//! The concrete syntax tree (CST) implements zero-copy
//! parsing of the DSL from a string reference. CST items
//! preserve info about their location in the source.
use crate::*;

/// CST stores strings as source references and expressions as [CstIter] instances.
#[derive(Debug, Clone, Default, PartialEq)]
pub struct Cst<'s>(pub CstIter<'s>);
impl<'s> Dsl for Cst<'s> {
    type Str = &'s str;
    type Exp = CstIter<'s>;
    fn nth (&self, index: usize) -> Option<DslVal<Self::Str, Self::Exp>> {
        self.0.nth(index)
    }
}

/// Parsed substring with range and value.
#[derive(Debug, Copy, Clone, Default, PartialEq)]
pub struct CstVal<'s> {
    /// Meaning of token.
    pub value:  DslVal<&'s str, CstIter<'s>>,
    /// Reference to source text.
    pub source: &'s str,
    /// Index of 1st character of token.
    pub start:  usize,
    /// Length of token.
    pub length: usize,
}
impl<'s> Dsl for CstVal<'s> {
    type Str = &'s str;
    type Exp = CstIter<'s>;
    fn nth (&self, index: usize) -> Option<DslVal<Self::Str, Self::Exp>> {
        todo!()
    }
}

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

/// Provides a native [Iterator] API over [CstConstIter],
/// emitting [Cst] items.
///
/// [Cst::next] returns just the [Cst] and mutates `self`,
/// instead of returning an updated version of the struct as [CstConstIter::next] does.
#[derive(Copy, Clone, Debug, Default, PartialEq)]
pub struct CstIter<'s>(pub CstConstIter<'s>);
impl<'s> Dsl for CstIter<'s> {
    type Str = &'s str;
    type Exp = Self;
    fn nth (&self, index: usize) -> Option<DslVal<Self::Str, Self::Exp>> {
        self.0.nth(index).map(|x|dsl_val(x))
    }
}
impl<'s> CstIter<'s> {
    pub const fn new (source: &'s str) -> Self {
        Self(CstConstIter::new(source))
    }
    pub const fn peek (&self) -> Option<CstVal<'s>> {
        self.0.peek()
    }
}
impl<'s> Iterator for CstIter<'s> {
    type Item = CstVal<'s>;
    fn next (&mut self) -> Option<CstVal<'s>> {
        self.0.next().map(|(item, rest)|{
            self.0 = rest;
            item
        })
    }
}
impl<'s> Into<Vec<CstVal<'s>>> for CstIter<'s> {
    fn into (self) -> Vec<CstVal<'s>> {
        self.collect()
    }
}
impl<'s> Into<Vec<Ast>> for CstIter<'s> {
    fn into (self) -> Vec<Ast> {
        self.map(Into::into).collect()
    }
}

/// Owns a reference to the source text.
/// [CstConstIter::next] emits subsequent pairs of:
/// * a [Cst] and
/// * the source text remaining
///   * [ ] TODO: maybe [CstConstIter::next] should wrap the remaining source in `Self` ?
#[derive(Copy, Clone, Debug, Default, PartialEq)]
pub struct CstConstIter<'s>(pub &'s str);
impl<'s> Dsl for CstConstIter<'s> {
    type Str = &'s str;
    type Exp = Self;
    fn nth (&self, mut index: usize) -> Option<DslVal<Self::Str, Self::Exp>> {
        use DslVal::*;
        let mut iter = self.clone();
        for i in 0..index {
            iter = iter.next()?.1
        }
        iter.next().map(|(x, _)|dsl_val(x.value))
    }
}
impl<'s> CstConstIter<'s> {
    pub const fn new (source: &'s 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<(CstVal<'s>, Self)> {
        Self::next_mut(&mut self)
    }
    pub const fn peek (&self) -> Option<CstVal<'s>> {
        peek_src(self.0)
    }
    pub const fn next_mut (&mut self) -> Option<(CstVal<'s>, Self)> {
        match self.peek() {
            Some(token) => Some((token, self.chomp(token.end()))),
            None => None
        }
    }
}
impl<'s> From<CstConstIter<'s>> for CstIter<'s> {
    fn from (iter: CstConstIter<'s>) -> Self {
        Self(iter)
    }
}
impl<'s> From<CstIter<'s>> for CstConstIter<'s> {
    fn from (iter: CstIter<'s>) -> Self {
        iter.0
    }
}

/// Implement the const iterator pattern.
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;
        }
    }
}

const_iter!(<'s>|self: CstConstIter<'s>|
    => CstVal<'s>
    => self.next_mut().map(|(result, _)|result));

/// Static iteration helper used by [cst].
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 <'s> (source: &'s str) -> Option<CstVal<'s>> {
    use DslVal::*;
    let mut token: CstVal<'s> = CstVal::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(),
            '(' =>
                CstVal::new(source, start, 1, Exp(1, CstIter::new(str_range(source, start, start + 1)))),
            '"' =>
                CstVal::new(source, start, 1, Str(str_range(source, start, start + 1))),
            ':'|'@' =>
                CstVal::new(source, start, 1, Sym(str_range(source, start, start + 1))),
            '/'|'a'..='z' =>
                CstVal::new(source, start, 1, Key(str_range(source, start, start + 1))),
            '0'..='9' =>
                CstVal::new(source, start, 1, match to_digit(c) {
                    Ok(c) => DslVal::Num(c),
                    Result::Err(e) => DslVal::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))
    })
}