset += rep->substring()->start; + rep = rep->substring()->child; + } + + CordRepSubstring* substring = new CordRepSubstring(); + substring->length = n; + substring->tag = SUBSTRING; + substring->start = offset; + substring->child = CordRep::Ref(rep); + return substring; +} + +inline CordRep* Substring(CordRep* rep, size_t offset) { + return Substring(rep, offset, rep->length - offset); +} + +} // namespace + +CordRepBtreeNavigator::Position CordRepBtreeNavigator::Skip(size_t n) { + int height = 0; + size_t index = index_[0]; + CordRepBtree* node = node_[0]; + CordRep* edge = node->Edge(index); + + // Overall logic: Find an edge of at least the length we need to skip. + // We consume all edges which are smaller (i.e., must be 100% skipped). + // If we exhausted all edges on the current level, we move one level + // up the tree, and repeat until we either find the edge, or until we hit + // the top of the tree meaning the skip exceeds tree->length. + while (n >= edge->length) { + n -= edge->length; + while (++index == node->end()) { + if (++height > height_) return {nullptr, n}; + node = node_[height]; + index = index_[height]; + } + edge = node->Edge(index); + } + + // If we moved up the tree, descend down to the leaf level, consuming all + // edges that must be skipped. + while (height > 0) { + node = edge->btree(); + index_[height] = index; + node_[--height] = node; + index = node->begin(); + edge = node->Edge(index); + while (n >= edge->length) { + n -= edge->length; + ++index; + assert(index != node->end()); + edge = node->Edge(index); + } + } + index_[0] = index; + return {edge, n}; +} + +ReadResult CordRepBtreeNavigator::Read(size_t edge_offset, size_t n) { + int height = 0; + size_t length = edge_offset + n; + size_t index = index_[0]; + CordRepBtree* node = node_[0]; + CordRep* edge = node->Edge(index); + assert(edge_offset < edge->length); + + if (length < edge->length) { + return {Substring(edge, edge_offset, n), length}; + } + + // Similar to 'Skip', we consume all edges that are inside the 'length' of + // data that needs to be read. If we exhaust the current level, we move one + // level up the tree and repeat until we hit the final edge that must be + // (partially) read. We consume all edges into `subtree`. + CordRepBtree* subtree = CordRepBtree::New(Substring(edge, edge_offset)); + size_t subtree_end = 1; + do { + length -= edge->length; + while (++index == node->end()) { + index_[height] = index; + if (++height > height_) { + subtree->set_end(subtree_end); + if (length == 0) return {subtree, 0}; + CordRep::Unref(subtree); + return {nullptr, length}; + } + if (length != 0) { + subtree->set_end(subtree_end); + subtree = CordRepBtree::New(subtree); + subtree_end = 1; + } + node = node_[height]; + index = index_[height]; + } + edge = node->Edge(index); + if (length >= edge->length) { + subtree->length += edge->length; + subtree->edges_[subtree_end++] = CordRep::Ref(edge); + } + } while (length >= edge->length); + CordRepBtree* tree = subtree; + subtree->length += length; + + // If we moved up the tree, descend down to the leaf level, consuming all + // edges that must be read, adding 'down' nodes to `subtree`. + while (height > 0) { + node = edge->btree(); + index_[height] = index; + node_[--height] = node; + index = node->begin(); + edge = node->Edge(index); + + if (length != 0) { + CordRepBtree* right = CordRepBtree::New(height); + right->length = length; + subtree->edges_[subtree_end++] = right; + subtree->set_end(subtree_end); + subtree = right; + subtree_end = 0; + while (length >= edge->length) { + subtree->edges_[subtree_end++] = CordRep::Ref(edge); + length -= edge->length; + edge = node->Edge(++index); + } + } + } + // Add any (partial) edge still remaining at the leaf level. + if (length != 0) { + subtree->edges_[subtree_end++] = Substring(edge, 0, length); + } + subtree->set_end(subtree_end); + index_[0] = index; + return {tree, length}; +} + +} // namespace cord_internal +ABSL_NAMESPACE_END +} // namespace absl |