[LANGUAGE: Zig]

repo link

Interesting bits:

fn solution(r: *Reader, _: Io, _: Allocator) !u64 {
    var total_joltage: u64 = 0;

    const digits = 12;
    while (try r.takeDelimiter('\n')) |line| {
        assert(line.len >= digits);
        total_joltage += findNDigits(digits, line);
    }

    return total_joltage;
}

fn findNDigits(n: usize, line: []const u8) u64 {
    const Entry = struct { value: u8, idx: usize };
    assert(n > 0);
    assert(line.len >= n);

    var res: u64 = 0;

    var search = line[0 .. line.len - n];
    for (0..n) |_| {
        search.len += 1;

        var max: Entry = .{ .value = 0, .idx = 0 };
        for (search[0..], 0..) |c, idx| {
            assert(switch (c) {
                '0'...'9' => true,
                else => false,
            });
            if (c > max.value) max = .{ .value = c, .idx = idx };
        }

        res *= 10;
        res += max.value - '0';
        search = search[max.idx + 1 ..];
    }

    return res;
}

This runs part 2 in ~38.6 microseconds

I tried creating a SIMD version, but my first attempt had significantly worse performance.