# -*- coding: utf-8 -*-
"""
Dictionary-based maximal matching word segmentation, constrained with
Thai Character Cluster (TCC) boundaries.
The code is based on the notebooks created by Korakot Chaovavanich,
with heuristic graph size limit added to avoid exponential wait time.
:See Also:
* \
https://colab.research.google.com/notebook#fileId=1V1Z657_5eSWPo8rLfVRwA0A5E4vkg7SI
* \
https://colab.research.google.com/drive/14Ibg-ngZXj15RKwjNwoZlOT32fQBOrBx#scrollTo=MYZ7NzAR7Dmw
"""
import re
from collections import defaultdict
from heapq import heappop, heappush
from typing import Generator, List
from pythainlp.tokenize import DEFAULT_WORD_DICT_TRIE
from pythainlp.util import Trie
from pythainlp.tokenize.tcc import tcc_pos
# match non-Thai tokens
_PAT_NONTHAI = re.compile(
r"""(?x)
[-a-zA-Z]+| # Latin characters
\d+([,\.]\d+)*| # number
[ \t]+| # space
\r?\n # newline
"""
)
# match 2-consonant Thai tokens
_PAT_THAI_TWOCHARS = re.compile("[ก-ฮ]{,2}$")
# maximum graph size before cutoff
_MAX_GRAPH_SIZE = 50
# window size for safe mode
_TEXT_SCAN_POINT = 120
_TEXT_SCAN_LEFT = 20
_TEXT_SCAN_RIGHT = 20
_TEXT_SCAN_BEGIN = _TEXT_SCAN_POINT - _TEXT_SCAN_LEFT
_TEXT_SCAN_END = _TEXT_SCAN_POINT + _TEXT_SCAN_RIGHT
del _TEXT_SCAN_POINT
del _TEXT_SCAN_LEFT
del _TEXT_SCAN_RIGHT
def _bfs_paths_graph(
graph: defaultdict, start: int, goal: int
) -> Generator[List[int], None, None]:
queue = [(start, [start])]
while queue:
(vertex, path) = queue.pop(0)
for pos in graph[vertex]:
if pos == goal:
yield path + [pos]
else:
queue.append((pos, path + [pos]))
def _onecut(text: str, custom_dict: Trie) -> Generator[str, None, None]:
# main data structure:
# - key is begin position (int)
# - value is possible end positions (List[int])
# if key is not found, value is empty list
graph = defaultdict(list)
graph_size = 0 # keep track of graph size, if too big will force cutoff
valid_poss = tcc_pos(text) # breaking positions that are TCC-valid
len_text = len(text)
pos_list = [0] # priority queue of possible breaking positions
end_pos = 0
while pos_list[0] < len_text:
begin_pos = heappop(pos_list)
for word in custom_dict.prefixes(text[begin_pos:]):
end_pos_candidate = begin_pos + len(word)
if end_pos_candidate in valid_poss:
graph[begin_pos].append(end_pos_candidate)
graph_size = graph_size + 1
if end_pos_candidate not in pos_list:
heappush(pos_list, end_pos_candidate)
if graph_size > _MAX_GRAPH_SIZE:
break
len_pos_list = len(pos_list)
if len_pos_list == 1: # one candidate, no longer ambiguous
end_pos_candidates = next(
_bfs_paths_graph(graph, end_pos, pos_list[0])
)
graph_size = 0
for pos in end_pos_candidates[1:]:
yield text[end_pos:pos]
end_pos = pos
elif len_pos_list == 0: # no candidate, deal with non-dictionary word
m = _PAT_NONTHAI.match(text[begin_pos:])
if m: # non-Thai token, skip to the end
end_pos = begin_pos + m.end()
else: # Thai token, find minimum skip
for pos in range(begin_pos + 1, len_text):
if pos in valid_poss:
prefix = text[pos:]
words = [
word
for word in custom_dict.prefixes(prefix)
if (
(pos + len(word) in valid_poss)
and not _PAT_THAI_TWOCHARS.match(word)
)
]
if words: # is a Thai token that longer than 2 chars
end_pos = pos
break
# is a non-Thai token
if _PAT_NONTHAI.match(prefix):
end_pos = pos
break
else:
end_pos = len_text
graph[begin_pos].append(end_pos)
graph_size = graph_size + 1
yield text[begin_pos:end_pos]
heappush(pos_list, end_pos)
[docs]def segment(
text: str,
custom_dict: Trie = DEFAULT_WORD_DICT_TRIE,
safe_mode: bool = False,
) -> List[str]:
"""Maximal-matching word segmentation, Thai Character Cluster constrained.
A dictionary-based word segmentation using maximal matching algorithm,
constrained to Thai Character Cluster boundaries.
A custom dictionary can be supplied.
:param text: text to be tokenized
:type text: str
:param custom_dict: tokenization dictionary,\
defaults to DEFAULT_WORD_DICT_TRIE
:type custom_dict: Trie, optional
:param safe_mode: reduce chance for long processing time in long text\
with many ambiguous breaking points, defaults to False
:type safe_mode: bool, optional
:return: list of tokens
:rtype: List[str]
"""
if not text or not isinstance(text, str):
return []
if not custom_dict:
custom_dict = DEFAULT_WORD_DICT_TRIE
if not safe_mode or len(text) < _TEXT_SCAN_END:
return list(_onecut(text, custom_dict))
# if the text is longer than the limit,
# breaks them into smaller chunks then tokenizes each chunk
text_parts = []
while len(text) >= _TEXT_SCAN_END:
sample = text[_TEXT_SCAN_BEGIN:_TEXT_SCAN_END]
# find possible break positions
cut_pos = _TEXT_SCAN_END
# try to break by space first
space_idx = sample.rfind(" ")
if space_idx >= 0:
cut_pos = space_idx + 1
else:
tokens = list(_onecut(sample, custom_dict))
token_max_idx = 0
token_max_len = 0
for i, token in enumerate(tokens):
if len(token) >= token_max_len:
token_max_len = len(token)
token_max_idx = i
# choose the position that covers longest token
cut_pos = _TEXT_SCAN_BEGIN
for i in range(0, token_max_idx):
cut_pos = cut_pos + len(tokens[i])
text_parts.append(text[:cut_pos])
text = text[cut_pos:]
# append remaining text
if len(text):
text_parts.append(text)
# tokenizes each text parts
tokens = []
for text_part in text_parts:
tokens.extend(list(_onecut(text_part, custom_dict)))
return tokens