wehub-resource-sync
50 lines
1.4 KiB
Python
50 lines
1.4 KiB
Python
from typing import Dict, TypeVar
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K = TypeVar("K")
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def collapse_transitive_map(d: Dict[K, K]) -> Dict[K, K]:
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"""Collapse transitive mappings in a dictionary. Given a mapping like
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{a: b, b: c, c: d}, returns {a: d}, removing intermediate b -> c, c -> d.
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Only keeps mappings where the key is NOT a value in another mapping (i.e., chain starting points).
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Args:
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d: Dictionary representing a mapping
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Returns:
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Dictionary with all transitive mappings collapsed, keeping only KV-pairs,
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such that K and V are starting and terminal point of a chain
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Examples:
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>>> collapse_transitive_map({"a": "b", "b": "c", "c": "d"})
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{'a': 'd'}
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>>> collapse_transitive_map({"a": "b", "x": "y"})
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{'a': 'b', 'x': 'y'}
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"""
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if not d:
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return {}
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collapsed = {}
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values_set = set(d.values())
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for k in d:
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# Skip mappings that are in the value-set, meaning that they are
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# part of the mapping chain (for ex, {a -> b, b -> c})
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if k in values_set:
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continue
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cur = k
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visited = {cur}
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# Follow the chain until we reach a key that's not in the mapping
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while cur in d:
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next = d[cur]
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if next in visited:
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raise ValueError(f"Detected a cycle in the mapping {d}")
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visited.add(next)
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cur = next
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collapsed[k] = cur
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return collapsed
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