// Package dehub TODO needs package docs
package dehub

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"

	"dehub.dev/src/dehub.git/fs"

	"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/cache"
	"gopkg.in/src-d/go-git.v4/plumbing/format/config"
	"gopkg.in/src-d/go-git.v4/storage"
	"gopkg.in/src-d/go-git.v4/storage/filesystem"
)

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

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

	// Main defines the name of the main branch.
	Main = "main"

	// MainRefName defines the reference name of the main branch.
	MainRefName = plumbing.NewBranchReferenceName(Main)
)

type openOpts struct {
	bare bool
}

// OpenOption is an option which can be passed to the OpenProject function to
// affect the Project's behavior.
type OpenOption func(*openOpts)

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

// Project implements accessing and modifying a local dehub project, as well as
// extending the functionality of the underlying git repo in ways which are
// specifically useful for dehub projects.
type Project struct {
	// GitRepo is the git repository which houses the project.
	GitRepo *git.Repository

	// GitDirFS corresponds to the .git directory (or the entire repo directory
	// if it's a bare repo)
	GitDirFS billy.Filesystem
}

func extractGitDirFS(storer storage.Storer) (billy.Filesystem, error) {
	dotGitFSer, ok := storer.(interface{ Filesystem() billy.Filesystem })
	if !ok {
		return nil, fmt.Errorf("git storage object of type %T does not expose its underlying filesystem",
			storer)
	}
	return dotGitFSer.Filesystem(), nil
}

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

	proj := Project{}
	var err error
	openOpts := &git.PlainOpenOptions{
		DetectDotGit: !opts.bare,
	}
	if proj.GitRepo, err = git.PlainOpenWithOptions(path, openOpts); err != nil {
		return nil, fmt.Errorf("opening git repo: %w", err)

	} else if proj.GitDirFS, err = extractGitDirFS(proj.GitRepo.Storer); err != nil {
		return nil, err
	}

	return &proj, nil
}

type initOpts struct {
	bare   bool
	remote bool
}

// InitOption is an option which can be passed into the Init functions to affect
// their behavior.
type InitOption func(*initOpts)

// InitBareRepo returns an InitOption which, if true is given, causes the Init
// function to initialize the project's git repo without a worktree.
func InitBareRepo(bare bool) InitOption {
	return func(o *initOpts) {
		o.bare = bare
	}
}

// InitRemoteRepo returns an InitOption which, if true is given, causes the Init
// function to initialize the project's git repo with certain git configuration
// options set which make the repo able to be used as a remote repo.
func InitRemoteRepo(remote bool) InitOption {
	return func(o *initOpts) {
		o.remote = remote
	}
}

// InitProject will initialize a new project at the given path. If bare is true
// then the project's git repo will not have a worktree.
func InitProject(path string, options ...InitOption) (*Project, error) {
	var opts initOpts
	for _, opt := range options {
		opt(&opts)
	}

	var proj Project
	var err error
	if proj.GitRepo, err = git.PlainInit(path, opts.bare); err != nil {
		return nil, fmt.Errorf("initializing git repo: %w", err)

	} else if proj.GitDirFS, err = extractGitDirFS(proj.GitRepo.Storer); err != nil {
		return nil, err

	} else if err = proj.init(opts); err != nil {
		return nil, fmt.Errorf("initializing repo with dehub defaults: %w", err)
	}

	return &proj, nil
}

// InitMemProject initializes an empty project which only exists in memory.
func InitMemProject(options ...InitOption) *Project {
	var opts initOpts
	for _, opt := range options {
		opt(&opts)
	}

	fs := memfs.New()
	dotGitFS, err := fs.Chroot(git.GitDirName)
	if err != nil {
		panic(err)
	}
	storage := filesystem.NewStorage(dotGitFS, cache.NewObjectLRUDefault())

	var worktree billy.Filesystem
	if !opts.bare {
		worktree = fs
	}

	r, err := git.Init(storage, worktree)
	if err != nil {
		panic(err)
	}

	proj := &Project{GitRepo: r, GitDirFS: dotGitFS}
	if err := proj.init(opts); err != nil {
		panic(err)
	}
	return proj
}

func (proj *Project) initRemotePreReceive(bare bool) error {
	if err := proj.GitDirFS.MkdirAll("hooks", 0755); err != nil {
		return fmt.Errorf("creating hooks directory: %w", err)
	}

	preRcvFlags := os.O_WRONLY | os.O_CREATE | os.O_TRUNC
	preRcv, err := proj.GitDirFS.OpenFile("hooks/pre-receive", preRcvFlags, 0755)
	if err != nil {
		return fmt.Errorf("opening hooks/pre-receive file: %w", err)
	}
	defer preRcv.Close()

	var preRcvBody string
	if bare {
		preRcvBody = "#!/bin/sh\nexec dehub hook -bare pre-receive\n"
	} else {
		preRcvBody = "#!/bin/sh\nexec dehub hook pre-receive\n"
	}

	if _, err := io.Copy(preRcv, bytes.NewBufferString(preRcvBody)); err != nil {
		return fmt.Errorf("writing to hooks/pre-receive: %w", err)
	}
	return nil
}

func (proj *Project) init(opts initOpts) error {
	headRef := plumbing.NewSymbolicReference(plumbing.HEAD, MainRefName)
	if err := proj.GitRepo.Storer.SetReference(headRef); err != nil {
		return fmt.Errorf("setting HEAD reference to %q: %w", MainRefName, err)
	}

	if opts.remote {
		cfg, err := proj.GitRepo.Config()
		if err != nil {
			return fmt.Errorf("opening git cfg: %w", err)
		}

		cfg.Raw = cfg.Raw.AddOption("http", config.NoSubsection, "receivepack", "true")
		if err := proj.GitRepo.Storer.SetConfig(cfg); err != nil {
			return fmt.Errorf("storing modified git config: %w", err)
		}

		if err := proj.initRemotePreReceive(opts.bare); err != nil {
			return fmt.Errorf("initializing pre-receive hook for remote-enabled repo: %w", err)
		}
	}

	return nil
}

func (proj *Project) billyFilesystem() (billy.Filesystem, error) {
	w, err := proj.GitRepo.Worktree()
	if err != nil {
		return nil, fmt.Errorf("opening git worktree: %w", err)
	}
	return w.Filesystem, nil
}

var errTraverseRefNoMatch = errors.New("failed to find reference matching given predicate")

// TraverseReferenceChain resolves a chain of references, calling the given
// predicate on each one, and returning the first one for which the predicate
// returns true. This method will return an error if it reaches the end of the
// chain and the predicate still has not returned true.
//
// If a reference name is encountered which does not actually exist, then it is
// assumed to be a hash reference to the zero hash.
func (proj *Project) TraverseReferenceChain(refName plumbing.ReferenceName, pred func(*plumbing.Reference) bool) (*plumbing.Reference, error) {
	// TODO infinite loop checking
	// TODO check that this (and the methods which use it) are actually useful
	for {
		ref, err := proj.GitRepo.Storer.Reference(refName)
		if errors.Is(err, plumbing.ErrReferenceNotFound) {
			ref = plumbing.NewHashReference(refName, plumbing.ZeroHash)
		} else if err != nil {
			return nil, fmt.Errorf("resolving reference %q: %w", refName, err)
		}

		if pred(ref) {
			return ref, nil
		} else if ref.Type() != plumbing.SymbolicReference {
			return nil, errTraverseRefNoMatch
		}
		refName = ref.Target()
	}
}

// ErrNoBranchReference is returned from ReferenceToBranchName if no reference
// in the reference chain is for a branch.
var ErrNoBranchReference = errors.New("no branch reference found")

// ReferenceToBranchName traverses a chain of references looking for the first
// branch reference, and returns that name, or returns ErrNoBranchReference if
// no branch reference is part of the chain.
func (proj *Project) ReferenceToBranchName(refName plumbing.ReferenceName) (plumbing.ReferenceName, error) {
	// first check if the given refName is a branch, if so just return that.
	if refName.IsBranch() {
		return refName, nil
	}

	ref, err := proj.TraverseReferenceChain(refName, func(ref *plumbing.Reference) bool {
		return ref.Target().IsBranch()
	})
	if errors.Is(err, errTraverseRefNoMatch) {
		return "", ErrNoBranchReference
	} else if err != nil {
		return "", fmt.Errorf("traversing reference chain: %w", err)
	}
	return ref.Target(), nil
}

// ReferenceToHash fully resolves a reference to a hash. If a reference cannot
// be resolved then plumbing.ZeroHash is returned.
func (proj *Project) ReferenceToHash(refName plumbing.ReferenceName) (plumbing.Hash, error) {
	ref, err := proj.TraverseReferenceChain(refName, func(ref *plumbing.Reference) bool {
		return ref.Type() == plumbing.HashReference
	})
	if errors.Is(err, errTraverseRefNoMatch) {
		return plumbing.ZeroHash, errors.New("no hash in reference chain (is this even possible???)")
	} else if errors.Is(err, plumbing.ErrReferenceNotFound) {
		return plumbing.ZeroHash, nil
	} else if err != nil {
		return plumbing.ZeroHash, fmt.Errorf("traversing reference chain: %w", err)
	}
	return ref.Hash(), nil
}

// headFS 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 (proj *Project) headFS() (fs.FS, error) {
	head, err := proj.GetHeadCommit()
	if errors.Is(err, ErrHeadIsZero) {
		bfs, err := proj.billyFilesystem()
		if err != nil {
			return nil, fmt.Errorf("getting 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(head.TreeObject), nil
}