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Fix Drew's day 16 solution #19

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Fix Drew's day 16 solution #19

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9edd5cd
Switch to a two-pass system so it works in the general case
drewbrew Dec 16, 2020
5d26446
Exclude tickets with a zero value from the part two candidates
drewbrew Dec 16, 2020
bc77e98
Add clarifying comment
drewbrew Dec 16, 2020
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26 changes: 18 additions & 8 deletions Drew/drew_day16.py
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Original file line number Diff line number Diff line change
Expand Up @@ -82,7 +82,9 @@ def part_two(puzzle_input: str) -> int:
rules = puzzle["rules"]
other_tickets = puzzle["nearby tickets"]
valid_tickets = [
ticket for ticket in other_tickets if not get_invalid_value(ticket, rules)
ticket
for ticket in other_tickets
if 0 not in ticket and not get_invalid_value(ticket, rules)
]
values_by_field = []
# think of this as a very ugly means of transposing a table
Expand All @@ -94,16 +96,23 @@ def part_two(puzzle_input: str) -> int:
if all(number_in_ranges(value, field_rules) for value in field_values):
field_map[field_name].append(field_index)
definite_fields = {}
for field_name, field_index_candidates in field_map.items():
for field_name, field_index_candidates in list(field_map.items()):
if len(field_index_candidates) == 1:
definite_fields[field_name] = field_index_candidates[0]
del field_map[field_name]
result = 1
while field_map:
# NOTE this runs indefinitely on mine. Let's just keep going until we get our 6
# departure fields isolated

# what we're doing is looping through each field to see if we have isolated the candidate
# field indexes down to one value

# first, process singletons
for field_name, field_index_candidates in list(field_map.items()):
if len(field_index_candidates) == 1:
if len(field_index_candidates) == 1:
# yay we've isolated one candidate
definite_fields[field_name] = field_index_candidates[0]
del field_map[field_name]
# then do the rest
for field_name, field_index_candidates in list(field_map.items()):
if field_name in definite_fields:
# we've already matched it. Kill it.
Expand All @@ -115,9 +124,9 @@ def part_two(puzzle_input: str) -> int:
field_index_candidates.remove(value)
except ValueError:
pass
if len(field_index_candidates) == 1:
# yay we've isolated one candidate
definite_fields[field_name] = field_index_candidates[0]
if not field_index_candidates:
raise ValueError(f"No viable candidates for {field_name}")

# by some miracle, have we gotten our 6?
departure_values = list(
val for key, val in definite_fields.items() if key.startswith("departure")
Expand All @@ -128,6 +137,7 @@ def part_two(puzzle_input: str) -> int:
result *= puzzle["your ticket"][i]
return result

# we only get here with the test input
return result


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