use std::{borrow::BorrowMut, collections::VecDeque};

use gix_object::{tree::EntryRef, FindExt};

use crate::{
    tree,
    tree::{visit::Change, TreeInfoPair},
};

/// The error returned by [`tree::Changes::needed_to_obtain()`].
#[derive(Debug, thiserror::Error)]
#[allow(missing_docs)]
pub enum Error {
    #[error(transparent)]
    Find(#[from] gix_object::find::existing_iter::Error),
    #[error("The delegate cancelled the operation")]
    Cancelled,
    #[error(transparent)]
    EntriesDecode(#[from] gix_object::decode::Error),
}

impl<'a> tree::Changes<'a> {
    /// Calculate the changes that would need to be applied to `self` to get `other` using `objects` to obtain objects as needed for traversal.
    ///
    /// * The `state` maybe owned or mutably borrowed to allow reuses allocated data structures through multiple runs.
    /// * `locate` is a function `f(object_id, &mut buffer) -> Option<TreeIter>` to return a `TreeIter` for the given object id backing
    ///   its data in the given buffer. Returning `None` is unexpected as these trees are obtained during iteration, and in a typical
    ///   database errors are not expected either which is why the error case is omitted. To allow proper error reporting, [`Error::Find`]
    ///   should be converted into a more telling error.
    /// * `delegate` will receive the computed changes, see the [`Visit`][`tree::Visit`] trait for more information on what to expect.
    ///
    /// # Notes
    ///
    /// * To obtain progress, implement it within the `delegate`.
    /// * Tree entries are expected to be ordered using [`tree-entry-comparison`][git_cmp_c] (the same [in Rust][git_cmp_rs])
    /// * it does a breadth first iteration as buffer space only fits two trees, the current one on the one we compare with.
    /// * does not do rename tracking but attempts to reduce allocations to zero (so performance is mostly determined
    ///   by the delegate implementation which should be as specific as possible. Rename tracking can be computed on top of the changes
    ///   received by the `delegate`.
    /// * cycle checking is not performed, but can be performed in the delegate which can return [`tree::visit::Action::Cancel`] to stop the traversal.
    /// * [`std::mem::ManuallyDrop`] is used because `Peekable` is needed. When using it as wrapper around our no-drop iterators, all of the sudden
    ///   borrowcheck complains as Drop is present (even though it's not)
    ///
    /// [git_cmp_c]: https://github.com/git/git/blob/311531c9de557d25ac087c1637818bd2aad6eb3a/tree-diff.c#L49:L65
    /// [git_cmp_rs]: https://github.com/Byron/gitoxide/blob/a4d5f99c8dc99bf814790928a3bf9649cd99486b/gix-object/src/mutable/tree.rs#L52-L55
    pub fn needed_to_obtain<R, StateMut>(
        mut self,
        other: gix_object::TreeRefIter<'_>,
        mut state: StateMut,
        objects: impl gix_object::Find,
        delegate: &mut R,
    ) -> Result<(), Error>
    where
        R: tree::Visit,
        StateMut: BorrowMut<tree::State>,
    {
        let state = state.borrow_mut();
        state.clear();
        let mut lhs_entries = peekable(self.0.take().unwrap_or_default());
        let mut rhs_entries = peekable(other);
        let mut pop_path = false;

        loop {
            if pop_path {
                delegate.pop_path_component();
            }
            pop_path = true;

            match (lhs_entries.next(), rhs_entries.next()) {
                (None, None) => {
                    match state.trees.pop_front() {
                        Some((None, Some(rhs))) => {
                            delegate.pop_front_tracked_path_and_set_current();
                            rhs_entries = peekable(objects.find_tree_iter(&rhs, &mut state.buf2)?);
                        }
                        Some((Some(lhs), Some(rhs))) => {
                            delegate.pop_front_tracked_path_and_set_current();
                            lhs_entries = peekable(objects.find_tree_iter(&lhs, &mut state.buf1)?);
                            rhs_entries = peekable(objects.find_tree_iter(&rhs, &mut state.buf2)?);
                        }
                        Some((Some(lhs), None)) => {
                            delegate.pop_front_tracked_path_and_set_current();
                            lhs_entries = peekable(objects.find_tree_iter(&lhs, &mut state.buf1)?);
                        }
                        Some((None, None)) => unreachable!("BUG: it makes no sense to fill the stack with empties"),
                        None => return Ok(()),
                    };
                    pop_path = false;
                }
                (Some(lhs), Some(rhs)) => {
                    use std::cmp::Ordering::*;
                    let (lhs, rhs) = (lhs?, rhs?);
                    match compare(&lhs, &rhs) {
                        Equal => handle_lhs_and_rhs_with_equal_filenames(lhs, rhs, &mut state.trees, delegate)?,
                        Less => catchup_lhs_with_rhs(&mut lhs_entries, lhs, rhs, &mut state.trees, delegate)?,
                        Greater => catchup_rhs_with_lhs(&mut rhs_entries, lhs, rhs, &mut state.trees, delegate)?,
                    }
                }
                (Some(lhs), None) => {
                    let lhs = lhs?;
                    delete_entry_schedule_recursion(lhs, &mut state.trees, delegate)?;
                }
                (None, Some(rhs)) => {
                    let rhs = rhs?;
                    add_entry_schedule_recursion(rhs, &mut state.trees, delegate)?;
                }
            }
        }
    }
}

fn compare(a: &EntryRef<'_>, b: &EntryRef<'_>) -> std::cmp::Ordering {
    let common = a.filename.len().min(b.filename.len());
    a.filename[..common].cmp(&b.filename[..common]).then_with(|| {
        let a = a.filename.get(common).or_else(|| a.mode.is_tree().then_some(&b'/'));
        let b = b.filename.get(common).or_else(|| b.mode.is_tree().then_some(&b'/'));
        a.cmp(&b)
    })
}

fn delete_entry_schedule_recursion<R: tree::Visit>(
    entry: EntryRef<'_>,
    queue: &mut VecDeque<TreeInfoPair>,
    delegate: &mut R,
) -> Result<(), Error> {
    delegate.push_path_component(entry.filename);
    if delegate
        .visit(Change::Deletion {
            entry_mode: entry.mode,
            oid: entry.oid.to_owned(),
        })
        .cancelled()
    {
        return Err(Error::Cancelled);
    }
    if entry.mode.is_tree() {
        delegate.pop_path_component();
        delegate.push_back_tracked_path_component(entry.filename);
        queue.push_back((Some(entry.oid.to_owned()), None));
    }
    Ok(())
}

fn add_entry_schedule_recursion<R: tree::Visit>(
    entry: EntryRef<'_>,
    queue: &mut VecDeque<TreeInfoPair>,
    delegate: &mut R,
) -> Result<(), Error> {
    delegate.push_path_component(entry.filename);
    if delegate
        .visit(Change::Addition {
            entry_mode: entry.mode,
            oid: entry.oid.to_owned(),
        })
        .cancelled()
    {
        return Err(Error::Cancelled);
    }
    if entry.mode.is_tree() {
        delegate.pop_path_component();
        delegate.push_back_tracked_path_component(entry.filename);
        queue.push_back((None, Some(entry.oid.to_owned())))
    }
    Ok(())
}
fn catchup_rhs_with_lhs<R: tree::Visit>(
    rhs_entries: &mut IteratorType<gix_object::TreeRefIter<'_>>,
    lhs: EntryRef<'_>,
    rhs: EntryRef<'_>,
    queue: &mut VecDeque<TreeInfoPair>,
    delegate: &mut R,
) -> Result<(), Error> {
    use std::cmp::Ordering::*;
    add_entry_schedule_recursion(rhs, queue, delegate)?;
    loop {
        match rhs_entries.peek() {
            Some(Ok(rhs)) => match compare(&lhs, rhs) {
                Equal => {
                    let rhs = rhs_entries.next().transpose()?.expect("the peeked item to be present");
                    delegate.pop_path_component();
                    handle_lhs_and_rhs_with_equal_filenames(lhs, rhs, queue, delegate)?;
                    break;
                }
                Greater => {
                    let rhs = rhs_entries.next().transpose()?.expect("the peeked item to be present");
                    delegate.pop_path_component();
                    add_entry_schedule_recursion(rhs, queue, delegate)?;
                }
                Less => {
                    delegate.pop_path_component();
                    delete_entry_schedule_recursion(lhs, queue, delegate)?;
                    break;
                }
            },
            Some(Err(err)) => return Err(Error::EntriesDecode(err.to_owned())),
            None => {
                delegate.pop_path_component();
                delete_entry_schedule_recursion(lhs, queue, delegate)?;
                break;
            }
        }
    }
    Ok(())
}

fn catchup_lhs_with_rhs<R: tree::Visit>(
    lhs_entries: &mut IteratorType<gix_object::TreeRefIter<'_>>,
    lhs: EntryRef<'_>,
    rhs: EntryRef<'_>,
    queue: &mut VecDeque<TreeInfoPair>,
    delegate: &mut R,
) -> Result<(), Error> {
    use std::cmp::Ordering::*;
    delete_entry_schedule_recursion(lhs, queue, delegate)?;
    loop {
        match lhs_entries.peek() {
            Some(Ok(lhs)) => match compare(lhs, &rhs) {
                Equal => {
                    let lhs = lhs_entries.next().expect("the peeked item to be present")?;
                    delegate.pop_path_component();
                    handle_lhs_and_rhs_with_equal_filenames(lhs, rhs, queue, delegate)?;
                    break;
                }
                Less => {
                    let lhs = lhs_entries.next().expect("the peeked item to be present")?;
                    delegate.pop_path_component();
                    delete_entry_schedule_recursion(lhs, queue, delegate)?;
                }
                Greater => {
                    delegate.pop_path_component();
                    add_entry_schedule_recursion(rhs, queue, delegate)?;
                    break;
                }
            },
            Some(Err(err)) => return Err(Error::EntriesDecode(err.to_owned())),
            None => {
                delegate.pop_path_component();
                add_entry_schedule_recursion(rhs, queue, delegate)?;
                break;
            }
        }
    }
    Ok(())
}

fn handle_lhs_and_rhs_with_equal_filenames<R: tree::Visit>(
    lhs: EntryRef<'_>,
    rhs: EntryRef<'_>,
    queue: &mut VecDeque<TreeInfoPair>,
    delegate: &mut R,
) -> Result<(), Error> {
    match (lhs.mode.is_tree(), rhs.mode.is_tree()) {
        (true, true) => {
            delegate.push_back_tracked_path_component(lhs.filename);
            if lhs.oid != rhs.oid
                && delegate
                    .visit(Change::Modification {
                        previous_entry_mode: lhs.mode,
                        previous_oid: lhs.oid.to_owned(),
                        entry_mode: rhs.mode,
                        oid: rhs.oid.to_owned(),
                    })
                    .cancelled()
            {
                return Err(Error::Cancelled);
            }
            queue.push_back((Some(lhs.oid.to_owned()), Some(rhs.oid.to_owned())));
        }
        (_, true) => {
            delegate.push_back_tracked_path_component(lhs.filename);
            if delegate
                .visit(Change::Deletion {
                    entry_mode: lhs.mode,
                    oid: lhs.oid.to_owned(),
                })
                .cancelled()
            {
                return Err(Error::Cancelled);
            };
            if delegate
                .visit(Change::Addition {
                    entry_mode: rhs.mode,
                    oid: rhs.oid.to_owned(),
                })
                .cancelled()
            {
                return Err(Error::Cancelled);
            };
            queue.push_back((None, Some(rhs.oid.to_owned())));
        }
        (true, _) => {
            delegate.push_back_tracked_path_component(lhs.filename);
            if delegate
                .visit(Change::Deletion {
                    entry_mode: lhs.mode,
                    oid: lhs.oid.to_owned(),
                })
                .cancelled()
            {
                return Err(Error::Cancelled);
            }
            if delegate
                .visit(Change::Addition {
                    entry_mode: rhs.mode,
                    oid: rhs.oid.to_owned(),
                })
                .cancelled()
            {
                return Err(Error::Cancelled);
            };
            queue.push_back((Some(lhs.oid.to_owned()), None));
        }
        (false, false) => {
            delegate.push_path_component(lhs.filename);
            debug_assert!(lhs.mode.is_no_tree() && lhs.mode.is_no_tree());
            if lhs.oid != rhs.oid
                && delegate
                    .visit(Change::Modification {
                        previous_entry_mode: lhs.mode,
                        previous_oid: lhs.oid.to_owned(),
                        entry_mode: rhs.mode,
                        oid: rhs.oid.to_owned(),
                    })
                    .cancelled()
            {
                return Err(Error::Cancelled);
            }
        }
    };
    Ok(())
}

type IteratorType<I> = std::mem::ManuallyDrop<std::iter::Peekable<I>>;

fn peekable<I: Iterator>(iter: I) -> IteratorType<I> {
    std::mem::ManuallyDrop::new(iter.peekable())
}

#[cfg(test)]
mod tests {
    use std::cmp::Ordering;

    use gix_object::tree::EntryKind;

    use super::*;

    #[test]
    fn compare_select_samples() {
        let null = gix_hash::ObjectId::null(gix_hash::Kind::Sha1);
        let actual = compare(
            &EntryRef {
                mode: EntryKind::Blob.into(),
                filename: "plumbing-cli.rs".into(),
                oid: &null,
            },
            &EntryRef {
                mode: EntryKind::Tree.into(),
                filename: "plumbing".into(),
                oid: &null,
            },
        );
        assert_eq!(actual, Ordering::Less);
        let actual = compare(
            &EntryRef {
                mode: EntryKind::Tree.into(),
                filename: "plumbing-cli.rs".into(),
                oid: &null,
            },
            &EntryRef {
                mode: EntryKind::Blob.into(),
                filename: "plumbing".into(),
                oid: &null,
            },
        );
        assert_eq!(actual, Ordering::Greater);
    }
}