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