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defmodule Day2 do
def checksum(ids) do
{two_count, three_count} =
ids
|> String.split("\n", trim: true)
|> Enum.reduce({0, 0}, fn id, {two_count, three_count} ->
char_counts =
id
|> String.graphemes()
|> Enum.group_by(& &1)
|> Enum.map(fn {_c, vals} -> Enum.count(vals) end)
|> Enum.uniq()
{incr_if_count(char_counts, two_count, 2), incr_if_count(char_counts, three_count, 3)}
end)
two_count * three_count
end
defp incr_if_count(counts, prev_count, matcher) do
if matcher in counts do
prev_count + 1
else
prev_count
end
end
def find_common_boxes(ids) do
ids =
ids
|> String.split("\n", trim: true)
|> Enum.map(&String.graphemes/1)
{lhs, rhs} = find_similar(ids)
intersection(lhs, rhs)
end
defp intersection(lhs, rhs) do
Enum.zip(lhs, rhs)
|> Enum.map(fn {l, l} -> l; _ -> nil end)
|> Enum.filter(& &1)
|> Enum.join
end
defp find_similar([lhs_id | ids]) do
matching_id =
Enum.find(ids, fn rhs_id ->
diff_count(lhs_id, rhs_id) == 1
end)
if matching_id do
{lhs_id, matching_id}
else
find_similar(ids)
end
end
defp diff_count(lhs, rhs) do
Enum.zip(lhs, rhs)
|> Enum.map(fn
{x, x} -> 0
_ -> 1
end)
|> Enum.sum()
end
end
ExUnit.start()
defmodule Day2Test do
use ExUnit.Case
import Day2
test "#checksum" do
assert checksum("""
abcdef
bababc
abbcde
abcccd
aabcdd
abcdee
ababab
""") == 12
assert File.read!("input.txt") |> checksum == 4693
end
test "#find_common_boxes" do
assert find_common_boxes("""
abcde
fghij
klmno
pqrst
fguij
axcye
wvxyz
""") == "fgij"
assert File.read!("input.txt") |> find_common_boxes == "pebjqsalrdnckzfihvtxysomg"
end
end
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