48 lines
1 KiB
Elixir
48 lines
1 KiB
Elixir
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import AOC
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aoc 2021, 6 do
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use AOCHelpers
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@doc """
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Emulates the fish literally as described by the problem but
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adds new (starting at 8) fish next to their parent instead
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of at the end of list.
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"""
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def tick_literal([]), do: []
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def tick_literal([0 | xs]), do: [6, 8 | tick_literal(xs)]
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def tick_literal([x | xs]), do: [x - 1 | tick_literal(xs)]
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@doc """
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"Ticks" a day-frequency encoded list of fish counts to the next day.
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Formula for the next-day count:
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fish(8) = fish(0)
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fish(6) = fish(7) + fish(0)
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fish(n) = fish(n + 1)
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"""
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def tick_encoded([f0, f1, f2, f3, f4, f5, f6, f7, f8]) do
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# 0, 1, 2, 3, 4, 5, 6, 7, 8
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[f1, f2, f3, f4, f5, f6, f7 + f0, f8, f0]
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end
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def p1 do
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start = input_number_list()
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iterate(80, start, &tick_literal/1)
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|> length()
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end
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def p2 do
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start = input_number_list()
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freqs =
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start
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|> Enum.frequencies()
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encoded_fish = for i <- 0..8, do: Map.get(freqs, i, 0)
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iterate(256, encoded_fish, &tick_encoded/1)
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|> Enum.sum()
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end
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end
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