#[derive(PartialEq)] enum Token<'a> { Number(i32), NonNumber(&'a str), } fn apply_op<'a, 'b>(num1: i32, words: &'a [Token<'b>]) -> Option<(i32, &'a [Token<'b>])> { let number_pos = words.iter().position(|w| match w { Token::Number(_) => true, Token::NonNumber(_) => false, })?; let (op_and_num, mut remainder) = words.split_at(number_pos + 1); let (op, num2) = op_and_num.split_at(number_pos); let &num2 = match num2 { [Token::Number(i)] => i, _ => unreachable!( "We split at a Number above, so num2 is surely a single-element slice w/ a number" ), }; match op { [Token::NonNumber("plus")] => Some(num1 + num2), [Token::NonNumber("minus")] => Some(num1 - num2), [Token::NonNumber("multiplied"), Token::NonNumber("by")] => Some(num1 * num2), [Token::NonNumber("divided"), Token::NonNumber("by")] => Some(num1 / num2), [ Token::NonNumber("raised"), Token::NonNumber("to"), Token::NonNumber("the"), ] => { if Some(&Token::NonNumber("power")) == remainder.first() { remainder = remainder.get(1..)?; Some(num1.pow(num2 as u32)) } else { None } } _ => None, } .map(|n| (n, remainder)) } pub fn answer(c: &str) -> Option { let words = c .trim_end_matches('?') .split_whitespace() .map(|word| { if let Ok(i) = word.parse::() { Token::Number(i) } else if let Some(i) = word .get(..word.len() - 2) .and_then(|word| word.parse::().ok()) { Token::Number(i) } else { Token::NonNumber(word) } }) .collect::>(); if words.len() < 3 { return None; } let mut result: i32 = match words[0..3] { [ Token::NonNumber("What"), Token::NonNumber("is"), Token::Number(i), ] => i, _ => return None, }; let mut words = words.split_at(3).1; while !words.is_empty() { let tmp = apply_op(result, words)?; result = tmp.0; words = tmp.1; } Some(result) }