use std::collections::HashMap;

use proc_macro2::Span;
use syn::{
    spanned::Spanned, GenericParam, Generics, Ident, PathArguments, Type, TypeParamBound, TypePath,
};

use crate::{
    parse_trait_sig::{MethodParseError, TypeTransform},
    syn_utils::{find_in_path, find_in_type},
    As, AssocTypeMatcher,
};

#[derive(Default)]
pub struct AssocTypeConversions<'a>(pub HashMap<Ident, &'a Type>);

pub enum TransformError {
    UnconvertibleAssocType(Span),
    AssocTypeInUnsupportedType(Span),
}

impl AssocTypeConversions<'_> {
    pub fn parse_type_path(&self, type_: &mut Type) -> Result<TypeTransform, TransformError> {
        let assoc_span = match find_in_type(type_, &AssocTypeMatcher) {
            Some(path) => path.span(),
            None => return Ok(TypeTransform::NoOp),
        };

        if let Type::Path(TypePath { path, qself: None }) = type_ {
            let ident = &path.segments.first().unwrap().ident;

            // TODO: support &mut dyn Iterator<Item = Self::A>
            // conversion to  Box<dyn Iterator<Item = Whatever>> via .map(Into::into)

            if ident == "Self" && path.segments.len() == 2 {
                let ident = &path.segments.last().unwrap().ident;
                *type_ = (*self
                    .0
                    .get(ident)
                    .ok_or_else(|| TransformError::UnconvertibleAssocType(ident.span()))?)
                .clone();
                return Ok(TypeTransform::Into);
            } else if ident == "Option" && path.segments.len() == 1 {
                let first_seg = path.segments.first_mut().unwrap();

                if let Some(args) = first_seg.arguments.get_as_mut() {
                    if args.args.len() == 1 {
                        if let Some(generic_type) = args.args.first_mut().unwrap().get_as_mut() {
                            if find_in_type(generic_type, &AssocTypeMatcher).is_some() {
                                return Ok(TypeTransform::Map(
                                    self.parse_type_path(generic_type)?.into(),
                                ));
                            }
                        }
                    }
                }
            } else if ident == "Result" && path.segments.len() == 1 {
                let first_seg = path.segments.first_mut().unwrap();
                if let Some(args) = first_seg.arguments.get_as_mut() {
                    if args.args.len() == 2 {
                        let mut args_iter = args.args.iter_mut();
                        if let (Some(ok_type), Some(err_type)) = (
                            args_iter.next().unwrap().get_as_mut(),
                            args_iter.next().unwrap().get_as_mut(),
                        ) {
                            if find_in_type(ok_type, &AssocTypeMatcher).is_some()
                                || find_in_type(err_type, &AssocTypeMatcher).is_some()
                            {
                                return Ok(TypeTransform::Result(
                                    self.parse_type_path(ok_type)?.into(),
                                    self.parse_type_path(err_type)?.into(),
                                ));
                            }
                        }
                    }
                }
            } else {
                let last_seg = &path.segments.last().unwrap();
                if last_seg.ident == "Result" {
                    let last_seg = path.segments.last_mut().unwrap();
                    if let Some(args) = last_seg.arguments.get_as_mut() {
                        if args.args.len() == 1 {
                            if let Some(generic_type) = args.args.first_mut().unwrap().get_as_mut()
                            {
                                if find_in_type(generic_type, &AssocTypeMatcher).is_some() {
                                    return Ok(TypeTransform::Map(
                                        self.parse_type_path(generic_type)?.into(),
                                    ));
                                }
                            }
                        }
                    }
                }
            }
        }

        // the type contains an associated type but we
        // don't know how to deal with it so we abort
        Err(TransformError::AssocTypeInUnsupportedType(assoc_span))
    }
}

pub fn dynamize_function_bounds(
    generics: &mut Generics,
    assoc_type_conversions: &AssocTypeConversions,
) -> Result<HashMap<Ident, Vec<TypeTransform>>, (Span, MethodParseError)> {
    let mut type_param_transforms = HashMap::new();

    for generic_param in &mut generics.params {
        if let GenericParam::Type(type_param) = generic_param {
            for bound in &mut type_param.bounds {
                if let TypeParamBound::Trait(bound) = bound {
                    if bound.path.segments.len() == 1 {
                        let segment = bound.path.segments.first_mut().unwrap();

                        if let PathArguments::Parenthesized(args) = &mut segment.arguments {
                            if segment.ident == "Fn"
                                || segment.ident == "FnOnce"
                                || segment.ident == "FnMut"
                            {
                                let mut transforms = Vec::new();
                                for input_type in &mut args.inputs {
                                    match assoc_type_conversions.parse_type_path(input_type) {
                                        Ok(ret_type) => {
                                            transforms.push(ret_type);
                                        }
                                        Err(TransformError::UnconvertibleAssocType(span)) => {
                                            return Err((
                                                span,
                                                MethodParseError::UnconvertibleAssocType,
                                            ));
                                        }
                                        Err(TransformError::AssocTypeInUnsupportedType(span)) => {
                                            return Err((
                                                span,
                                                MethodParseError::UnconvertibleAssocTypeInFnInput,
                                            ));
                                        }
                                    }
                                }
                                if transforms.iter().any(|t| !matches!(t, TypeTransform::NoOp)) {
                                    type_param_transforms
                                        .insert(type_param.ident.clone(), transforms);
                                }
                            }
                        }
                    }
                    if let Some(path) = find_in_path(&bound.path, &AssocTypeMatcher) {
                        return Err((
                            path.span(),
                            MethodParseError::UnconvertibleAssocTypeInTraitBound,
                        ));
                    }
                }
            }
        }
    }
    Ok(type_param_transforms)
}