FINALLY GOT IT
defmodule Day14Part2Only do
use Bitwise
@moduledoc false
@input File.read!("lib/input")
defmodule MyParser do
import NimbleParsec
mask =
ignore(string("mask = "))
|> ascii_string([?1, ?0, ?X], 36)
|> ignore(string("\n"))
mem =
ignore(string("mem["))
|> integer(min: 1)
|> ignore(string("] = "))
|> integer(min: 1)
|> ignore(string("\n"))
|> reduce({List, :to_tuple, []})
group =
mask
|> repeat(mem)
|> reduce({List, :wrap, []})
defparsec(:nimble_parse, repeat(group))
end
def nimble_parse do
MyParser.nimble_parse(@input)
|> elem(1)
|> Enum.map(fn [mask | assignments] -> {mask, assignments} end)
end
def initialize(version \\ 1) do
nimble_parse()
|> build_mem_map(version)
end
def build_mem_map(processed_lines, version \\ 1) do
processed_lines
|> Enum.reduce(%{}, fn {mask, assignments}, memory ->
assign_memory(mask, assignments, memory, version)
end)
end
def assign_memory(mask, assignments, memory, 2 = version) do
assignments
|> Enum.map(&apply_mask(&1, mask, version))
|> Enum.reduce(memory, fn {addresses, val}, acc ->
update_memory({addresses, val}, acc, version)
end)
end
def apply_mask({address, value}, mask, 2) do
{address
|> Integer.to_string(2)
|> String.pad_leading(36, "0")
|> String.codepoints()
|> Enum.zip(String.codepoints(mask))
|> Enum.reduce([0], fn {a, b}, acc ->
case b do
"0" -> Enum.map(acc, fn addr -> addr * 2 + String.to_integer(a) end)
"1" -> Enum.map(acc, fn addr -> addr * 2 + 1 end)
"X" -> fork(acc)
end
end), value}
end
def fork(acc) do
acc
|> Enum.flat_map(fn addr -> [addr * 2, addr * 2 + 1] end)
end
def update_memory({addresses, val}, memory, 2) do
addresses
|> Enum.reduce(memory, fn address, acc ->
Map.put(acc, address, val)
end)
end
def run2 do
initialize(2)
|> sum_memory()
|> IO.puts()
end
def sum_memory(mem_map) do
mem_map
|> Map.values()
|> Enum.sum()
end
end
Day14Part2Only.run2()
I’ve done a half dozen approaches to the branching/flat mapping portion of applying the mask, but in the end there was something squirrelly about how I was processing the input. Once I broke down and used Nimble Parsec it just worked. UGGGGH.