88 lines
1.9 KiB
Elixir
88 lines
1.9 KiB
Elixir
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defmodule Oasis do
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@moduledoc """
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Day 9 of Advent of Code 2023.
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Link: https://adventofcode.com/2023/day/9.
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"""
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def solve() do
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input =
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File.stream!("input.txt")
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|> Stream.map(&String.trim_trailing/1)
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# Get lists of strings.
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|> Stream.map(&String.split/1)
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# Get lists of integers.
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|> Stream.map(fn list -> Enum.map(list, &String.to_integer/1) end)
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# Predict next value for each list.
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input
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|> Stream.map(fn list -> predict(list, &predict_next_value/2) end)
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|> Enum.sum()
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|> IO.puts()
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# Predict previous value for each list.
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input
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|> Stream.map(fn list -> predict(list, &predict_prev_value/2) end)
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|> Enum.sum()
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|> IO.puts()
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end
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@doc """
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Predict a value for the list.
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Predictor_fun is a function that takes current row and previous prediction result and returns new prediction.
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Examples:
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iex> Oasis.predict([0, 3, 6, 9, 12, 15], &Oasis.predict_next_value/2)
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18
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iex> Oasis.predict([1, 3, 6, 10, 15, 21], &Oasis.predict_next_value/2)
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28
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iex> Oasis.predict([10, 13, 16, 21, 30, 45], &Oasis.predict_prev_value/2)
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5
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"""
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def predict(list, predictor_fun) do
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diff = get_diff(list)
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if zeros_only?(diff) do
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predictor_fun.(list, 0)
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else
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predictor_fun.(list, predict(diff, predictor_fun))
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end
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end
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def predict_next_value(list, prev_result) do
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Enum.at(list, -1) + prev_result
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end
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def predict_prev_value(list, prev_result) do
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hd(list) - prev_result
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end
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@doc """
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Get a list of differences between values.
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Examples:
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iex> Oasis.get_diff([1, 3, 6, 10, 15, 21])
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[2, 3, 4, 5, 6]
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"""
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def get_diff([head | tail]) do
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case tail do
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[] -> []
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[val | _] -> [val - head | get_diff(tail)]
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end
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end
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def zeros_only?(list) do
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case list do
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[] ->
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true
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[head | tail] ->
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if head != 0, do: false, else: zeros_only?(tail)
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end
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end
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end
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