use hashbrown::HashSet;
use itertools::Itertools;
use orchid_base::error::{OrcErrv, OrcRes, Reporter, mk_errv};
use orchid_base::interner::{Interner, Tok};
use orchid_base::location::Pos;
use orchid_base::name::VName;

/// Errors produced by absolute_path
#[derive(Clone, Debug, Hash, PartialEq, Eq)]
pub enum AbsPathError {
	/// `super` of root requested, for example, `app::main` referenced
	/// `super::super::super::std`
	TooManySupers,
	/// root selected, for example, `app::main` referenced exactly `super::super`.
	/// The empty path also triggers this.
	RootPath,
}
impl AbsPathError {
	pub async fn err_obj(self, i: &Interner, pos: Pos, path: &str) -> OrcErrv {
		let (descr, msg) = match self {
			AbsPathError::RootPath => (
				i.i("Path ends on root module").await,
				format!(
					"{path} is equal to the empty path. You cannot directly reference the root. \
					Use one fewer 'super::' or add more segments to make it valid."
				),
			),
			AbsPathError::TooManySupers => (
				i.i("Too many 'super::' steps in path").await,
				format!("{path} is leading outside the root."),
			),
		};
		mk_errv(descr, msg, [pos])
	}
}

/// Turn a relative (import) path into an absolute path.
/// If the import path is empty, the return value is also empty.
///
/// # Errors
///
/// if the relative path contains as many or more `super` segments than the
/// length of the absolute path.
pub async fn absolute_path(
	mut cwd: &[Tok<String>],
	mut rel: &[Tok<String>],
	i: &Interner,
) -> Result<VName, AbsPathError> {
	let i_self = i.i("self").await;
	let i_super = i.i("super").await;
	let mut relative = false;
	if let Some((_, tail)) = rel.split_first().filter(|(h, _)| **h == i_self) {
		rel = tail;
		relative = true;
	} else {
		while let Some((_, tail)) = rel.split_first().filter(|(h, _)| **h == i_super) {
			cwd = cwd.split_last().ok_or(AbsPathError::TooManySupers)?.1;
			rel = tail;
			relative = true;
		}
	}
	if relative { VName::new(cwd.iter().chain(rel).cloned()) } else { VName::new(rel.to_vec()) }
		.map_err(|_| AbsPathError::RootPath)
}

pub struct DealiasCtx<'a> {
	pub i: &'a Interner,
	pub rep: &'a Reporter,
}

pub async fn resolv_glob<Mod: Tree>(
	cwd: &[Tok<String>],
	root: &Mod,
	abs_path: &[Tok<String>],
	pos: Pos,
	i: &Interner,
	ctx: &mut Mod::Ctx<'_>,
) -> OrcRes<HashSet<Tok<String>>> {
	let coprefix_len = cwd.iter().zip(abs_path).take_while(|(a, b)| a == b).count();
	let (co_prefix, diff_path) = abs_path.split_at(abs_path.len().min(coprefix_len + 1));
	let fst_diff =
		walk(root, false, co_prefix.iter().cloned(), ctx).await.expect("Invalid step in cwd");
	let target_module = match walk(fst_diff, true, diff_path.iter().cloned(), ctx).await {
		Ok(t) => t,
		Err(e) => {
			let path = abs_path[..=coprefix_len + e.pos].iter().join("::");
			let (tk, msg) = match e.kind {
				ChildErrorKind::Constant => ("Invalid import path", format!("{path} is a const")),
				ChildErrorKind::Missing => ("Invalid import path", format!("{path} not found")),
				ChildErrorKind::Private => ("Import inaccessible", format!("{path} is private")),
			};
			return Err(mk_errv(i.i(tk).await, msg, [pos]));
		},
	};
	Ok(target_module.children(coprefix_len < abs_path.len()))
}

pub type ChildResult<'a, T> = Result<&'a T, ChildErrorKind>;

pub trait Tree {
	type Ctx<'a>;
	#[must_use]
	fn children(&self, public_only: bool) -> HashSet<Tok<String>>;
	#[must_use]
	fn child(
		&self,
		key: Tok<String>,
		public_only: bool,
		ctx: &mut Self::Ctx<'_>,
	) -> impl Future<Output = ChildResult<'_, Self>>;
}
#[derive(Clone, Debug, Hash, PartialEq, Eq)]
pub enum ChildErrorKind {
	Missing,
	/// Only thrown if public_only is true
	Private,
	Constant,
}
#[derive(Clone, Debug, Hash, PartialEq, Eq)]
pub struct ChildError {
	pub pos: usize,
	pub kind: ChildErrorKind,
}

// Problem: walk should take into account aliases and visibility
//
// help: since every alias is also its own import, visibility only has to be
// checked on the top level
//
// idea: do a simple stack machine like below with no visibility for aliases and
// call it from an access-checking implementation for just the top level
//
// caveat: we need to check EVERY IMPORT to ensure that all
// errors are raised
pub async fn walk<'a, T: Tree>(
	root: &'a T,
	public_only: bool,
	path: impl IntoIterator<Item = Tok<String>>,
	ctx: &mut T::Ctx<'_>,
) -> Result<&'a T, ChildError> {
	let mut cur = root;
	for (i, item) in path.into_iter().enumerate() {
		match cur.child(item, public_only, ctx).await {
			ChildResult::Ok(v) => cur = v,
			ChildResult::Err(kind) => return Err(ChildError { pos: i, kind }),
		}
	}
	Ok(cur)
}