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aoc25/02-gift-shop/solve.py

96 lines
2.2 KiB
Python
Executable File

#!/usr/bin/env python3
import sys
from typing import Optional, Iterator
def magnitude(x : int) -> int:
assert x >= 0
return len(str(x))
def delta(order : int) -> int:
return 10**order + 1
def sum_1_to_excl(x : int) -> int:
return x * (x - 1) // 2
def sum_1_to_incl(x : int) -> int:
return x * (x + 1) // 2
# incl. both bounds
def sum_from_to_incl(lo : int, hi : int) -> int:
if lo > hi:
return 0
return sum_1_to_incl(hi) - sum_1_to_excl(lo)
def div2(x : int) -> Optional[int]:
if x % 2:
return None
else:
return x // 2
# inclusive both bounds
def sum_invalid_ids(lo : int, hi : int) -> int:
total = 0
while True:
magnitude_lo = magnitude(lo)
order_lo = div2(magnitude_lo)
if order_lo is None:
# no invalid IDs in that order of magnitude
lo = 10**magnitude_lo
continue
magnitude_hi = magnitude(hi)
d = delta(order_lo)
if magnitude_lo > magnitude_hi:
# this can happen with odd magnitude_hi
break
elif magnitude_lo == magnitude_hi:
assert lo <= hi, f"{lo} <= {hi}"
total += d * sum_from_to_incl((lo-1)//d + 1, hi // d)
break
assert magnitude_lo < magnitude_hi
surrogate_hi = 10**magnitude_lo - 1
assert lo <= surrogate_hi, f"{lo} <= {surrogate_hi}"
total += d * sum_from_to_incl((lo-1)//d + 1, surrogate_hi // d)
lo = surrogate_hi + 1
return total
def consolidate(rngs : list[tuple[int, int]]) -> Iterator[tuple[int, int]]:
rngs = sorted(rngs)
cur = None
for lo, hi in rngs:
match cur:
case None:
cur = lo, hi
case cur_lo, cur_hi:
if lo <= cur_hi:
cur = cur_lo, hi
else:
yield cur
cur = lo, hi
assert cur
yield cur
if __name__ == '__main__':
ranges = [
(int(lo_s), int(hi_s))
for lo_s, hi_s in (
rng.split('-')
for rng in sys.stdin.read().strip().split(',')
)
]
total = 0
for lo, hi in consolidate(ranges):
total += sum_invalid_ids(lo, hi)
print(total)