259 lines
8.4 KiB
Cython
259 lines
8.4 KiB
Cython
# cython: infer_types=True
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"""Implements the projectivize/deprojectivize mechanism in Nivre & Nilsson 2005
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for doing pseudo-projective parsing implementation uses the HEAD decoration
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scheme.
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"""
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from copy import copy
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from cython.operator cimport dereference as deref
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from cython.operator cimport preincrement as incr
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from libc.limits cimport INT_MAX
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from libc.stdlib cimport abs
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from libcpp cimport bool
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from libcpp.string cimport string, to_string
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from libcpp.unordered_set cimport unordered_set
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from libcpp.vector cimport vector
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from ...tokens.doc cimport Doc, set_children_from_heads
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from ...errors import Errors
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DELIMITER = '||'
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def ancestors(tokenid, heads):
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# Returns all words going from the word up the path to the root. The path
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# to root cannot be longer than the number of words in the sentence. This
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# function ends after at most len(heads) steps, because it would otherwise
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# loop indefinitely on cycles.
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head = tokenid
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cnt = 0
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while heads[head] != head and cnt < len(heads):
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head = heads[head]
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cnt += 1
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yield head
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if head is None:
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break
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def contains_cycle(heads):
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# in an acyclic tree, the path from each word following the head relation
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# upwards always ends at the root node
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for tokenid in range(len(heads)):
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seen = set([tokenid])
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for ancestor in ancestors(tokenid, heads):
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if ancestor in seen:
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return seen
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seen.add(ancestor)
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return None
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def is_nonproj_arc(tokenid, heads):
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cdef vector[int] c_heads = _heads_to_c(heads)
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return _is_nonproj_arc(tokenid, c_heads)
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cdef bool _is_nonproj_arc(int tokenid, const vector[int]& heads) nogil except *:
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# definition (e.g. Havelka 2007): an arc h -> d, h < d is non-projective
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# if there is a token k, h < k < d such that h is not
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# an ancestor of k. Same for h -> d, h > d
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head = heads[tokenid]
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if head == tokenid: # root arcs cannot be non-projective
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return False
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elif head < 0: # unattached tokens cannot be non-projective
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return False
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cdef int start, end
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if head < tokenid:
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start, end = (head+1, tokenid)
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else:
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start, end = (tokenid+1, head)
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for k in range(start, end):
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if not _has_head_as_ancestor(k, head, heads):
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return True
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return False
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cdef bool _has_head_as_ancestor(int tokenid, int head, const vector[int]& heads) nogil except *:
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ancestor = tokenid
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cdef unordered_set[int] seen_tokens
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seen_tokens.insert(ancestor)
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while True:
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# Reached the head or a disconnected node
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if heads[ancestor] == head or heads[ancestor] < 0:
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return True
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# Reached the root
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if heads[ancestor] == ancestor:
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return False
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ancestor = heads[ancestor]
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result = seen_tokens.insert(ancestor)
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# Found cycle
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if not result.second:
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raise_domain_error(heads_to_string(heads))
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return False
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cdef string heads_to_string(const vector[int]& heads) nogil:
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cdef vector[int].const_iterator citer
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cdef string cycle_str
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cycle_str.append("Found cycle in dependency graph: [")
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# FIXME: Rewrite using ostringstream when available in Cython.
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citer = heads.const_begin()
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while citer != heads.const_end():
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if citer != heads.const_begin():
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cycle_str.append(", ")
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cycle_str.append(to_string(deref(citer)))
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incr(citer)
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cycle_str.append("]")
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return cycle_str
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def is_nonproj_tree(heads):
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cdef vector[int] c_heads = _heads_to_c(heads)
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# a tree is non-projective if at least one arc is non-projective
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return any(_is_nonproj_arc(word, c_heads) for word in range(len(heads)))
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def decompose(label):
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return label.partition(DELIMITER)[::2]
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def is_decorated(label):
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return DELIMITER in label
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def count_decorated_labels(gold_data):
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freqs = {}
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for example in gold_data:
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proj_heads, deco_deps = projectivize(example.get_aligned("HEAD"),
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example.get_aligned("DEP"))
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# set the label to ROOT for each root dependent
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deco_deps = [
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'ROOT' if head == i else deco_deps[i]
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for i, head in enumerate(proj_heads)
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]
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# count label frequencies
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for label in deco_deps:
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if is_decorated(label):
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freqs[label] = freqs.get(label, 0) + 1
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return freqs
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def projectivize(heads, labels):
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# Use the algorithm by Nivre & Nilsson 2005. Assumes heads to be a proper
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# tree, i.e. connected and cycle-free. Returns a new pair (heads, labels)
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# which encode a projective and decorated tree.
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proj_heads = copy(heads)
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cdef int new_head
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cdef vector[int] c_proj_heads = _heads_to_c(proj_heads)
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cdef int smallest_np_arc = _get_smallest_nonproj_arc(c_proj_heads)
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if smallest_np_arc == -1: # this sentence is already projective
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return proj_heads, copy(labels)
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while smallest_np_arc != -1:
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new_head = _lift(smallest_np_arc, proj_heads)
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c_proj_heads[smallest_np_arc] = new_head
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smallest_np_arc = _get_smallest_nonproj_arc(c_proj_heads)
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deco_labels = _decorate(heads, proj_heads, labels)
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return proj_heads, deco_labels
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cdef vector[int] _heads_to_c(heads):
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cdef vector[int] c_heads
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for head in heads:
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if head is None:
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c_heads.push_back(-1)
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else:
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assert head < len(heads)
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c_heads.push_back(head)
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return c_heads
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cpdef deprojectivize(Doc doc):
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# Reattach arcs with decorated labels (following HEAD scheme). For each
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# decorated arc X||Y, search top-down, left-to-right, breadth-first until
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# hitting a Y then make this the new head.
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for i in range(doc.length):
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label = doc.vocab.strings[doc.c[i].dep]
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if DELIMITER in label:
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new_label, head_label = label.split(DELIMITER)
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new_head = _find_new_head(doc[i], head_label)
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doc.c[i].head = new_head.i - i
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doc.c[i].dep = doc.vocab.strings.add(new_label)
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set_children_from_heads(doc.c, 0, doc.length)
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return doc
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def _decorate(heads, proj_heads, labels):
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# uses decoration scheme HEAD from Nivre & Nilsson 2005
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if (len(heads) != len(proj_heads)) or (len(proj_heads) != len(labels)):
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raise ValueError(Errors.E082.format(n_heads=len(heads),
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n_proj_heads=len(proj_heads),
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n_labels=len(labels)))
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deco_labels = []
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for tokenid, head in enumerate(heads):
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if head != proj_heads[tokenid]:
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deco_labels.append(f"{labels[tokenid]}{DELIMITER}{labels[head]}")
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else:
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deco_labels.append(labels[tokenid])
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return deco_labels
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def get_smallest_nonproj_arc_slow(heads):
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cdef vector[int] c_heads = _heads_to_c(heads)
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return _get_smallest_nonproj_arc(c_heads)
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cdef int _get_smallest_nonproj_arc(const vector[int]& heads) nogil except -2:
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# return the smallest non-proj arc or None
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# where size is defined as the distance between dep and head
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# and ties are broken left to right
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cdef int smallest_size = INT_MAX
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# -1 means its already projective.
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cdef int smallest_np_arc = -1
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cdef int size
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cdef int tokenid
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cdef int head
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for tokenid in range(heads.size()):
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head = heads[tokenid]
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size = abs(tokenid-head)
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if size < smallest_size and _is_nonproj_arc(tokenid, heads):
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smallest_size = size
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smallest_np_arc = tokenid
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return smallest_np_arc
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cpdef int _lift(tokenid, heads):
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# reattaches a word to it's grandfather
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head = heads[tokenid]
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ghead = heads[head]
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cdef int new_head = ghead if head != ghead else tokenid
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# attach to ghead if head isn't attached to root else attach to root
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heads[tokenid] = new_head
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return new_head
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def _find_new_head(token, headlabel):
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# search through the tree starting from the head of the given token
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# returns the id of the first descendant with the given label
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# if there is none, return the current head (no change)
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queue = [token.head]
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while queue:
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next_queue = []
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for qtoken in queue:
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for child in qtoken.children:
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if child.is_space:
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continue
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if child == token:
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continue
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if child.dep_ == headlabel:
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return child
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next_queue.append(child)
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queue = next_queue
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return token.head
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