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import torch
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from abc import ABC, abstractmethod
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from typing import List, Optional, Tuple
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from torch import Tensor
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from torch.nn.utils.rnn import pad_sequence
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class BaseTokenizer(ABC):
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def __init__(self, charset: str, specials_first: tuple = (), specials_last: tuple = ()) -> None:
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self._itos = specials_first + tuple(charset + '[UNK]') + specials_last
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self._stoi = {s: i for i, s in enumerate(self._itos)}
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def __len__(self):
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return len(self._itos)
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def _tok2ids(self, tokens: str) -> List[int]:
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return [self._stoi[s] for s in tokens]
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def _ids2tok(self, token_ids: List[int], join: bool = True) -> str:
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tokens = [self._itos[i] for i in token_ids]
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return ''.join(tokens) if join else tokens
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@abstractmethod
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def encode(self, labels: List[str], device: Optional[torch.device] = None) -> Tensor:
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raise NotImplementedError
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@abstractmethod
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def _filter(self, probs: Tensor, ids: Tensor) -> Tuple[Tensor, List[int]]:
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"""Internal method which performs the necessary filtering prior to decoding."""
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raise NotImplementedError
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def decode(self, token_dists: Tensor, beam_width: int = 1, raw: bool = False) -> Tuple[List[str], List[Tensor]]:
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if beam_width > 1:
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return self.beam_search_decode(token_dists, beam_width, raw)
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else:
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return self.greedy_decode(token_dists, raw)
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def greedy_decode(self, token_dists: Tensor, raw: bool = False) -> Tuple[List[str], List[Tensor]]:
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batch_tokens = []
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batch_probs = []
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for dist in token_dists:
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probs, ids = dist.max(-1)
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if not raw:
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probs, ids = self._filter(probs, ids)
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tokens = self._ids2tok(ids, not raw)
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batch_tokens.append(tokens)
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batch_probs.append(probs)
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return batch_tokens, batch_probs
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def beam_search_decode(self, token_dists: Tensor, beam_width: int, raw: bool) -> Tuple[List[str], List[Tensor]]:
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batch_tokens = []
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batch_probs = []
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for dist in token_dists:
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sequences = [([], 1.0)]
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for step_dist in dist:
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all_candidates = []
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for seq, score in sequences:
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top_probs, top_ids = step_dist.topk(beam_width)
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for i in range(beam_width):
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candidate = (seq + [top_ids[i].item()],
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score * top_probs[i].item())
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all_candidates.append(candidate)
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ordered = sorted(all_candidates, key=lambda x: x[1], reverse=True)
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sequences = ordered[:beam_width]
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best_sequence, best_score = sequences[0]
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if not raw:
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best_score_tensor = torch.tensor([best_score])
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best_sequence_tensor = torch.tensor(best_sequence)
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best_score_tensor, best_sequence = self._filter(
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best_score_tensor, best_sequence_tensor)
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best_score = best_score_tensor.item()
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tokens = self._ids2tok(best_sequence, not raw)
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batch_tokens.append(tokens)
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batch_probs.append(best_score)
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return batch_tokens, batch_probs
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class Tokenizer(BaseTokenizer):
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BOS = '[B]'
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EOS = '[E]'
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PAD = '[P]'
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def __init__(self, charset: str) -> None:
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specials_first = (self.EOS,)
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specials_last = (self.BOS, self.PAD)
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super().__init__(charset, specials_first, specials_last)
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self.eos_id, self.bos_id, self.pad_id = [
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self._stoi[s] for s in specials_first + specials_last]
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def encode(self, labels: List[str], device: Optional[torch.device] = None) -> Tensor:
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batch = [torch.as_tensor([self.bos_id] + self._tok2ids(y) + [self.eos_id], dtype=torch.long, device=device)
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for y in labels]
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return pad_sequence(batch, batch_first=True, padding_value=self.pad_id)
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def _filter(self, probs: Tensor, ids: Tensor) -> Tuple[Tensor, List[int]]:
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ids = ids.tolist()
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try:
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eos_idx = ids.index(self.eos_id)
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except ValueError:
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eos_idx = len(ids)
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ids = ids[:eos_idx]
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probs = probs[:eos_idx + 1]
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return probs, ids
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