use crate::*;

/// Implement the const iterator pattern.
#[macro_export] macro_rules! const_iter {
    ($(<$l:lifetime>)?|$self:ident: $Struct:ty| => $Item:ty => $expr:expr) => {
        impl$(<$l>)? Iterator for $Struct {
            type Item = $Item;
            fn next (&mut $self) -> Option<$Item> { $expr }
        }
        impl$(<$l>)? ConstIntoIter for $Struct {
            type Kind = IsIteratorKind;
            type Item = $Item;
            type IntoIter = Self;
        }
    }
}

/// Provides a native [Iterator] API over the [ConstIntoIter] [SourceIter]
/// [TokenIter::next] returns just the [Token] and mutates `self`,
/// instead of returning an updated version of the struct as [SourceIter::next] does.
#[derive(Copy, Clone, Debug, Default, PartialEq)]
pub struct TokenIter<'a>(
    pub SourceIter<'a>
);

impl<'a> TokenIter<'a> {
    pub const fn new (source: &'a str) -> Self {
        Self(SourceIter::new(source))
    }
    pub const fn peek (&self) -> Option<Token<'a>> {
        self.0.peek()
    }
}

impl<'a> Iterator for TokenIter<'a> {
    type Item = Token<'a>;
    fn next (&mut self) -> Option<Token<'a>> {
        self.0.next().map(|(item, rest)|{self.0 = rest; item})
    }
}

impl<'a> From<&'a str> for TokenIter<'a> {
    fn from (source: &'a str) -> Self{
        Self(SourceIter(source))
    }
}

impl<'a> From<SourceIter<'a>> for TokenIter<'a> {
    fn from (source: SourceIter<'a>) -> Self{
        Self(source)
    }
}

/// Owns a reference to the source text.
/// [SourceIter::next] emits subsequent pairs of:
///   * a [Token] and
///   * the source text remaining
///     * [ ] TODO: maybe [SourceIter::next] should wrap the remaining source in `Self` ?
#[derive(Copy, Clone, Debug, Default, PartialEq)]
pub struct SourceIter<'a>(pub &'a str);

const_iter!(<'a>|self: SourceIter<'a>| => Token<'a> => self.next_mut().map(|(result, _)|result));

impl<'a> From<&'a str> for SourceIter<'a> {
    fn from (source: &'a str) -> Self{
        Self::new(source)
    }
}

impl<'a> SourceIter<'a> {
    pub const fn new (source: &'a str) -> Self {
        Self(source)
    }
    pub const fn chomp (&self, index: usize) -> Self {
        Self(split_at(self.0, index).1)
    }
    pub const fn next (mut self) -> Option<(Token<'a>, Self)> {
        Self::next_mut(&mut self)
    }
    pub const fn peek (&self) -> Option<Token<'a>> {
        peek_src(self.0)
    }
    pub const fn next_mut (&mut self) -> Option<(Token<'a>, Self)> {
        match self.peek() {
            Some(token) => Some((token, self.chomp(token.end()))),
            None => None
        }
    }
}

/// 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<Token<'a>> {
    let mut token: Token<'a> = Token::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(),
            '(' =>
                Token::new(source, start, 1, Exp(1, TokenIter::new(str_range(source, start, start + 1)))),
            '"' =>
                Token::new(source, start, 1, Str(str_range(source, start, start + 1))),
            ':'|'@' =>
                Token::new(source, start, 1, Sym(str_range(source, start, start + 1))),
            '/'|'a'..='z' =>
                Token::new(source, start, 1, Key(str_range(source, start, start + 1))),
            '0'..='9' =>
                Token::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),
    }
}

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))
    })
}