aoc2020/day2/part1/main.d

import std;

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

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

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

bool isPasswordValid(PasswordWithPolicy password)
{
    immutable numberOfRequiredLetter = password[3].byChar.filter!(c => c == password[2]).count;
    return numberOfRequiredLetter >= password[0] && numberOfRequiredLetter <= password[1];
}

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

    assert(input.findNumberOfValidPasswords == 2);
}