70 lines
1.7 KiB
Python
70 lines
1.7 KiB
Python
verbose = False
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def indices(lst, element):
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"""
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See <https://stackoverflow.com/questions/6294179/how-to-find-all-occurrences-of-an-element-in-a-list>.
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"""
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result = []
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offset = -1
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while True:
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try:
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offset = lst.index(element, offset+1)
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except ValueError:
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return result
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result.append(offset)
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def load_input(filename="input"):
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with open(filename, "r") as fp:
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lines = fp.read().splitlines()
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# Find the empty line/separator.
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sep = lines.index("")
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# Handle rules
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rule_lines = lines[:sep]
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rules = [tuple(map(int, l.split("|"))) for l in rule_lines]
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# Handle lines
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update_lines = lines[sep + 1:]
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updates = [list(map(int, l.split(","))) for l in update_lines]
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return rules, updates
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def is_update_legal(update, rules, print_offence=False) -> bool:
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for rule in rules:
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# Find indices of left hand side and right hand side
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lhs, rhs = rule
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if not lhs in update or not rhs in update:
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continue
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# This reminds me of limes inferior and limes superior.
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# Thanks, Mueger.
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if max(indices(update, lhs)) > min(indices(update, rhs)):
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if print_offence:
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print(f"Rule", rule, "offended by update", update)
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return False
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return True
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def middle_page_num(update) -> int:
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return update[len(update) // 2]
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if __name__ == "__main__":
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rules, updates = load_input()
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indices_legal = []
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middle_sum = 0
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for idx, update in enumerate(updates):
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if is_update_legal(update, rules):
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if verbose:
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print(update, " is legal")
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indices_legal.append(idx)
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middle_sum += middle_page_num(update)
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else:
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if verbose:
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print(update, " is illegal")
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if verbose:
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print(indices_legal)
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print(f"The sum of middle page numbers of legal updates is {middle_sum}.")
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