use rand::prelude::*;

enum Tok {
    Roll(u32),
    Flat(u32),
}

use Tok::*;

/// Rolls dice corresponding to a ttrpg dice specifier
/// ```
/// assert_eq!(roll("4d6 + 1d4 + 1").1.len(), 2)
/// ```
pub fn roll(spec: &str) -> (u32, Vec<u32>) {
    let mut toks = spec.split_whitespace();
    let mut rolls = vec![];
    let mut total = 0;
    
    let first = toks.next().unwrap();
    match parse_tok(first) {
	Roll(v) => {
	    total += v;
	    rolls.push(v);
	}
	Flat(v) => total += v,
    };
    let toks : Vec<&str> = toks.collect();
    let mut toks = toks.chunks(2);
    while let Some(&[op, tok]) = toks.next() {
	let val = match parse_tok(tok) {
	    Roll(v) => {
		rolls.push(v);
		v
	    }
	    Flat(v) => v,
	};
	if op == "-" {
	    total -= val;
	} else {
	    total += val;
	}
    }
    (total, rolls)
}

fn parse_tok(tok: &str) -> Tok {
    let mut rng = thread_rng();
    match *tok.split('d')
        .map(|x| x.parse().unwrap())
	.collect::<Vec<_>>()
	.as_slice() {
	[v] => Flat(v),
	[n, d] => {
	    let mut total = 0;
	    for _ in 0..n {
		total += rng.gen_range(1..=d)
	    }
	    Roll(total)
	}
	_ => panic!()
    }
}