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// Copyright 2016 Google Inc.
//
// 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
//
// http://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 <functional>
#include <memory>
#include <queue>
#include <random>
#include <utility>
#include <vector>
#include "bpe_model.h"
#include "freelist.h"
#include "third_party/absl/container/flat_hash_map.h"
#include "util.h"
namespace sentencepiece {
namespace bpe {
Model::Model(const ModelProto &model_proto) {
model_proto_ = &model_proto;
InitializePieces();
}
Model::~Model() {}
std::vector<std::pair<absl::string_view, int>> Model::SampleEncode(
absl::string_view normalized, float alpha) const {
if (!status().ok() || normalized.empty()) {
return {};
}
struct SymbolPair {
int left; // left index of this pair
int right; // right index of this pair
float score; // score of this pair. large is better.
size_t size; // length of this piece
};
class SymbolPairComparator {
public:
const bool operator()(SymbolPair *h1, SymbolPair *h2) {
return (h1->score < h2->score ||
(h1->score == h2->score && h1->left > h2->left));
}
};
struct Symbol {
int prev; // prev index of this symbol. -1 for BOS.
int next; // next index of tihs symbol. -1 for EOS.
bool freeze; // this symbol is never be merged.
absl::string_view piece;
};
using Agenda = std::priority_queue<SymbolPair *, std::vector<SymbolPair *>,
SymbolPairComparator>;
Agenda agenda;
std::vector<Symbol> symbols;
symbols.reserve(normalized.size());
// Reverse merge rules.
// key: merged symbol, value: pair of original symbols.
absl::flat_hash_map<absl::string_view,
std::pair<absl::string_view, absl::string_view>,
string_util::string_view_hash>
rev_merge;
// Pre-allocates SymbolPair for efficiency.
constexpr size_t kPreallocateSymbolPairSize = 256;
model::FreeList<SymbolPair> symbol_pair_allocator(kPreallocateSymbolPairSize);
// Lookup new symbol pair at [left, right] and inserts it to agenda.
auto MaybeAddNewSymbolPair = [this, &symbol_pair_allocator, &symbols, &agenda,
&rev_merge](int left, int right) {
if (left == -1 || right == -1 || symbols[left].freeze ||
symbols[right].freeze)
return;
const absl::string_view piece(
symbols[left].piece.data(),
symbols[left].piece.size() + symbols[right].piece.size());
const auto it = pieces_.find(piece);
if (it == pieces_.end()) {
return;
}
auto *h = symbol_pair_allocator.Allocate();
h->left = left;
h->right = right;
h->score = GetScore(it->second);
h->size = piece.size();
agenda.push(h);
// Makes `rev_merge` for resegmentation.
if (IsUnusedInlined(it->second)) {
rev_merge[piece] =
std::make_pair(symbols[left].piece, symbols[right].piece);
}
};
// Splits the input into character sequence
int index = 0;
while (!normalized.empty()) {
Symbol s;
const int mblen = matcher_->PrefixMatch(normalized, &s.freeze);
s.piece = absl::string_view(normalized.data(), mblen);
s.prev = index == 0 ? -1 : index - 1;
normalized.remove_prefix(mblen);
s.next = normalized.empty() ? -1 : index + 1;
++index;
symbols.emplace_back(s);
}
if (symbols.empty()) {
return {};
}
// Lookup all bigrams.
for (size_t i = 1; i < symbols.size(); ++i) {
MaybeAddNewSymbolPair(i - 1, i);
}
// BPE-dropout: https://arxiv.org/pdf/1910.13267.pdf
std::mt19937 *rand_gen = nullptr;
auto skip_merge = [&]() {
if (alpha <= 0.0) return false;
if (rand_gen == nullptr) rand_gen = random::GetRandomGenerator();
std::uniform_real_distribution<> gen(0.0, 1.0);
return gen(*rand_gen) < alpha;
};
// Main loop.
while (!agenda.empty()) {
SymbolPair *top = agenda.top();
agenda.pop();
// `top` is no longer available.
if (symbols[top->left].piece.empty() || symbols[top->right].piece.empty() ||
symbols[top->left].piece.size() + symbols[top->right].piece.size() !=
top->size) {
continue;
}
// Note that orignal BPE-dropout paper assumes that all merged symbols are
// pre computed, but here we randomly skip merge opration inside this loop.
// This implemenation is theoretically equivalent to the original one.
if (skip_merge()) continue;
// Replaces symbols with `top` rule.
symbols[top->left].piece = absl::string_view(
symbols[top->left].piece.data(),
symbols[top->left].piece.size() + symbols[top->right].piece.size());
// Updates prev/next pointers.
symbols[top->left].next = symbols[top->right].next;
if (symbols[top->right].next >= 0) {
symbols[symbols[top->right].next].prev = top->left;
}
symbols[top->right].piece = absl::string_view("");
// Adds new symbol pairs which are newly added after symbol replacement.
MaybeAddNewSymbolPair(symbols[top->left].prev, top->left);
MaybeAddNewSymbolPair(top->left, symbols[top->left].next);
}
std::function<void(absl::string_view, EncodeResult *)> resegment;
resegment = [this, &resegment, &rev_merge](absl::string_view w,
EncodeResult *output) -> void {
const int id = PieceToId(w);
if (id == -1 || !IsUnusedInlined(id)) {
output->emplace_back(w, id);
return;
}
const auto p = rev_merge.find(w);
if (p == rev_merge.end()) {
// This block will never be called, as `rev_merge` stores all the
// resegmentation info for unused id.
output->emplace_back(w, id);
return;
}
// Recursively resegment left and right symbols.
resegment(p->second.first, output);
resegment(p->second.second, output);
};
EncodeResult output;
for (int index = 0; index != -1; index = symbols[index].next) {
CHECK_GE(index, 0);
CHECK_LT(index, static_cast<int>(symbols.size()));
resegment(symbols[index].piece, &output);
}
return output;
}
} // namespace bpe
} // namespace sentencepiece
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