Solution to http://security.stackexchange.com/a/102794/1898

main.rs

// http://security.stackexchange.com/questions/102703/help-generating-wordlist-using-crunch
// I need to configure crunch so that it produces a wordlist with the following set of rules:
//
//     Exactly 10 characters long
//     Contains exactly 4 letters, which have to be a-f (all lowercase)
//     Contains exactly 6 numbers, which have to be 2-9
//     The letters and numbers can be anywhere in the word
//
// Is there any way to add a function to that program that disallows character repetition above 2?

fn all_words(prefix: &str, letters: u32, numbers: u32) {
    let prefix_length = prefix.len();
    if prefix_length >= 3 {
        // abort early is last three chars are identical
        let c1 = prefix.chars().last();
        let c2 = prefix.chars().nth(prefix_length-2);
        let c3 = prefix.chars().nth(prefix_length-3);

        if c1 == c2 && c1 == c3 {
            return;
        }
    }

    if letters == 0 && numbers == 0 {
        println!("{}", prefix);
        return;
    }

    if letters > 0 {
        for c in b'a'..b'f'+1 {
            let mut new_prefix = String::from(prefix);
            new_prefix.push(c as char);
            all_words(&new_prefix, letters-1, numbers);
        }
    }
    if numbers > 0 {
        for n in b'2'..b'9'+1 {
            let mut new_prefix = String::from(prefix);
            new_prefix.push(n as char);
            all_words(&new_prefix, letters, numbers-1);
        }
    }
}

fn main() {
    all_words("", 4, 6);
}