use crate::*;
use std::error::Error;

/// Standard result type for DSL-specific operations.
pub type DslResult<T> = Result<T, DslErr>;

/// DSL-specific error codes.
#[derive(Error, Debug, Copy, Clone, PartialEq)] pub enum DslErr {
    #[error("parse failed: not implemented")]
    Unimplemented,
    #[error("parse failed: empty")]
    Empty,
    #[error("parse failed: incomplete")]
    Incomplete,
    #[error("parse failed: unexpected character '{0}'")]
    Unexpected(char),
    #[error("parse failed: error #{0}")]
    Code(u8),
}

/// Enumeration of possible DSL tokens.
/// Generic over string and expression storage.
///
/// * [ ] FIXME: Value may be [Err] which may shadow [Result::Err]
/// * [DslVal::Exp] wraps an expression depth and a [CstIter]
///   with the remaining part of the expression.
///   * expression depth other that 0 mean unclosed parenthesis.
///   * closing and unopened parenthesis panics during reading.
///   * [ ] TODO: signed depth might be interesting
/// * [DslVal::Sym] and [DslVal::Key] are stringish literals
///   with slightly different parsing rules.
/// * [DslVal::Num] is an unsigned integer literal.
#[derive(Clone, Debug, PartialEq, Default)]
pub enum DslVal<Str, Exp> {
    #[default]
    Nil,
    Err(DslErr),
    Num(usize),
    Sym(Str),
    Key(Str),
    Str(Str),
    Exp(usize, Exp),
}

pub fn dsl_val <A: Into<X>, B: Into<Y>, X, Y> (val: DslVal<A, B>) -> DslVal<X, Y> {
    use DslVal::*;
    match val {
        Nil => Nil,
        Err(e) => Err(e),
        Num(u) => Num(u),
        Sym(s) => Sym(s.into()),
        Key(s) => Key(s.into()),
        Str(s) => Str(s.into()),
        Exp(d, x) => Exp(d, x.into()),
    }
}

pub trait Dsl: Debug {
    type Str: PartialEq + Clone + Default + Debug + AsRef<str>;
    type Exp: PartialEq + Clone + Default + Debug + Dsl;
    fn nth (&self, index: usize) -> Option<DslVal<Self::Str, Self::Exp>>;
    fn val (&self) -> DslVal<Self::Str, Self::Exp> {
        self.nth(0).unwrap_or(DslVal::Nil)
    }
    // exp-only nth here?
}

impl<
    Str: PartialEq + Clone + Default + Debug + AsRef<str>,
    Exp: PartialEq + Clone + Default + Debug + Dsl,
> Dsl for DslVal<Str, Exp> {
    type Str = Str;
    type Exp = Exp;
    fn val (&self) -> DslVal<Str, Exp> {
        self.clone()
    }
    fn nth (&self, _index: usize) -> Option<DslVal<Str, Exp>> {
        todo!()
    }
}

/// May construct self from state and DSL.
pub trait DslFrom<State>: Sized {
    fn try_dsl_from (state: &State, value: &impl Dsl) -> Perhaps<Self>;
    fn dsl_from (
        state: &State, value: &impl Dsl, error: impl Fn()->Box<dyn Error>
    ) -> Usually<Self> {
        match Self::try_dsl_from(state, value)? {
            Some(value) => Ok(value),
            _ => Err(error())
        }
    }
}

/// May construct another from self and DSL.
pub trait DslInto<Item> {
    fn try_dsl_into (&self, dsl: &impl Dsl) -> Perhaps<Item>;
    fn dsl_into (
        &self, value: &impl Dsl, error: impl Fn()->Box<dyn Error>
    ) -> Usually<Item> {
        match Self::try_dsl_into(self, value)? {
            Some(value) => Ok(value),
            _ => Err(error())
        }
    }
}

impl<Str, Exp> DslVal<Str, Exp> {
    pub fn is_nil (&self) -> bool {
        matches!(self, Self::Nil)
    }
    pub fn as_err (&self) -> Option<&DslErr> {
        if let Self::Err(e) = self { Some(e) } else { None }
    }
    pub fn as_num (&self) -> Option<usize> {
        if let Self::Num(n) = self { Some(*n) } else { None }
    }
    pub fn as_exp (&self) -> Option<&Exp> {
        if let Self::Exp(_, x) = self { Some(x) } else { None }
    }
    pub fn exp_depth (&self) -> Option<usize> {
        todo!()
    }
    pub fn exp_head (&self) -> Option<&Self> {
        todo!()
    } // TODO
    pub fn exp_tail (&self) -> Option<&Exp> {
        todo!()
    } // TODO
    pub fn peek (&self) -> Option<Self> {
        todo!()
    }
    pub fn next (&mut self) -> Option<Self> {
        todo!()
    }
    pub fn rest (self) -> Vec<Self> {
        todo!()
    }
}

impl<Str: Copy, Exp: Copy> Copy for DslVal<Str, Exp> {}

impl<Str: AsRef<str>, Exp> DslVal<Str, Exp> {
    pub fn as_sym (&self) -> Option<&str> {
        if let Self::Sym(s) = self { Some(s.as_ref()) } else { None }
    }
    pub fn as_key (&self) -> Option<&str> {
        if let Self::Key(k) = self { Some(k.as_ref()) } else { None }
    }
    pub fn as_str (&self) -> Option<&str> {
        if let Self::Str(s) = self { Some(s.as_ref()) } else { None }
    }
    pub fn exp_match <T, F> (&self, namespace: &str, cb: F) -> Perhaps<T>
    where F: Fn(&str, &Exp)-> Perhaps<T> {
        if let Some(Self::Key(key)) = self.exp_head()
        && key.as_ref().starts_with(namespace)
        && let Some(tail) = self.exp_tail() {
            cb(key.as_ref().split_at(namespace.len()).1, tail)
        } else {
            Ok(None)
        }
    }
}

macro_rules! from_str {
    ($Struct:ty |$source:ident| $expr:expr) => {
        impl<'s> From<&'s str> for $Struct {
            fn from ($source: &'s str) -> Self {
                $expr 
            }
        }
    }
}

from_str!(Ast|source|Self::from(CstIter::from(source)));
from_str!(Cst<'s>|source|Self(CstIter(CstConstIter(source))));
from_str!(CstIter<'s>|source|Self(CstConstIter(source)));
from_str!(CstConstIter<'s>|source|Self::new(source));