aoc2020/day2/part2/main.d

module day2.part2.main;

import std;

void main()
{
    const passwords = slurp!(size_t, size_t, char, string)("input", "%s-%s %s: %s");
    passwords.findNumberOfValidPasswords.writeln;
}

alias PasswordWithPolicy = Tuple!(size_t, size_t, char, string);

ulong findNumberOfValidPasswords(const PasswordWithPolicy[] passwords)
{
    return passwords.filter!isPasswordValid.count;
}

bool isPasswordValid(const PasswordWithPolicy password)
{
    immutable pos1 = password[0] - 1;
    immutable pos2 = password[1] - 1;
    immutable character = password[2];
    immutable actualPassword = password[3];
    return (actualPassword[pos1] == character) != (actualPassword[pos2] == character);
}

unittest
{
    immutable input = [
        tuple(1UL, 3UL, 'a', "abcde"),
        tuple(1UL, 3UL, 'b', "cdefg"),
        tuple(2UL, 9UL, 'c', "ccccccccc")
    ];

    assert(input.findNumberOfValidPasswords == 1);
}