//! Type system for working with git branches

use crate::{Commit, HashId};
use git2::Repository;
use std::{mem, sync::Arc};

/// Abstraction for a branch history slice
///
/// Git implements an acyclical graph, where branches can be split,
/// and re-merge later.  Traversal always happens from some point
/// onwards, backwards through the history.  Because git repositories
/// can get quite large and this is a recursive process, it's very
/// quickly possible to overflow your program stack.  To avoid this,
/// `supergit` uses an iterator design to enumerate commits.
///
/// Use the API on this type to specify your starting point.  By
/// default, it will be the head of the branch you are looking at.
/// Note: not all branches have names!
///
/// After creating a `BranchIter` you can then call `next()` on it,
/// yielding `BranchCommit` objects.  These can either be single
/// commits, or various types of merge commits.  Each merge commit
/// yields some set of `Branch` handles, that you can either traverse
/// by building another `BranchIter`.
///
/// A branch iterator is therefore always first-parent, meaning that
/// merged branches can simply be ignored by only ever inspecting the
/// current `Commit` contained by a `BranchCommit`.
#[derive(Clone)]
pub struct Branch {
    repo: Arc<Repository>,
    name: Option<String>,
    head: HashId,
}

impl Branch {
    /// Create a new branch handle
    pub(crate) fn new(repo: &Arc<Repository>, name: String, head: HashId) -> Self {
        Self {
            repo: Arc::clone(repo),
            name: Some(name),
            head,
        }
    }

    pub(crate) fn without_name(repo: &Arc<Repository>, head: HashId) -> Self {
        Self {
            repo: Arc::clone(repo),
            name: None,
            head,
        }
    }

    /// Get a branch handle starting at a certain commit
    // TODO: do we want to check if this is actually a child?
    pub fn skip_to(&self, from: HashId) -> Self {
        match self.name {
            Some(ref name) => Self::new(&self.repo, name.clone(), from),
            None => Self::without_name(&self.repo, from),
        }
    }

    /// Create a branch handle that skips a certain number of commits
    ///
    /// This walker always picks the first parent.
    pub fn skip(&self, num: usize) -> Self {
        let mut head = self.repo.find_commit(self.head.clone().into()).unwrap();
        for _ in 0..num {
            if let Ok(p) = head.parent(0) {
                head = p;
            }
        }

        match self.name {
            Some(ref name) => Self::new(&self.repo, name.clone(), head.id().into()),
            None => Self::without_name(&self.repo, head.id().into()),
        }
    }

    /// Create a branch iterator that stops when reaching a commit
    pub fn get_to(&self, commit: HashId) -> BranchIter {
        BranchIter::new(
            Arc::clone(&self.repo),
            self.head.clone(),
            SegLimit::Commit(false, commit),
        )
    }

    /// Create a step-limited branch iterator
    ///
    /// This type of iterator is especially useful when combined with
    /// `skip()`, to create a paginated view onto commits.
    pub fn get(&self, num: usize) -> BranchIter {
        BranchIter::new(
            Arc::clone(&self.repo),
            self.head.clone(),
            SegLimit::Length(0, num),
        )
    }

    /// Create an endless branch iterator
    ///
    /// While the creation of the iterator is instantanious, actually
    /// enumerating all commits in a repository can be quite
    /// computationally intensive and is almost never what you
    /// actually want.
    pub fn get_all(&self) -> BranchIter {
        BranchIter::new(Arc::clone(&self.repo), self.head.clone(), SegLimit::None)
    }

    /// Get the current HEAD commit
    pub fn head(&self) -> Commit {
        Commit::new(&self.repo, self.head.clone()).unwrap()
    }

    /// Get the branch name, if it exists
    pub fn name(&self) -> Option<String> {
        self.name.clone()
    }
}

/// A branch slice iterator, created via `Branch` handle
///
/// This iterator yields `BranchCommit` objects, that can either be
/// simple commits, or various types of merge commits with new Branch
/// handles.  This means that without explicitly branching, this
/// iterator is first-parent.
pub struct BranchIter {
    repo: Arc<Repository>,
    curr: Option<HashId>,
    limit: SegLimit,
}

impl BranchIter {
    /// Create a new branch segment iterator
    fn new(repo: Arc<Repository>, last: HashId, limit: SegLimit) -> Self {
        Self {
            repo,
            curr: Some(last),
            limit,
        }
    }

    pub fn current(&self) -> Commit {
        Commit::new(&self.repo, self.curr.as_ref().unwrap().clone()).unwrap()
    }

    /// Get a commit object, if it exists
    fn find_commit(&self, id: &HashId) -> Option<Commit> {
        Commit::new(&self.repo, id.clone())
    }

    /// For a current commit, get it's parents if they exists
    fn parents(&self, curr: &Commit) -> (Option<Commit>, Option<Commit>) {
        (curr.first_parent(), curr.parent(1))
    }

    /// Take an optional commit and turn it into a branch commit
    fn make_branch_commit(&self, curr: Commit) -> BranchCommit {
        match curr.parent_count() {
            0 | 1 => BranchCommit::Commit(curr),
            2 => {
                let p2 = self.parents(&curr).1.unwrap();
                BranchCommit::Merge(curr, Branch::without_name(&self.repo, p2.id))
            }
            _ => BranchCommit::Octopus(
                curr.clone(),
                curr.parents()
                    .into_iter()
                    .map(|c| Branch::without_name(&self.repo, c.id))
                    .collect(),
            ),
        }
    }

    /// Get the current commit
    ///
    /// This function looks either at the "curr" field, or takes the
    /// ID from `cmd`, if it is set to `IterCmd::Jump(...)`, which
    /// indicates that the previous commit was a merge, and we need to escape
    fn set_next(&mut self, current: Commit) -> Commit {
        self.curr = match current.first_parent() {
            Some(p1) => Some(p1.id),
            None => None,
        };

        current
    }
}

impl Iterator for BranchIter {
    type Item = BranchCommit;

    fn next(&mut self) -> Option<Self::Item> {
        mem::replace(&mut self.curr, None)
            .and_then(|id| self.find_commit(&id))
            .map(|c| self.set_next(c))
            .and_then(|c| match self.limit {
                SegLimit::None => Some(c),
                SegLimit::Commit(ended, _) if ended => None,
                SegLimit::Commit(ref mut b, ref target) => {
                    if &c.id == target {
                        *b = true;
                    }

                    Some(c)
                }
                SegLimit::Length(ref mut curr, ref max) if *curr < *max => {
                    *curr += 1;
                    Some(c)
                }
                SegLimit::Length(ref curr, ref mut max) if curr >= max => None,
                SegLimit::Length(_, _) => unreachable!(), // oh rustc :)
            })
            .map(|c| self.make_branch_commit(c))
    }
}
/// Thelimit applied to a branch segment
pub enum SegLimit {
    /// No limit, enumerating all children
    None,
    /// Run until a certain commit is found
    Commit(bool, HashId),
    /// Run to collect a certain number of commits
    Length(usize, usize),
}

/// A commit represented as a relationship to a branch
///
/// Most commits will be simple, meaning they are in sequence on the
/// branch.  Two types of merge commits exist: normal, and octopus.
/// All branches leading into this branch are a reverse tree
pub enum BranchCommit {
    /// A single commit
    Commit(Commit),
    /// A merge commit from one other branch
    Merge(Commit, Branch),
    /// An octopus merge with multiple branches
    Octopus(Commit, Vec<Branch>),
}

impl BranchCommit {
    pub fn id(&self) -> HashId {
        use BranchCommit::*;
        match self {
            Commit(ref c) => &c.id,
            Merge(_, ref b) => &b.head,
            Octopus(ref c, _) => &c.id,
        }
        .clone()
    }

    /// Get the underlying commit, regardless of type
    pub fn commit(&self) -> &Commit {
        use BranchCommit::*;
        match self {
            Commit(ref c) => c,
            Merge(ref c, _) => c,
            Octopus(ref c, _) => c,
        }
    }
}