83 lines
1.9 KiB
Nix
83 lines
1.9 KiB
Nix
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{lib, ...}: input: let
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inherit (builtins) elemAt match filter foldl';
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inherit (lib.strings) toIntBase10;
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inherit (lib) mod;
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acost = 3;
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bcost = 1;
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part2offset = 10000000000000;
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machines = input
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|> lib.trim
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|> lib.splitString "\n\n"
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|> map (text: let
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parts = text |> lib.splitString "\n";
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buttona = elemAt parts 0;
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buttonb = elemAt parts 1;
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prize = elemAt parts 2;
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in {
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a = buttonToVec buttona;
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b = buttonToVec buttonb;
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prize = prizeToVec prize;
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}
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)
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;
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buttonToVec = text: text
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|> match ''Button .: X\+([0-9]+), Y\+([0-9]+)''
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|> map toIntBase10
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|> (nums: {
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x = elemAt nums 0;
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y = elemAt nums 1;
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})
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;
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prizeToVec = text: text
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|> match ''Prize: X=([0-9]+), Y=([0-9]+)''
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|> map toIntBase10
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|> (nums: {
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x = elemAt nums 0;
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y = elemAt nums 1;
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})
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;
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getPresses = {a, b, prize}:
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let
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# see https://www.desmos.com/calculator/qfzdjjffso
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# ma = 1.0 * a.y / a.x;
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# mb = 1.0 * b.y / b.x;
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# c = prize.y - prize.x * mb;
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# x = c / (ma - mb);
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# rearranged to avoid precision errors
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x = (b.x * prize.y - prize.x * b.y) * a.x / (b.x * a.y - a.x * b.y);
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apresses = x / a.x;
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bpresses = (prize.x - x) / b.x;
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in {
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a = apresses;
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b = bpresses;
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doable = a.x * apresses + b.x * bpresses == prize.x &&
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a.y * apresses + b.y * bpresses == prize.y;
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};
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in {
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inherit machines buttonToVec prizeToVec getPresses;
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part1result = machines
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|> map getPresses
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|> filter ({doable, ...}: doable)
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|> foldl' (cost: {a,b,...}: cost + a * acost + b * bcost) 0
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;
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part2result = machines
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|> map (machine: machine // {prize = {
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x = machine.prize.x + part2offset;
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y = machine.prize.y + part2offset;
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};})
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|> map getPresses
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|> filter ({doable, ...}: doable)
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|> foldl' (cost: {a,b,...}: cost + a * acost + b * bcost) 0
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;
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}
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