Interview prep

atoi.rs

#[derive(PartialEq)]
pub enum State {
    BeforePrefixOp,
    DigitStart,
}

fn convert_to_int(s: &Vec<u8>) -> i32 {
    if s.is_empty() {
        return 0;
    }
    let mut mult = 10_i32.pow((s.len() - 1) as u32);
    let mut result = 0;
    for b in s.iter() {
        let digit = (*b - b'0') as i32;
        result += digit * mult;
        mult /= 10;
    }
    
    result
}

pub fn atoi(s: String) -> i32 {
    let mut num_string = Vec::with_capacity(s.len());
    let mut neg = false;
    let mut state = State::BeforePrefixOp;
    
    for b in s.bytes() {
        match b {
            b'+' if state == State::BeforePrefixOp => {
                state = State::DigitStart;
            }
            b'-' if state == State::BeforePrefixOp => {
                neg = true;
                state = State::DigitStart;
            }
            b' ' if state == State::BeforePrefixOp => {}
            ch => {
                if ch <= b'9' && ch >= b'0' {
                    num_string.push(ch);
                    state = State::DigitStart;
                } else if state == State::BeforePrefixOp {
                    return 0;
                } else {
                    break;
                }
            }
        }
    }
    
    let num = convert_to_int(&num_string);
    if neg { -num } else { num }
}

fn main() {
    println!("Atoi: {}", atoi("123".to_string()));
    println!("Atoi: {}", atoi("    123".to_string()));
    println!("Atoi: {}", atoi("    -123".to_string()));
    println!("Atoi: {}", atoi("     -123bb".to_string()));
    println!("Atoi: {}", atoi("     +123bb".to_string()));
    println!("Atoi: {}", atoi("       ".to_string()));
    println!("Atoi: {}", atoi("abcde".to_string()));
}