number_to_string.rs

const DEMO_MODE : bool = false;

fn get_digit(num: u32, index: u32) -> u8 {
    (num / 10u32.pow(index) % 10) as u8
}

/* Returns true if a number was emitted */
fn emit_units_tens_hundreds(num: u32, str: &mut String) -> bool {
    let units_names = ["", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"];
    let tens_names = ["", "ten", "twenty", "thirty", "fourty", "fifty", "sixty", "seventy", "eighty", "ninety"];
    let tens_single_names = ["", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"];

    let units = get_digit(num, 0) as usize;
    let tens = get_digit(num, 1) as usize;
    let hundreds = get_digit(num, 2) as usize;

    let has_units = units > 0;
    let has_tens = tens > 0;
    let has_hundreds = hundreds > 0;

    let is_teens = has_units && tens == 1;

    if has_hundreds {
        *str += format!("{} hundred ", units_names[hundreds]).as_str();
    }
    if has_tens {
        if is_teens {
            *str += format!("{} ", tens_single_names[units]).as_str();
        } else {
            *str += format!("{}{}", tens_names[tens], if has_units {"-"}  else {" "}).as_str();
        }
    }
    if has_units & !is_teens {
        *str += format!("{} ", units_names[units]).as_str();
    }

    return has_units || has_tens || has_hundreds;
}

fn main() {
    let input_number;

    if DEMO_MODE {
        input_number = 1234567890;
    } else {
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).expect("Can't read line");
        input_number = line.trim().parse::<u32>().expect("Invalid number entered");
    }

    let mut number_str = String::new();

    if input_number != 0 {
        // Digits 7 to 9
        let has_billions = emit_units_tens_hundreds(input_number / (1000 * 1000 * 1000), &mut number_str);
        if has_billions {
            number_str += "billion ";
        }

        // Digits 7 to 9
        let has_millions = emit_units_tens_hundreds(input_number / (1000 * 1000), &mut number_str);
        if has_millions {
            number_str += "million ";
        }

        // Digits 4 to 6
        let has_thousands = emit_units_tens_hundreds(input_number / 1000, &mut number_str);
        if has_thousands {
            number_str += "thousand ";
        }

        // Digits 1 to 3
        emit_units_tens_hundreds(input_number, &mut number_str);
    } else {
        number_str += "zero";
    }

    println!("{}: {}", input_number, number_str);
}