// Package dehub TODO needs package docs
package dehub

import (
	"dehub/fs"
	"errors"
	"fmt"
	"path/filepath"

	"gopkg.in/src-d/go-billy.v4"
	"gopkg.in/src-d/go-billy.v4/memfs"
	"gopkg.in/src-d/go-git.v4"
	"gopkg.in/src-d/go-git.v4/plumbing"
	"gopkg.in/src-d/go-git.v4/plumbing/object"
	"gopkg.in/src-d/go-git.v4/storage"
	"gopkg.in/src-d/go-git.v4/storage/memory"
)

const (
	// DehubDir defines the name of the directory where all dehub-related files are
	// expected to be found.
	DehubDir = ".dehub"
)

var (
	// ConfigPath defines the expected path to the Repo's configuration file.
	ConfigPath = filepath.Join(DehubDir, "config.yml")

	// Trunk defines the name of the trunk branch.
	Trunk = "trunk"

	// TrunkRefName defines the reference name of the trunk branch.
	TrunkRefName = plumbing.NewBranchReferenceName(Trunk)
)

type repoOpts struct {
	bare bool
}

// OpenOption is an option which can be passed to the OpenRepo function to
// affect the Repo's behavior.
type OpenOption func(*repoOpts)

// OpenBare returns an OpenOption which, if true is given, causes the OpenRepo
// function to expect to open a bare repo.
func OpenBare(bare bool) OpenOption {
	return func(o *repoOpts) {
		o.bare = bare
	}
}

// Repo is an object which allows accessing and modifying the dehub repo.
type Repo struct {
	GitRepo *git.Repository
	Storer  storage.Storer
}

// OpenRepo opens the dehub repo in the given directory and returns the object
// for it.
//
// The given path is expected to have a git repo and .dehub folder already
// initialized.
func OpenRepo(path string, options ...OpenOption) (*Repo, error) {
	var opts repoOpts
	for _, opt := range options {
		opt(&opts)
	}

	r := Repo{}
	var err error
	openOpts := &git.PlainOpenOptions{
		DetectDotGit: !opts.bare,
	}
	if r.GitRepo, err = git.PlainOpenWithOptions(path, openOpts); err != nil {
		return nil, fmt.Errorf("could not open git repo: %w", err)
	}

	return &r, nil
}

// InitMemRepo initializes an empty repository which only exists in memory.
func InitMemRepo() *Repo {
	r, err := git.Init(memory.NewStorage(), memfs.New())
	if err != nil {
		panic(err)
	}

	repo := &Repo{GitRepo: r}
	if err := repo.init(); err != nil {
		panic(err)
	}
	return repo
}

func (r *Repo) init() error {
	h := plumbing.NewSymbolicReference(plumbing.HEAD, TrunkRefName)
	if err := r.GitRepo.Storer.SetReference(h); err != nil {
		return fmt.Errorf("could not set HEAD to %q: %w", TrunkRefName, err)
	}
	return nil
}

func (r *Repo) billyFilesystem() (billy.Filesystem, error) {
	w, err := r.GitRepo.Worktree()
	if err != nil {
		return nil, fmt.Errorf("could not open git worktree: %w", err)
	}
	return w.Filesystem, nil
}

// CheckedOutBranch returns the name of the currently checked out branch.
func (r *Repo) CheckedOutBranch() (plumbing.ReferenceName, error) {
	// Head() can't be used for this, because it doesn't handle the case of a
	// newly initialized repo very well.
	ogRef, err := r.GitRepo.Storer.Reference(plumbing.HEAD)
	if err != nil {
		return "", fmt.Errorf("couldn't de-reference HEAD (is it a bare repo?): %w", err)
	}

	ref := ogRef
	for {
		if ref.Type() != plumbing.SymbolicReference {
			break
		}

		target := ref.Target()
		if target.IsBranch() {
			return target, nil
		}

		ref, err = r.GitRepo.Storer.Reference(target)
		if err != nil {
			break
		}
	}

	return "", fmt.Errorf("could not de-reference HEAD to a branch: %w", err)
}

func (r *Repo) head() (*object.Commit, *object.Tree, error) {
	head, err := r.GitRepo.Head()
	if err != nil {
		return nil, nil, fmt.Errorf("could not get repo HEAD: %w", err)
	}

	headHash := head.Hash()
	headCommit, err := r.GitRepo.CommitObject(headHash)
	if err != nil {
		return nil, nil, fmt.Errorf("could not get commit at HEAD (%q): %w", headHash, err)
	}

	headTree, err := r.GitRepo.TreeObject(headCommit.TreeHash)
	if err != nil {
		return nil, nil, fmt.Errorf("could not get tree object at HEAD (commit:%q tree:%q): %w",
			headHash, headCommit.TreeHash, err)
	}

	return headCommit, headTree, nil
}

// headOrRawFS returns an FS based on the HEAD commit, or if there is no HEAD
// commit (it's an empty repo) an FS based on the raw filesystem.
func (r *Repo) headOrRawFS() (fs.FS, error) {
	_, headTree, err := r.head()
	if errors.Is(err, plumbing.ErrReferenceNotFound) {
		bfs, err := r.billyFilesystem()
		if err != nil {
			return nil, fmt.Errorf("could not get underlying filesystem: %w", err)
		}
		return fs.FromBillyFilesystem(bfs), nil
	} else if err != nil {
		return nil, fmt.Errorf("could not get HEAD tree: %w", err)
	}
	return fs.FromTree(headTree), nil
}