use crate::lexer::Lexer;
use crate::num;
use crate::num::Num;
use num_bigint::BigInt;
use std::str::FromStr;

enum ParseError {
    Empty,
    Consumed,
}

fn seq<T>(r: Result<T, ParseError>) -> Result<T, ParseError> {
    match r {
        Err(ParseError::Empty) => Err(ParseError::Consumed),
        x => x,
    }
}

fn or<T>(r: Result<T, ParseError>) -> Result<Option<T>, ParseError> {
    match r {
        Ok(x) => Ok(Some(x)),
        Err(ParseError::Empty) => Ok(None),
        Err(ParseError::Consumed) => Err(ParseError::Consumed),
    }
}

struct Parser<'a> {
    r: Lexer<'a>,
    first_token: Option<&'a str>,
}

impl<'a> Parser<'a> {
    fn peek(&mut self) -> Result<&'a str, ParseError> {
        if let Some(t) = self.first_token {
            return Ok(t);
        }
        let Some(t) = self.r.next() else {
            return Err(ParseError::Empty);
        };
        self.first_token = Some(t);
        Ok(t)
    }

    fn next(&mut self) -> Result<&'a str, ParseError> {
        if let Some(t) = std::mem::take(&mut self.first_token) {
            return Ok(t);
        }
        let Some(t) = self.r.next() else {
            return Err(ParseError::Empty);
        };
        Ok(t)
    }

    fn symbol(&mut self, s: &str) -> bool {
        let Ok(t) = self.peek() else {
            return false;
        };
        if t != s {
            return false;
        }
        let _ = self.next();
        true
    }

    fn parse_num(&mut self) -> Result<Num, ParseError> {
        let negative = self.symbol("-");
        let t = seq(self.next())?;
        if t.contains('.') || t.contains('e') || t.contains('E') {
            let Ok(f) = f64::from_str(t) else {
                return Err(ParseError::Consumed);
            };
            if negative {
                return Ok(num::float(-f));
            }
            return Ok(num::float(f));
        }
        let Ok(i) = BigInt::from_str(t) else {
            return Err(ParseError::Consumed);
        };
        if negative {
            return Ok(num::int(-i));
        }
        Ok(num::int(i))
    }

    fn paren_expr(&mut self) -> Result<Num, ParseError> {
        if !self.symbol("(") {
            return Err(ParseError::Empty);
        }
        let n = seq(self.expr())?;
        if !self.symbol(")") {
            return Err(ParseError::Consumed);
        }
        Ok(n)
    }

    fn parse_const(&mut self) -> Result<Num, ParseError> {
        if self.symbol("e") {
            return Ok(num::float(std::f64::consts::E));
        }
        if self.symbol("pi") {
            return Ok(num::float(std::f64::consts::PI));
        }
        Err(ParseError::Empty)
    }

    fn atom(&mut self) -> Result<Num, ParseError> {
        if let Some(n) = or(self.paren_expr())? {
            return Ok(n);
        }
        if let Some(n) = or(self.parse_const())? {
            return Ok(n);
        }
        self.parse_num()
    }

    fn parse_fun(&mut self) -> Result<Num, ParseError> {
        if self.symbol("sin") {
            return Ok(self.atom()?.sin());
        }
        if self.symbol("cos") {
            return Ok(self.atom()?.cos());
        }
        if self.symbol("tan") {
            return Ok(self.atom()?.tan());
        }
        if self.symbol("sqrt") {
            return Ok(self.atom()?.sqrt());
        }
        if self.symbol("log") || self.symbol("ln") {
            return Ok(self.atom()?.log());
        }
        if self.symbol("floor") {
            return Ok(self.atom()?.floor());
        }
        if self.symbol("ceil") || self.symbol("ceiling") {
            return Ok(self.atom()?.ceil());
        }
        if self.symbol("round") {
            return Ok(self.atom()?.round());
        }
        Err(ParseError::Empty)
    }

    fn app_expr_fold(&mut self, e1: Num) -> Result<Num, ParseError> {
        if let Some(e2) = or(self.parse_fun())? {
            return self.app_expr_fold(e1.mul(e2));
        }
        if let Some(e2) = or(self.paren_expr())? {
            return self.app_expr_fold(e1.mul(e2));
        }
        if let Some(e2) = or(self.parse_const())? {
            return self.app_expr_fold(e1.mul(e2));
        }
        Ok(e1)
    }

    fn app_expr(&mut self) -> Result<Num, ParseError> {
        if let Some(e1) = or(self.parse_fun())? {
            return Ok(e1);
        }
        let e1 = self.atom()?;
        self.app_expr_fold(e1)
    }

    fn expt_expr_fold(&mut self, e1: Num) -> Result<Num, ParseError> {
        if !self.symbol("^") {
            return Ok(e1);
        }
        let e2 = seq(self.app_expr())?;
        let e2 = self.expt_expr_fold(e2)?;
        Ok(e1.pow(e2))
    }

    fn expt_expr(&mut self) -> Result<Num, ParseError> {
        let n = self.app_expr()?;
        self.expt_expr_fold(n)
    }

    fn mul_expr_fold(&mut self, e1: Num) -> Result<Num, ParseError> {
        if self.symbol("*") {
            let e2 = seq(self.expt_expr())?;
            let e = e1.mul(e2);
            return self.mul_expr_fold(e);
        }
        if self.symbol("/") {
            let e2 = seq(self.expt_expr())?;
            let e = e1.div(e2);
            return self.mul_expr_fold(e);
        }
        if self.symbol("//") {
            let e2 = seq(self.expt_expr())?;
            let e = e1.int_div(e2);
            return self.mul_expr_fold(e);
        }
        Ok(e1)
    }

    fn mul_expr(&mut self) -> Result<Num, ParseError> {
        let n = self.expt_expr()?;
        self.mul_expr_fold(n)
    }

    fn add_expr_fold(&mut self, e1: Num) -> Result<Num, ParseError> {
        if self.symbol("+") {
            let e2 = seq(self.mul_expr())?;
            let e = e1.add(e2);
            return self.add_expr_fold(e);
        }
        if self.symbol("-") {
            let e2 = seq(self.mul_expr())?;
            let e = e1.sub(e2);
            return self.add_expr_fold(e);
        }
        Ok(e1)
    }

    fn expr(&mut self) -> Result<Num, ParseError> {
        let n = self.mul_expr()?;
        self.add_expr_fold(n)
    }
}

pub fn parse(expr: &str) -> Option<Num> {
    let mut p = Parser {
        r: Lexer { buf: expr },
        first_token: None,
    };
    let n = p.expr().ok()?;
    if p.r.next().is_some() {
        // Trailing characters.
        return None;
    }
    Some(n)
}