My Advent of Code solutions in Python.
kevinyap.ca/2019/12/going-fast-in-advent-of-code/
advent-of-code
python
Add 2021/18
+157
+157
2021/day18.py
+157
2021/day18.py
···
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import fileinput
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from itertools import permutations
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class Snail:
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def __init__(self, val, depth):
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self.val = val
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self.depth = depth
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def __repr__(self):
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return 'S({} @ {})'.format(self.val, self.depth)
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def process_snailnum(s):
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"""Turns a written snailfish number into a List[Snail]."""
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depth = -1
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snails = []
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for c in s:
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if c == '[':
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depth += 1
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elif c == ']':
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depth -= 1
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elif '0' <= c <= '9':
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snails.append(Snail(int(c), depth))
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return snails
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def reduce_num(snails):
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"""
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Reduces a snailfish number by iteratively going through
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it until no more explosions or splits can be performed.
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"""
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while True:
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did_explode = did_split = False
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i = 0
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while i < len(snails):
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s = snails[i]
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# Prefer explode
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if s.depth >= 4:
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# print "ex",
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l = snails[i]
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n = snails[i+1]
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# Process left explosion
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leftmost = False
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if i == 0:
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snails[i] = Snail(0, s.depth - 1)
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leftmost = True
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else:
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p = snails[i-1]
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snails[i-1] = Snail(p.val + l.val, p.depth)
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# Process right explosion
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rightmost = False
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if i+1 == len(snails) - 1:
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snails[i+1] = Snail(0, n.depth - 1)
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rightmost = True
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else:
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nn = snails[i+2]
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snails[i+2] = Snail(n.val + nn.val, nn.depth)
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if leftmost:
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del snails[1]
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elif rightmost:
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del snails[-2]
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else:
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del snails[i+1]
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snails[i] = Snail(0, l.depth - 1)
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did_explode = True
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break
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i += 1
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if not did_explode:
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i = 0
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while i < len(snails):
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s = snails[i]
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if s.val >= 10:
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a = s.val // 2
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b = (s.val + 1) // 2
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assert a + b == s.val
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snails = snails[:i] + [Snail(a, s.depth + 1), Snail(b, s.depth + 1)] + snails[i+1:]
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did_split = True
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break
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i += 1
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if not did_explode and not did_split:
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break
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return snails
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def add_snails(la, lb):
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"""Adds two snailfish numbers together."""
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res = []
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for x in la:
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res.append(Snail(x.val, x.depth + 1))
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for x in lb:
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res.append(Snail(x.val, x.depth + 1))
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res = reduce_num(res)
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return res
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def magnitude(snails):
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"""Returns the magnitude of a snailnum (iterative)."""
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while True:
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max_depth = 0
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i = 0
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for i in range(len(snails)):
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s = snails[i]
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max_depth = max(max_depth, s.depth)
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# Nothing more to reduce
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if max_depth == 0:
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break
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# Else, search and expand the leftmost pair
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expanded = False
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for i in range(len(snails)):
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s = snails[i]
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if s.depth == max_depth:
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n = snails[i+1]
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assert s.depth == n.depth
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ns = Snail(s.val * 3 + n.val * 2, s.depth - 1)
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snails = snails[:i] + [ns] + snails[i+2:]
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expanded = True
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break
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return snails[0].val * 3 + snails[1].val * 2
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# Parse problem input.
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SNAILS = [process_snailnum(l.strip()) for l in fileinput.input()]
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# Solve part 1.
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part_1_snails = reduce_num(SNAILS[0])
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for s in SNAILS[1:]:
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part_1_snails = add_snails(part_1_snails, s)
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print "Part 1:", magnitude(part_1_snails)
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# Solve part 2.
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print "Part 2:", max(magnitude(add_snails(a, b)) for a, b in permutations(SNAILS, 2))