Day 21

21.py

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+#!/usr/bin/python3
+
+"""Solution for day 21 of Advent of Code 2016.
+
+Each operation has an entry in the ops dictionary, which mangles arguments and does pre-processing before handing off
+to one of the main methods to perform the actual transformation.
+
+For part two, the reverse_ops dictionary maps most operations to a reverse of their original. The only exception is
+'rotate based on position', which is just brute-forced by rotating the letters and seeing if the forward instruction
+puts them back to the expected positions.
+"""
+
+
+from functools import reduce
+
+ops = {
+    'swap position':     lambda l, a: swap_positions(l, int(a[0]), int(a[3])),
+    'swap letter':       lambda l, a: swap_letter(l, a[0], a[3]),
+    'rotate left':       lambda l, a: rotate(l, -int(a[0])),
+    'rotate right':      lambda l, a: rotate(l, int(a[0])),
+    'rotate based':      lambda l, a: rotate(l, 1 + l.index(a[4]) + (1 if l.index(a[4]) > 3 else 0)),
+    'reverse positions': lambda l, a: reverse(l, int(a[0]), int(a[2])),
+    'move position':     lambda l, a: move(l, int(a[0]), int(a[3])),
+}
+
+reverse_ops = {
+    'swap position':     lambda l, a: swap_positions(l, int(a[3]), int(a[0])),
+    'swap letter':       ops['swap letter'],
+    'rotate left':       ops['rotate right'],
+    'rotate right':      ops['rotate left'],
+    'rotate based':      lambda l, a: reverse_rotation(l, a),
+    'reverse positions': ops['reverse positions'],
+    'move position':     lambda l, a: move(l, int(a[3]), int(a[0])),
+}
+
+
+def swap_positions(letters, x, y):
+    letters[x], letters[y] = letters[y], letters[x]
+    return letters
+
+
+def swap_letter(letters, a, b):
+    return [a if l == b else b if l == a else l for l in letters]
+
+
+def rotate(letters, num):
+    num %= len(letters)
+    return letters[-num:] + letters[:-num]
+
+
+def reverse(letters, start, end):
+    return letters[:start] + letters[start:end+1][::-1] + letters[end+1:]
+
+
+def move(letters, start, end):
+    letters.insert(end, letters.pop(start))
+    return letters
+
+
+def reverse_rotation(l, a):
+    return [rotate(l, i) for i in range(len(l)) if ops['rotate based'](rotate(l, i), a) == l][0]
+
+
+with open('data/21.txt', 'r') as file:
+    instr = list(map(lambda x: [' '.join(x.split(' ')[0:2])] + x.strip().split(' ')[2:], file.readlines()))
+    print('Part one: %s' % ''.join(reduce(lambda l, o: ops[o[0]](l, o[1:]), instr, list('abcdefgh'))))
+    print('Part two: %s' % ''.join(reduce(lambda l, o: reverse_ops[o[0]](l, o[1:]), instr[::-1], list('fbgdceah'))))

data/21.txt

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+move position 2 to position 6
+move position 0 to position 5
+move position 6 to position 4
+reverse positions 3 through 7
+move position 1 to position 7
+swap position 6 with position 3
+swap letter g with letter b
+swap position 2 with position 3
+move position 4 to position 3
+move position 6 to position 3
+swap position 4 with position 1
+swap letter b with letter f
+reverse positions 3 through 4
+swap letter f with letter e
+reverse positions 2 through 7
+rotate based on position of letter h
+rotate based on position of letter a
+rotate based on position of letter e
+rotate based on position of letter h
+rotate based on position of letter c
+move position 5 to position 7
+swap letter a with letter d
+move position 5 to position 6
+swap position 4 with position 0
+swap position 4 with position 6
+rotate left 6 steps
+rotate right 4 steps
+rotate right 5 steps
+swap letter f with letter e
+swap position 2 with position 7
+rotate based on position of letter e
+move position 4 to position 5
+swap position 4 with position 2
+rotate right 1 step
+swap letter b with letter f
+rotate based on position of letter b
+reverse positions 3 through 5
+move position 3 to position 1
+rotate based on position of letter g
+swap letter c with letter e
+swap position 7 with position 3
+move position 0 to position 3
+rotate right 6 steps
+reverse positions 1 through 3
+swap letter d with letter e
+reverse positions 3 through 5
+move position 0 to position 3
+swap letter c with letter e
+move position 2 to position 7
+swap letter g with letter b
+rotate right 0 steps
+reverse positions 1 through 3
+swap letter h with letter d
+move position 4 to position 0
+move position 6 to position 3
+swap letter a with letter c
+reverse positions 3 through 6
+swap letter h with letter g
+move position 7 to position 2
+rotate based on position of letter h
+swap letter b with letter h
+reverse positions 2 through 6
+move position 6 to position 7
+rotate based on position of letter a
+rotate right 7 steps
+reverse positions 1 through 6
+move position 1 to position 6
+rotate based on position of letter g
+rotate based on position of letter d
+move position 0 to position 4
+rotate based on position of letter e
+rotate based on position of letter d
+rotate based on position of letter a
+rotate based on position of letter a
+rotate right 4 steps
+rotate based on position of letter b
+reverse positions 0 through 4
+move position 1 to position 7
+rotate based on position of letter e
+move position 1 to position 7
+swap letter f with letter h
+move position 5 to position 1
+rotate based on position of letter f
+reverse positions 0 through 1
+move position 2 to position 4
+rotate based on position of letter a
+swap letter b with letter d
+move position 6 to position 0
+swap letter e with letter b
+rotate right 7 steps
+move position 2 to position 7
+rotate left 4 steps
+swap position 6 with position 1
+move position 3 to position 5
+rotate right 7 steps
+reverse positions 0 through 6
+swap position 2 with position 1
+reverse positions 4 through 6
+rotate based on position of letter g
+move position 6 to position 4