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+#!/usr/bin/python3
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+
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+"""Solution for day 23 of Advent of Code 2016.
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+
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+This is loosely based on day 12's solution. As instructions can now be modified (by the TGL opcode), they're kept
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+in a 'register' as well.
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+
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+For part two to work in a reasonable amount of time, try_multiply implements a look-ahead optimisation for
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+basic multiplication. Instructions of the form
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+
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+ cpy b c
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+ inc a
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+ dec c
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+ jnz c -2
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+ dec d
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+ jnz d -5
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+
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+Are actually incrementing a by b*d, and setting both c and d to 0. The optimisation supports any registers, as long
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+as they're distinct. It also only applies if the whole block of six instructions is reached naturally (i.e., execution
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+doesn't jump into the middle of it).
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+"""
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+
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+
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+def value(registers, x):
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+ try:
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+ return int(x)
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+ except ValueError:
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+ return registers[x]
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+
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+
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+def cpy(registers, src, dst):
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+ registers[dst] = value(registers, src)
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+
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+
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+def inc(registers, reg):
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+ registers[reg] += 1
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+
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+
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+def dec(registers, reg):
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+ registers[reg] -= 1
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+
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+
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+def jnz(registers, val, tar):
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+ registers['pc'] += 0 if value(registers, val) == 0 else value(registers, tar) - 1
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+
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+
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+def tgl(registers, ins):
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+ mappings = {inc: dec, dec: inc, tgl: inc, jnz: cpy, cpy: jnz}
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+ target = registers['pc'] + value(registers, ins)
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+ registers['in'][target] = (mappings[registers['in'][target][0]], registers['in'][target][1])
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+
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+
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+def try_multiply(registers):
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+ try:
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+ pc = registers['pc']
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+ chunk = registers['in'][pc:pc+6]
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+ instr, args = map(list, zip(*chunk))
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+ if instr == [cpy, inc, dec, jnz, dec, jnz] \
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+ and args[0][1] == args[2][0] == args[3][0] \
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+ and args[4][0] == args[5][0] \
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+ and args[1][0] != args[0][1] != args[4][0] != args[0][0] \
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+ and args[3][1] == '-2' and args[5][1] == '-5':
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+ registers[args[1][0]] += value(registers, args[0][0]) * value(registers, args[4][0])
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+ registers[args[2][0]] = 0
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+ registers[args[4][0]] = 0
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+ registers['pc'] += 5
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+ return True
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+ except:
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+ pass
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+ return False
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+
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+
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+def step(registers):
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+ if not try_multiply(registers):
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+ try:
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+ registers['in'][registers['pc']][0](registers, *registers['in'][registers['pc']][1])
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+ except:
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+ pass
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+
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+
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+def run(values):
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+ registers = values
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+ registers['pc'] = 0
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+ registers['in'] = instr.copy()
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+
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+ while registers['pc'] < len(instr):
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+ step(registers)
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+ registers['pc'] += 1
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+
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+ return registers['a']
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+
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+
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+with open('data/23.txt', 'r') as file:
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+ instr = map(str.split, map(str.strip, file.readlines()))
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+ instr = [(globals()[i[0]], i[1:]) for i in instr]
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+
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+print("Stage 1: %s" % run({'a': 7, 'b': 0, 'c': 0, 'd': 0}))
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+print("Stage 2: %s" % run({'a': 12, 'b': 0, 'c': 0, 'd': 0}))
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