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Clean up trace_alternating_paths()

This commit is contained in:
Joris van Rantwijk 2023-02-08 19:21:34 +01:00
parent f491d6dcec
commit d1b79c1cde
1 changed files with 11 additions and 13 deletions
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24
python/max_weight_matching.py
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@ -840,18 +840,17 @@ class _MatchingContext:
# "xedges" is a list of edges used while tracing from "x".
# "yedges" is a list of edges used while tracing from "y".
xedges: list[tuple[int, int]] = []
yedges: list[tuple[int, int]] = []
# Pre-load the edge (x, y) on both lists.
xedges: list[tuple[int, int]] = [(x, y)]
yedges: list[tuple[int, int]] = [(y, x)]
# Pre-load the edge between "x" and "y" so it will end up in the right
# place in the final path.
xedges.append((x, y))
# "first_common" is the first common ancestor of "x" and "y"
# in the alternating tree, or None if there is no common ancestor.
first_common: Optional[_Blossom] = None
# Alternate between tracing the path from "x" and the path from "y".
# This ensures that the search time is bounded by the size of the
# newly found blossom.
# TODO : this code is a bit shady; maybe reconsider the swapping trick
first_common: Optional[_Blossom] = None
while x != -1 or y != -1:
# Check if we found a common ancestor.
@ -882,17 +881,16 @@ class _MatchingContext:
b.marker = False
# If we found a common ancestor, trim the paths so they end there.
# TODO : also this is just plain ugly - try to rework
if first_common is not None:
assert self.vertex_top_blossom[xedges[-1][0]] is first_common
while yedges and (self.vertex_top_blossom[yedges[-1][0]]
is not first_common):
while self.vertex_top_blossom[yedges[-1][0]] is not first_common:
yedges.pop()
# Fuse the two paths.
# Flip the order of one path and the edge tuples in the other path
# to obtain a continuous path with correctly ordered edge tuples.
path_edges = xedges[::-1] + [(y, x) for (x, y) in yedges]
# Flip the order of one path, and flip the edge tuples in the other
# path to obtain a continuous path with correctly ordered edge tuples.
# Skip the duplicate edge in one of the paths.
path_edges = xedges[::-1] + [(y, x) for (x, y) in yedges[1:]]
# Any S-to-S alternating path must have odd length.
assert len(path_edges) % 2 == 1