const std = @import("std");

const PATH = "input/day25.txt";

const DENOMINATOR = 20201227;

pub fn first(allocator: ?std.mem.Allocator) anyerror!usize {
    _ = allocator;

    var lines = std.mem.tokenize(u8, @embedFile(PATH), "\n");
    const pub1 = try std.fmt.parseUnsigned(usize, lines.next().?, 10);
    const pub2 = try std.fmt.parseUnsigned(usize, lines.next().?, 10);

    var pkey1: usize = 1;
    var pkey2: usize = 1;
    var ekey1: usize = 1;
    var ekey2: usize = 1;
    while (true) {
        pkey1 = pkey1 * 7 % DENOMINATOR;
        pkey2 = pkey2 * 7 % DENOMINATOR;
        ekey1 = ekey1 * pub1 % DENOMINATOR;
        ekey2 = ekey2 * pub2 % DENOMINATOR;
        if (pkey1 == pub2) return ekey1;
        if (pkey2 == pub1) return ekey2;
    }

    unreachable;
}

test "day25a" {
    try std.testing.expectEqual(@as(usize, 16457981), try first(std.testing.allocator));
}

const test_input =
    \\5764801
    \\17807724
;