use std::collections::BTreeSet;

use git_hash::ObjectId;

use crate::{
    packed, peel,
    raw::Reference,
    store_impl::{file, file::log},
    Target,
};

pub trait Sealed {}
impl Sealed for crate::Reference {}

/// A trait to extend [Reference][crate::Reference] with functionality requiring a [file::Store].
pub trait ReferenceExt: Sealed {
    /// A step towards obtaining forward or reverse iterators on reference logs.
    fn log_iter<'a, 's>(&'a self, store: &'s file::Store) -> log::iter::Platform<'a, 's>;

    /// For details, see [Reference::log_exists()].
    fn log_exists(&self, store: &file::Store) -> bool;

    /// For details, see [Reference::peel_to_id_in_place()].
    fn peel_to_id_in_place<E: std::error::Error + Send + Sync + 'static>(
        &mut self,
        store: &file::Store,
        find: impl FnMut(git_hash::ObjectId, &mut Vec<u8>) -> Result<Option<(git_object::Kind, &[u8])>, E>,
    ) -> Result<ObjectId, peel::to_id::Error>;

    /// For details, see [Reference::peel_to_id_in_place()], with support for a known stable packed buffer.
    fn peel_to_id_in_place_packed<E: std::error::Error + Send + Sync + 'static>(
        &mut self,
        store: &file::Store,
        find: impl FnMut(git_hash::ObjectId, &mut Vec<u8>) -> Result<Option<(git_object::Kind, &[u8])>, E>,
        packed: Option<&packed::Buffer>,
    ) -> Result<ObjectId, peel::to_id::Error>;

    /// Follow this symbolic reference one level and return the ref it refers to.
    ///
    /// Returns `None` if this is not a symbolic reference, hence the leaf of the chain.
    fn follow(&self, store: &file::Store) -> Option<Result<Reference, file::find::existing::Error>>;

    /// Follow this symbolic reference one level and return the ref it refers to,
    /// possibly providing access to `packed` references for lookup if it contains the referent.
    ///
    /// Returns `None` if this is not a symbolic reference, hence the leaf of the chain.
    fn follow_packed(
        &self,
        store: &file::Store,
        packed: Option<&packed::Buffer>,
    ) -> Option<Result<Reference, file::find::existing::Error>>;
}

impl ReferenceExt for Reference {
    fn log_iter<'a, 's>(&'a self, store: &'s file::Store) -> log::iter::Platform<'a, 's> {
        log::iter::Platform {
            store,
            name: self.name.as_ref(),
            buf: Vec::new(),
        }
    }

    fn log_exists(&self, store: &file::Store) -> bool {
        store
            .reflog_exists(self.name.as_ref())
            .expect("infallible name conversion")
    }

    fn peel_to_id_in_place<E: std::error::Error + Send + Sync + 'static>(
        &mut self,
        store: &file::Store,
        find: impl FnMut(git_hash::ObjectId, &mut Vec<u8>) -> Result<Option<(git_object::Kind, &[u8])>, E>,
    ) -> Result<ObjectId, peel::to_id::Error> {
        let packed = store.assure_packed_refs_uptodate().map_err(|err| {
            peel::to_id::Error::Follow(file::find::existing::Error::Find(file::find::Error::PackedOpen(err)))
        })?;
        self.peel_to_id_in_place_packed(store, find, packed.as_ref().map(|b| &***b))
    }

    fn peel_to_id_in_place_packed<E: std::error::Error + Send + Sync + 'static>(
        &mut self,
        store: &file::Store,
        mut find: impl FnMut(git_hash::ObjectId, &mut Vec<u8>) -> Result<Option<(git_object::Kind, &[u8])>, E>,
        packed: Option<&packed::Buffer>,
    ) -> Result<ObjectId, peel::to_id::Error> {
        match self.peeled {
            Some(peeled) => {
                self.target = Target::Peeled(peeled.to_owned());
                Ok(peeled)
            }
            None => {
                if self.target.kind() == crate::Kind::Symbolic {
                    let mut seen = BTreeSet::new();
                    let cursor = &mut *self;
                    while let Some(next) = cursor.follow_packed(store, packed) {
                        let next = next?;
                        if seen.contains(&next.name) {
                            return Err(peel::to_id::Error::Cycle {
                                start_absolute: store.reference_path(cursor.name.as_ref()),
                            });
                        }
                        *cursor = next;
                        seen.insert(cursor.name.clone());
                        const MAX_REF_DEPTH: usize = 5;
                        if seen.len() == MAX_REF_DEPTH {
                            return Err(peel::to_id::Error::DepthLimitExceeded {
                                max_depth: MAX_REF_DEPTH,
                            });
                        }
                    }
                };
                let mut buf = Vec::new();
                let mut oid = self.target.try_id().expect("peeled ref").to_owned();
                let peeled_id = loop {
                    let (kind, data) = find(oid, &mut buf)
                        .map_err(|err| Box::new(err) as Box<dyn std::error::Error + Send + Sync + 'static>)?
                        .ok_or_else(|| peel::to_id::Error::NotFound {
                            oid,
                            name: self.name.0.clone(),
                        })?;
                    match kind {
                        git_object::Kind::Tag => {
                            oid = git_object::TagRefIter::from_bytes(data).target_id().map_err(|_err| {
                                peel::to_id::Error::NotFound {
                                    oid,
                                    name: self.name.0.clone(),
                                }
                            })?;
                        }
                        _ => break oid,
                    };
                };
                self.peeled = Some(peeled_id);
                self.target = Target::Peeled(peeled_id);
                Ok(peeled_id)
            }
        }
    }

    fn follow(&self, store: &file::Store) -> Option<Result<Reference, file::find::existing::Error>> {
        let packed = match store
            .assure_packed_refs_uptodate()
            .map_err(|err| file::find::existing::Error::Find(file::find::Error::PackedOpen(err)))
        {
            Ok(packed) => packed,
            Err(err) => return Some(Err(err)),
        };
        self.follow_packed(store, packed.as_ref().map(|b| &***b))
    }

    fn follow_packed(
        &self,
        store: &file::Store,
        packed: Option<&packed::Buffer>,
    ) -> Option<Result<Reference, file::find::existing::Error>> {
        match self.peeled {
            Some(peeled) => Some(Ok(Reference {
                name: self.name.clone(),
                target: Target::Peeled(peeled),
                peeled: None,
            })),
            None => match &self.target {
                Target::Peeled(_) => None,
                Target::Symbolic(full_name) => match store.try_find_packed(full_name.as_ref(), packed) {
                    Ok(Some(next)) => Some(Ok(next)),
                    Ok(None) => Some(Err(file::find::existing::Error::NotFound {
                        name: full_name.to_path().to_owned(),
                    })),
                    Err(err) => Some(Err(file::find::existing::Error::Find(err))),
                },
            },
        }
    }
}