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day14.py
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day14.py
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#!/usr/bin/env python3
"""
--- Day 14: Docking Data ---
As your ferry approaches the sea port, the captain asks for your help again. The
computer system that runs this port isn't compatible with the docking program on
the ferry, so the docking parameters aren't being correctly initialized in the
docking program's memory.
After a brief inspection, you discover that the sea port's computer system uses
a strange bitmask system in its initialization program. Although you don't have
the correct decoder chip handy, you can emulate it in software!
The initialization program (your puzzle input) can either update the bitmask or
write a value to memory. Values and memory addresses are both 36-bit unsigned
integers. For example, ignoring bitmasks for a moment, a line like mem[8] = 11
would write the value 11 to memory address 8.
The bitmask is always given as a string of 36 bits, written with the most
significant bit (representing 2^35) on the left and the least significant bit
(2^0, that is, the 1s bit) on the right. The current bitmask is applied to
values immediately before they are written to memory: a 0 or 1 overwrites the
corresponding bit in the value, while an X leaves the bit in the value
unchanged.
For example, consider the following program:
mask = XXXXXXXXXXXXXXXXXXXXXXXXXXXXX1XXXX0X
mem[8] = 11
mem[7] = 101
mem[8] = 0
This program starts by specifying a bitmask (mask = ....). The mask it specifies
will overwrite two bits in every written value: the 2s bit is overwritten with
0, and the 64s bit is overwritten with 1.
The program then attempts to write the value 11 to memory address 8. By
expanding everything out to individual bits, the mask is applied as follows:
value: 000000000000000000000000000000001011 (decimal 11)
mask: XXXXXXXXXXXXXXXXXXXXXXXXXXXXX1XXXX0X
result: 000000000000000000000000000001001001 (decimal 73)
So, because of the mask, the value 73 is written to memory address 8 instead.
Then, the program tries to write 101 to address 7:
value: 000000000000000000000000000001100101 (decimal 101)
mask: XXXXXXXXXXXXXXXXXXXXXXXXXXXXX1XXXX0X
result: 000000000000000000000000000001100101 (decimal 101)
This time, the mask has no effect, as the bits it overwrote were already the
values the mask tried to set. Finally, the program tries to write 0 to address
8:
value: 000000000000000000000000000000000000 (decimal 0)
mask: XXXXXXXXXXXXXXXXXXXXXXXXXXXXX1XXXX0X
result: 000000000000000000000000000001000000 (decimal 64)
64 is written to address 8 instead, overwriting the value that was there
previously.
To initialize your ferry's docking program, you need the sum of all values left
in memory after the initialization program completes. (The entire 36-bit address
space begins initialized to the value 0 at every address.) In the above example,
only two values in memory are not zero - 101 (at address 7) and 64 (at address
8) - producing a sum of 165.
Execute the initialization program. What is the sum of all values left in memory
after it completes?
--- Part Two ---
For some reason, the sea port's computer system still can't communicate with
your ferry's docking program. It must be using version 2 of the decoder chip!
A version 2 decoder chip doesn't modify the values being written at all.
Instead, it acts as a memory address decoder. Immediately before a value is
written to memory, each bit in the bitmask modifies the corresponding bit of the
destination memory address in the following way:
If the bitmask bit is 0, the corresponding memory address bit is unchanged.
If the bitmask bit is 1, the corresponding memory address bit is overwritten with 1.
If the bitmask bit is X, the corresponding memory address bit is floating.
A floating bit is not connected to anything and instead fluctuates
unpredictably. In practice, this means the floating bits will take on all
possible values, potentially causing many memory addresses to be written all at
once!
For example, consider the following program:
mask = 000000000000000000000000000000X1001X
mem[42] = 100
mask = 00000000000000000000000000000000X0XX
mem[26] = 1
When this program goes to write to memory address 42, it first applies the
bitmask:
address: 000000000000000000000000000000101010 (decimal 42)
mask: 000000000000000000000000000000X1001X
result: 000000000000000000000000000000X1101X
After applying the mask, four bits are overwritten, three of which are
different, and two of which are floating. Floating bits take on every possible
combination of values; with two floating bits, four actual memory addresses are
written:
000000000000000000000000000000011010 (decimal 26)
000000000000000000000000000000011011 (decimal 27)
000000000000000000000000000000111010 (decimal 58)
000000000000000000000000000000111011 (decimal 59)
Next, the program is about to write to memory address 26 with a different
bitmask:
address: 000000000000000000000000000000011010 (decimal 26)
mask: 00000000000000000000000000000000X0XX
result: 00000000000000000000000000000001X0XX
This results in an address with three floating bits, causing writes to eight
memory addresses:
000000000000000000000000000000010000 (decimal 16)
000000000000000000000000000000010001 (decimal 17)
000000000000000000000000000000010010 (decimal 18)
000000000000000000000000000000010011 (decimal 19)
000000000000000000000000000000011000 (decimal 24)
000000000000000000000000000000011001 (decimal 25)
000000000000000000000000000000011010 (decimal 26)
000000000000000000000000000000011011 (decimal 27)
The entire 36-bit address space still begins initialized to the value 0 at every
address, and you still need the sum of all values left in memory at the end of
the program. In this example, the sum is 208.
Execute the initialization program using an emulator for a version 2 decoder
chip. What is the sum of all values left in memory after it completes?
"""
import argparse
from collections import defaultdict
from itertools import product
parser = argparse.ArgumentParser(epilog=__doc__,
formatter_class=argparse.RawTextHelpFormatter)
parser.add_argument('input', type=argparse.FileType('rt'))
parser.add_argument('--part-two', action='store_true')
args = parser.parse_args()
mask_and = 0
mask_or = 0
memory = defaultdict(lambda: 0)
for line in args.input:
op, arg = line.strip().split(' = ')
if op == 'mask':
# mask = 00000X110010111111X000100XX01010000X
mask = arg
mask_and = int(mask.replace('X', '1'), 2)
mask_or = int(mask.replace('X', '0'), 2)
else:
# mem[13197]
address = int(op[4:-1])
value = int(arg)
if not args.part_two:
memory[address] = (value | mask_or) & mask_and
else:
# ignoring the Xs for now, the described operation is OR
address |= mask_or
floating_bits = mask.count('X')
bits = ('0', '1')
floating_bit_combinations = product(*([bits] * floating_bits))
# in Python you cannot replace a character by index in a string
def str_replace(s, repl, pos): return s[:pos] + repl + s[pos+1:]
for combination in floating_bit_combinations:
address_bits = f'{address:036b}'
start = 0
for bit in combination:
# find positions of X in mask
position = mask.index('X', start)
start = position + 1
# hardcode bit in address
address_bits = str_replace(address_bits, bit, position)
memory[address_bits] = value
print(sum(memory.values()))