Day 3 we wrote another hand written parser becaus regexes suck. It uses the match context which you can tell by compiling with the flag:
# ERL_COMPILER_OPTIONS=bin_opt_info mix compile --force
It looks more verbose because of this, but essentially we keep an index of where in the string we are and use that to then extract data.
Part 1
def day_3_1() do
"./day_3_input.txt"
|> File.read!()
|> parse(1, [], 0)
end
def parse(<<>>, _, _, total), do: total
@mul_start "mul("
@comma ","
@mul_end ")"
# If we see a mul start but the stack is not empty then it can't be valid.
def parse(<<@mul_start, rest::binary>>, current_index, [], total) do
new_current_index = current_index + 3
end_index = parse_number(rest, new_current_index)
if end_index == new_current_index do
parse(rest, end_index, [], total)
else
<<number::binary-size(end_index - new_current_index), rest::binary>> = rest
first_int = String.to_integer(number)
parse(rest, end_index, [first_int], total)
end
end
def parse(<<@comma, rest::binary>>, current_index, [first_int], total) do
new_current_index = current_index + 1
end_index = parse_number(rest, new_current_index)
if end_index == new_current_index do
parse(rest, current_index, [], total)
else
<<number::binary-size(end_index - new_current_index), rest::binary>> = rest
second_int = String.to_integer(number)
parse(rest, end_index, [second_int, first_int], total)
end
end
def parse(<<@mul_end, rest::binary>>, current_index, [first, second], total) do
parse(rest, current_index + 1, [], first * second + total)
end
# We reset the stack if we had started a mult that never happened.
def parse(<<_::binary-size(1), rest::binary>>, index, _stack, total) do
parse(rest, index + 1, [], total)
end
@all_digits ~c"0123456789"
for digit <- @all_digits do
def parse_number(<<unquote(digit), rest::bits>>, end_index) do
parse_number(rest, end_index + 1)
end
end
def parse_number(_rest, end_index), do: end_index
Part 2
Part two just adds a couple more functions to the mix, but repeated myself so part 1 still works on its own.
def day_3_2() do
"./day_3_input.txt"
|> File.read!()
|> parse_instructions(1, [], 0)
end
def parse_instructions(<<>>, _, _, total), do: total
@doo "do()"
@dont "don't()"
def parse_instructions(<<@dont, rest::binary>>, current_index, _, total) do
parse_instructions(rest, current_index + 6, [:dont], total)
end
def parse_instructions(<<@doo, rest::binary>>, current_index, [:dont], total) do
parse_instructions(rest, current_index + 3, [], total)
end
# Now if the stack has :dont in it we skip.
def parse_instructions(<<_::binary-size(1), rest::binary>>, current_index, [:dont], total) do
parse_instructions(rest, current_index + 1, [:dont], total)
end
def parse_instructions(<<@mul_start, rest::binary>>, current_index, [], total) do
new_current_index = current_index + 3
end_index = parse_number(rest, new_current_index)
if end_index == new_current_index do
parse_instructions(rest, end_index, [], total)
else
<<number::binary-size(end_index - new_current_index), rest::binary>> = rest
first_int = String.to_integer(number)
parse_instructions(rest, end_index, [first_int], total)
end
end
def parse_instructions(<<@comma, rest::binary>>, current_index, [first_int], total) do
new_current_index = current_index + 1
end_index = parse_number(rest, new_current_index)
if end_index == new_current_index do
parse_instructions(rest, current_index, [], total)
else
<<number::binary-size(end_index - new_current_index), rest::binary>> = rest
second_int = String.to_integer(number)
parse_instructions(rest, end_index, [second_int, first_int], total)
end
end
def parse_instructions(<<@mul_end, rest::binary>>, current_index, [first, second], total) do
parse_instructions(rest, current_index + 1, [], first * second + total)
end
def parse_instructions(<<_::binary-size(1), rest::binary>>, index, _stack, total) do
parse_instructions(rest, index + 1, [], total)
end
