46 lines
1.2 KiB
Python
46 lines
1.2 KiB
Python
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"""Generic tools for working with trees."""
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from math import ceil, log
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def build_n_ary_tree(leaves, n):
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"""Build N-ary tree from sequence of leaf nodes.
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Return a list of lists where each non-leaf node is a list containing
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max n nodes.
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"""
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if not leaves:
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return []
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assert n > 1
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depth = ceil(log(len(leaves), n))
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if depth <= 1:
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return list(leaves)
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# Fully populate complete subtrees of root until we have enough leaves left
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root = []
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unassigned = None
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full_step = n ** (depth - 1)
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for i in range(0, len(leaves), full_step):
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subtree = leaves[i : i + full_step]
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if len(subtree) < full_step:
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unassigned = subtree
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break
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while len(subtree) > n:
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subtree = [subtree[k : k + n] for k in range(0, len(subtree), n)]
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root.append(subtree)
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if unassigned:
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# Recurse to fill the last subtree, which is the only partially populated one
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subtree = build_n_ary_tree(unassigned, n)
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if len(subtree) <= n - len(root):
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# replace last subtree with its children if they can still fit
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root.extend(subtree)
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else:
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root.append(subtree)
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assert len(root) <= n
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return root
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