use crate::*;

/// CST stores strings as source references and expressions as new [SourceIter] instances.
pub type CstValue<'source> = Value<&'source str, SourceIter<'source>>;

/// Token sharing memory with source reference.
#[derive(Debug, Copy, Clone, Default, PartialEq)]
pub struct CstToken<'source>(pub CstValue<'source>, pub CstMeta<'source>);

/// Reference to the source slice.
#[derive(Debug, Copy, Clone, Default, PartialEq)] pub struct CstMeta<'source> {
    pub source: &'source str,
    pub start:  usize,
    pub length: usize,
}

impl<'source> CstToken<'source> {
    pub const fn new (
        source: &'source str, start: usize, length: usize, value: CstValue<'source>
    ) -> Self {
        Self(value, CstMeta { source, start, length })
    }
    pub const fn end (&self) -> usize {
        self.1.start.saturating_add(self.1.length)
    }
    pub const fn slice (&'source self) -> &'source str {
        self.slice_source(self.1.source)
    }
    pub const fn slice_source <'range> (&'source self, source: &'range str) -> &'range str {
        str_range(source, self.1.start, self.end())
    }
    pub const fn slice_source_exp <'range> (&'source self, source: &'range str) -> &'range str {
        str_range(source, self.1.start.saturating_add(1), self.end())
    }
    pub const fn with_value (self, value: CstValue<'source>) -> Self {
        Self(value, self.1)
    }
    pub const fn value (&self) -> CstValue<'source> {
        self.0
    }
    pub const fn error (self, error: DslError) -> Self {
        Self(Value::Err(error), self.1)
    }
    pub const fn grow (self) -> Self {
        Self(self.0, CstMeta { length: self.1.length.saturating_add(1), ..self.1 })
    }
    pub const fn grow_num (self, m: usize, c: char) -> Self {
        use Value::*;
        match to_digit(c) {
            Result::Ok(n)  => Self(Num(10*m+n), self.grow().1),
            Result::Err(e) => Self(Err(e), self.grow().1),
        }
    }
    pub const fn grow_key (self) -> Self {
        use Value::*;
        let token = self.grow();
        token.with_value(Key(token.slice_source(self.1.source)))
    }
    pub const fn grow_sym (self) -> Self {
        use Value::*;
        let token = self.grow();
        token.with_value(Sym(token.slice_source(self.1.source)))
    }
    pub const fn grow_str (self) -> Self {
        use Value::*;
        let token = self.grow();
        token.with_value(Str(token.slice_source(self.1.source)))
    }
    pub const fn grow_exp (self) -> Self {
        use Value::*;
        let token = self.grow();
        if let Exp(depth, _) = token.value() {
            token.with_value(Exp(depth, SourceIter::new(token.slice_source_exp(self.1.source))))
        } else {
            unreachable!()
        }
    }
    pub const fn grow_in (self) -> Self {
        let token = self.grow_exp();
        if let Value::Exp(depth, source) = token.value() {
            token.with_value(Value::Exp(depth.saturating_add(1), source))
        } else {
            unreachable!()
        }
    }
    pub const fn grow_out (self) -> Self {
        let token = self.grow_exp();
        if let Value::Exp(depth, source) = token.value() {
            if depth > 0 {
                token.with_value(Value::Exp(depth - 1, source))
            } else {
                return self.error(Unexpected(')'))
            }
        } else {
            unreachable!()
        }
    }
}

pub const fn to_number (digits: &str) -> DslResult<usize> {
    let mut value = 0;
    iterate!(char_indices(digits) => (_, c) => match to_digit(c) {
        Ok(digit) => value = 10 * value + digit,
        Result::Err(e) => return Result::Err(e)
    });
    Ok(value)
}

pub const fn to_digit (c: char) -> DslResult<usize> {
    Ok(match c {
        '0' => 0, '1' => 1, '2' => 2, '3' => 3, '4' => 4,
        '5' => 5, '6' => 6, '7' => 7, '8' => 8, '9' => 9,
        _ => return Result::Err(Unexpected(c))
    })
}

/// Static iteration helper.
#[macro_export] macro_rules! iterate {
    ($expr:expr => $arg: pat => $body:expr) => {
        let mut iter = $expr;
        while let Some(($arg, next)) = iter.next() {
            $body;
            iter = next;
        }
    }
}

pub const fn peek_src <'a> (source: &'a str) -> Option<CstToken<'a>> {
    use Value::*;
    let mut token: CstToken<'a> = CstToken::new(source, 0, 0, Nil);
    iterate!(char_indices(source) => (start, c) => token = match token.value() {
        Err(_) => return Some(token),
        Nil => match c {
            ' '|'\n'|'\r'|'\t' =>
                token.grow(),
            '(' =>
                CstToken::new(source, start, 1, Exp(1, SourceIter::new(str_range(source, start, start + 1)))),
            '"' =>
                CstToken::new(source, start, 1, Str(str_range(source, start, start + 1))),
            ':'|'@' =>
                CstToken::new(source, start, 1, Sym(str_range(source, start, start + 1))),
            '/'|'a'..='z' =>
                CstToken::new(source, start, 1, Key(str_range(source, start, start + 1))),
            '0'..='9' =>
                CstToken::new(source, start, 1, match to_digit(c) {
                    Ok(c) => Value::Num(c),
                    Result::Err(e) => Value::Err(e)
                }),
            _ => token.error(Unexpected(c))
        },
        Str(_) => match c {
            '"' => return Some(token),
            _ => token.grow_str(),
        },
        Num(n) => match c {
            '0'..='9' => token.grow_num(n, c),
            ' '|'\n'|'\r'|'\t'|')' => return Some(token),
            _ => token.error(Unexpected(c))
        },
        Sym(_) => match c {
            'a'..='z'|'A'..='Z'|'0'..='9'|'-' => token.grow_sym(),
            ' '|'\n'|'\r'|'\t'|')' => return Some(token),
            _ => token.error(Unexpected(c))
        },
        Key(_) => match c {
            'a'..='z'|'0'..='9'|'-'|'/' => token.grow_key(),
            ' '|'\n'|'\r'|'\t'|')' => return Some(token),
            _ => token.error(Unexpected(c))
        },
        Exp(depth, _) => match depth {
            0 => return Some(token.grow_exp()),
            _ => match c {
                ')' => token.grow_out(),
                '(' => token.grow_in(),
                 _  => token.grow_exp(),
            }
        },
    });
    match token.value() {
        Nil => None,
        _ => Some(token),
    }
}