#![feature(assert_matches)]
#![allow(dead_code)] //TODO eventually turn this off

type ParseResult<I, O, E> = Result<(O, I), E>;

trait Parser<I, O, E> {
    fn parse(&self, input: I) -> ParseResult<I, O, E>;
}

impl<I, O, E, F> Parser<I, O, E> for F
where
    F: Fn(I) -> ParseResult<I, O, E>,
{
    fn parse(&self, input: I) -> ParseResult<I, O, E> {
        self(input)
    }
}

fn literal(expected: &'static str) -> impl Fn(&str) -> ParseResult<&str, &str, &str> {
    move |input| match input.get(0..expected.len()) {
        Some(next) if next == expected => Ok((expected, &input[expected.len()..])),
        _ => Err(input),
    }
}

fn map<P, F, I, O1, O2, E>(parser: P, map_fn: F) -> impl Parser<I, O2, E>
where
    P: Parser<I, O1, E>,
    F: Fn(O1) -> O2,
{
    move |input| {
        parser
            .parse(input)
            .map(|(result, rest)| (map_fn(result), rest))
    }
}

fn seq<P1, P2, I, O1, O2, E>(parser1: P1, parser2: P2) -> impl Parser<I, (O1, O2), E>
where
    P1: Parser<I, O1, E>,
    P2: Parser<I, O2, E>,
{
    move |input| {
        parser1.parse(input).and_then(|(result1, rest1)| {
            parser2
                .parse(rest1)
                .map(|(result2, rest2)| ((result1, result2), rest2))
        })
    }
}

/// Parses a standard identifier in a programming language
fn identifier(input: &str) -> ParseResult<&str, String, &str> {
    let mut chars = input.chars();
    let mut buf = String::new();

    match chars.next() {
        Some(ch) if ch.is_alphabetic() => buf.push(ch),
        _ => return Err(input),
    }

    while let Some(next) = chars.next() {
        if next.is_alphanumeric() {
            buf.push(next);
        } else {
            break;
        }
    }

    let next_index = buf.len();
    Ok((buf, &input[next_index..]))
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::assert_matches::assert_matches;

    #[test]
    fn test_parsing() {
        let output = literal("a")("a yolo");
        assert_matches!(output.unwrap(), ("a", " yolo"));
    }

    #[test]
    fn test_identifier() {
        assert_matches!(identifier("bongo1beans"), Ok((s, "")) if s == "bongo1beans");
        assert_matches!(identifier("2bongo1beans"), Err("2bongo1beans"));
    }

    #[test]
    fn test_map() {
        let lit_a = literal("a");
        let output = map(lit_a, |s| s.to_uppercase()).parse("a yolo");
        assert_matches!(output.unwrap(), (s, " yolo") if s == "A");
    }

    #[test]
    fn test_seq() {
        let p = seq(identifier, seq(literal(" "), literal("ruts")));
        assert_matches!(p.parse("fort1 ruts"), Ok((r, "")) if r.0 == "fort1" && r.1 == (" ", "ruts") );
    }
}