package dehub

import (
	"bytes"
	"encoding/base64"
	"errors"
	"fmt"
	"reflect"
	"sort"
	"strings"
	"time"

	"dehub.dev/src/dehub.git/accessctl"
	"dehub.dev/src/dehub.git/fs"
	"dehub.dev/src/dehub.git/sigcred"
	"dehub.dev/src/dehub.git/typeobj"

	"gopkg.in/src-d/go-git.v4"
	"gopkg.in/src-d/go-git.v4/plumbing"
	"gopkg.in/src-d/go-git.v4/plumbing/object"
	yaml "gopkg.in/yaml.v2"
)

// CommitInterface describes the methods which must be implemented by the
// different commit types. None of the methods should modify the underlying
// object.
type CommitInterface interface {
	// MessageHead returns the head of the commit message (i.e. the first line).
	// The CommitCommon of the outer Commit is passed in for added context, if
	// necessary.
	MessageHead(CommitCommon) (string, error)

	// ExpectedHash returns the raw hash which Signifiers can sign to accredit
	// this commit. The ChangedFile objects given describe the file changes
	// between the parent commit and this commit.
	ExpectedHash([]ChangedFile) ([]byte, error)

	// StoredHash returns the signable Hash embedded in the commit, which should
	// hopefully correspond to the ExpectedHash.
	StoredHash() []byte
}

// CommitCommon describes the fields common to all Commit objects.
type CommitCommon struct {
	// Credentials represent all created Credentials for this commit, and can be
	// set on all Commit objects regardless of other fields being set.
	Credentials []sigcred.Credential `yaml:"credentials"`
}

func (cc CommitCommon) credIDs() []string {
	m := map[string]struct{}{}
	for _, cred := range cc.Credentials {
		if cred.AccountID != "" {
			m[cred.AccountID] = struct{}{}
		} else if cred.AnonID != "" {
			m[cred.AnonID] = struct{}{}
		}
	}
	s := make([]string, 0, len(m))
	for id := range m {
		s = append(s, id)
	}
	sort.Strings(s)
	return s
}

func abbrevCommitMessage(msg string) string {
	i := strings.Index(msg, "\n")
	if i > 0 {
		msg = msg[:i]
	}
	if len(msg) > 80 {
		msg = msg[:80] + "..."
	}
	return msg
}

// Commit represents a single Commit which is being added to a branch. Only one
// field should be set on a Commit, unless otherwise noted.
type Commit struct {
	Change     *CommitChange     `type:"change,default"`
	Credential *CommitCredential `type:"credential"`
	Comment    *CommitComment    `type:"comment"`

	Common CommitCommon `yaml:",inline"`
}

// MarshalYAML implements the yaml.Marshaler interface.
func (c Commit) MarshalYAML() (interface{}, error) {
	return typeobj.MarshalYAML(c)
}

// UnmarshalYAML implements the yaml.Unmarshaler interface.
func (c *Commit) UnmarshalYAML(unmarshal func(interface{}) error) error {
	return typeobj.UnmarshalYAML(c, unmarshal)
}

// Interface returns the CommitInterface instance encapsulated by this Commit
// object.
func (c Commit) Interface() (CommitInterface, error) {
	el, _, err := typeobj.Element(c)
	if err != nil {
		return nil, err
	}
	return el.(CommitInterface), nil
}

// Type returns the Commit's type (as would be used in its YAML "type" field).
func (c Commit) Type() (string, error) {
	_, typeStr, err := typeobj.Element(c)
	if err != nil {
		return "", err
	}
	return typeStr, nil
}

// MarshalText implements the encoding.TextMarshaler interface by returning the
// form the Commit object takes in the git commit message.
func (c Commit) MarshalText() ([]byte, error) {
	commitInt, err := c.Interface()
	if err != nil {
		return nil, fmt.Errorf("could not cast Commit %+v to interface : %w", c, err)
	}

	msgHead, err := commitInt.MessageHead(c.Common)
	if err != nil {
		return nil, fmt.Errorf("error constructing message head: %w", err)
	}

	msgBodyB, err := yaml.Marshal(c)
	if err != nil {
		return nil, fmt.Errorf("error marshaling commit %+v as yaml: %w", c, err)
	}

	w := new(bytes.Buffer)
	w.WriteString(msgHead)
	w.WriteString("\n\n---\n")
	w.Write(msgBodyB)
	return w.Bytes(), nil
}

// UnmarshalText implements the encoding.TextUnmarshaler interface by decoding a
// Commit object which has been encoded into a git commit message.
func (c *Commit) UnmarshalText(msg []byte) error {
	i := bytes.Index(msg, []byte("\n"))
	if i < 0 {
		return fmt.Errorf("commit message %q is malformed, it has no body", msg)
	}
	msgBody := msg[i:]

	if err := yaml.Unmarshal(msgBody, c); err != nil {
		return fmt.Errorf("could not unmarshal Commit message from yaml: %w", err)

	} else if reflect.DeepEqual(*c, Commit{}) {
		// a basic check, but worthwhile
		return errors.New("commit message is malformed, could not unmarshal yaml object")
	}

	return nil
}

// AccreditCommit returns the given Commit with an appended Credential provided
// by the given SignifierInterface.
func (r *Repo) AccreditCommit(commit Commit, sigInt sigcred.SignifierInterface) (Commit, error) {
	commitInt, err := commit.Interface()
	if err != nil {
		return commit, fmt.Errorf("could not cast commit %+v to interface: %w", commit, err)
	}

	headFS, err := r.headFS()
	if err != nil {
		return commit, fmt.Errorf("could not grab snapshot of HEAD fs: %w", err)
	}

	cred, err := sigInt.Sign(headFS, commitInt.StoredHash())
	if err != nil {
		return commit, fmt.Errorf("could not accredit change commit: %w", err)
	}
	commit.Common.Credentials = append(commit.Common.Credentials, cred)
	return commit, nil
}

// CommitBareParams are the parameters to the CommitBare method. All are
// required, unless otherwise noted.
type CommitBareParams struct {
	Commit     Commit
	Author     string
	ParentHash plumbing.Hash // can be zero if the commit has no parents (Q_Q)
	GitTree    *object.Tree
}

// CommitBare constructs a git commit object and and stores it, returning the
// resulting GitCommit. This method does not interact with HEAD at all.
func (r *Repo) CommitBare(params CommitBareParams) (GitCommit, error) {
	msgB, err := params.Commit.MarshalText()
	if err != nil {
		return GitCommit{}, fmt.Errorf("encoding %T to message string: %w",
			params.Commit, err)
	}

	author := object.Signature{
		Name: params.Author,
		When: time.Now(),
	}
	commit := &object.Commit{
		Author:    author,
		Committer: author,
		Message:   string(msgB),
		TreeHash:  params.GitTree.Hash,
	}
	if params.ParentHash != plumbing.ZeroHash {
		commit.ParentHashes = []plumbing.Hash{params.ParentHash}
	}

	commitObj := r.GitRepo.Storer.NewEncodedObject()
	if err := commit.Encode(commitObj); err != nil {
		return GitCommit{}, fmt.Errorf("encoding commit object: %w", err)
	}

	commitHash, err := r.GitRepo.Storer.SetEncodedObject(commitObj)
	if err != nil {
		return GitCommit{}, fmt.Errorf("setting encoded object: %w", err)
	}

	return r.GetGitCommit(commitHash)
}

// Commit uses the given Commit to create a git commit object and commits it to
// the current HEAD, returning the full GitCommit.
func (r *Repo) Commit(commit Commit) (GitCommit, error) {
	headRef, err := r.TraverseReferenceChain(plumbing.HEAD, func(ref *plumbing.Reference) bool {
		return ref.Type() == plumbing.HashReference
	})
	if err != nil {
		return GitCommit{}, fmt.Errorf("resolving HEAD to a hash reference: %w", err)
	}
	headRefName := headRef.Name()

	headHash, err := r.ReferenceToHash(headRefName)
	if err != nil {
		return GitCommit{}, fmt.Errorf("resolving ref %q (HEAD): %w", headRefName, err)
	}

	// TODO this is also used in the same way in NewCommitChange. It might make
	// sense to refactor this logic out, it might not be needed in fs at all.
	_, stagedTree, err := fs.FromStagedChangesTree(r.GitRepo)
	if err != nil {
		return GitCommit{}, fmt.Errorf("getting staged changes: %w", err)
	}

	gitCommit, err := r.CommitBare(CommitBareParams{
		Commit:     commit,
		Author:     strings.Join(commit.Common.credIDs(), ", "),
		ParentHash: headHash,
		GitTree:    stagedTree,
	})
	if err != nil {
		return GitCommit{}, err
	}

	// now set the branch to this new commit
	newHeadRef := plumbing.NewHashReference(headRefName, gitCommit.GitCommit.Hash)
	if err := r.GitRepo.Storer.SetReference(newHeadRef); err != nil {
		return GitCommit{}, fmt.Errorf("setting reference %q to new commit hash %q: %w",
			headRefName, gitCommit.GitCommit.Hash, err)
	}

	return gitCommit, nil
}

// HasStagedChanges returns true if there are file changes which have been
// staged (e.g. via "git add").
func (r *Repo) HasStagedChanges() (bool, error) {
	w, err := r.GitRepo.Worktree()
	if err != nil {
		return false, fmt.Errorf("error retrieving worktree: %w", err)
	}

	status, err := w.Status()
	if err != nil {
		return false, fmt.Errorf("error retrieving worktree status: %w", err)
	}

	var any bool
	for _, fileStatus := range status {
		if fileStatus.Staging != git.Unmodified &&
			fileStatus.Staging != git.Untracked {
			any = true
			break
		}
	}
	return any, nil
}

// VerifyCommits verifies that the given commits, which are presumably on the
// given branch, are gucci.
func (r *Repo) VerifyCommits(branch plumbing.ReferenceName, gitCommits []GitCommit) error {

	// First determine the root of the main branch. All commits need to be an
	// ancestor of it.
	var root plumbing.Hash
	mainGitCommit, err := r.GetGitRevision(plumbing.Revision(MainRefName))
	if err != nil {
		return fmt.Errorf("retrieving commit at HEAD of main: %w", err)
	}

	rootCommit := mainGitCommit.GitCommit
	for {
		if rootCommit.NumParents() == 0 {
			break
		} else if rootCommit.NumParents() > 1 {
			return fmt.Errorf("commit %q in main branch has more than one parent", root)
		} else if rootCommit, err = rootCommit.Parent(0); err != nil {
			return fmt.Errorf("retrieving parent commit of %q: %w", root, err)
		}
	}

	for i, gitCommit := range gitCommits {
		// It's not a requirement that the given GitCommits are in ancestral
		// order, but usually they are, so we can help verifyCommit not have to
		// calculate the parentTree if the previous commit is the parent of this
		// one, and not have to determine that each commit is an ancestor of
		// main manually.
		var parentTree *object.Tree
		if i > 0 && gitCommits[i-1].GitCommit.Hash == gitCommit.GitCommit.ParentHashes[0] {
			parentTree = gitCommits[i-1].GitTree

		} else if gitCommit.GitCommit.Hash == rootCommit.Hash {
			// looking at the root commit itself, assume it's ok

		} else if isAncestor, err := rootCommit.IsAncestor(gitCommit.GitCommit); err != nil {
			return fmt.Errorf("determining if %q is an ancestor of %q (root of main): %w",
				gitCommit.GitCommit.Hash, rootCommit.Hash, err)

		} else if !isAncestor {
			return fmt.Errorf("%q is not an ancestor of %q (root of main)",
				gitCommit.GitCommit.Hash, rootCommit.Hash)
		}

		if err := r.verifyCommit(branch, gitCommit, parentTree); err != nil {
			return fmt.Errorf("verifying commit %q: %w",
				gitCommit.GitCommit.Hash, err)
		}
	}
	return nil
}

// parentTree returns the tree of the parent commit of the given commit. If the
// given commit has no parents then a bare tree is returned.
func (r *Repo) parentTree(commitObj *object.Commit) (*object.Tree, error) {
	switch commitObj.NumParents() {
	case 0:
		return new(object.Tree), nil
	case 1:
		if parentCommitObj, err := commitObj.Parent(0); err != nil {
			return nil, fmt.Errorf("getting parent commit %q: %w",
				commitObj.ParentHashes[0], err)
		} else if parentTree, err := r.GitRepo.TreeObject(parentCommitObj.TreeHash); err != nil {
			return nil, fmt.Errorf("getting parent tree object %q: %w",
				parentCommitObj.TreeHash, err)
		} else {
			return parentTree, nil
		}
	default:
		return nil, errors.New("commit has multiple parents")
	}
}

// if parentTree is nil then it will be inferred.
func (r *Repo) verifyCommit(branch plumbing.ReferenceName, gitCommit GitCommit, parentTree *object.Tree) error {
	parentTree, err := r.parentTree(gitCommit.GitCommit)
	if err != nil {
		return fmt.Errorf("retrieving parent tree of commit: %w", err)
	}

	var sigFS fs.FS
	if gitCommit.Root() {
		sigFS = fs.FromTree(gitCommit.GitTree)
	} else {
		sigFS = fs.FromTree(parentTree)
	}

	cfg, err := r.loadConfig(sigFS)
	if err != nil {
		return fmt.Errorf("loading config of parent %q: %w",
			gitCommit.GitCommit.ParentHashes[0], err)
	}

	// assert access controls
	changedFiles, err := ChangedFilesBetweenTrees(parentTree, gitCommit.GitTree)
	if err != nil {
		return fmt.Errorf("calculating diff from tree %q to tree %q: %w",
			parentTree.Hash, gitCommit.GitTree.Hash, err)

	} else if len(changedFiles) > 0 && gitCommit.Commit.Change == nil {
		return errors.New("files changes but commit is not a change commit")
	}

	pathsChanged := make([]string, len(changedFiles))
	for i := range changedFiles {
		pathsChanged[i] = changedFiles[i].Path
	}

	commitType, err := gitCommit.Commit.Type()
	if err != nil {
		return fmt.Errorf("determining type of commit %+v: %w", gitCommit.Commit, err)
	}

	err = accessctl.AssertCanCommit(cfg.AccessControls, accessctl.CommitRequest{
		Type:         commitType,
		Branch:       branch.Short(),
		Credentials:  gitCommit.Commit.Common.Credentials,
		FilesChanged: pathsChanged,
	})
	if err != nil {
		return fmt.Errorf("asserting access controls: %w", err)
	}

	// ensure the hash is what it's expected to be
	storedCommitHash := gitCommit.Interface.StoredHash()
	expectedCommitHash, err := gitCommit.Interface.ExpectedHash(changedFiles)
	if err != nil {
		return fmt.Errorf("calculating expected commit hash: %w", err)
	} else if !bytes.Equal(storedCommitHash, expectedCommitHash) {
		return fmt.Errorf("unexpected hash in commit body, is %s but should be %s",
			base64.StdEncoding.EncodeToString(storedCommitHash),
			base64.StdEncoding.EncodeToString(expectedCommitHash))
	}

	// verify all credentials
	for _, cred := range gitCommit.Commit.Common.Credentials {
		if cred.AccountID == "" {
			if err := cred.SelfVerify(expectedCommitHash); err != nil {
				return fmt.Errorf("verifying credential %+v: %w", cred, err)
			}
		} else {
			sig, err := r.signifierForCredential(sigFS, cred)
			if err != nil {
				return fmt.Errorf("finding signifier for credential %+v: %w", cred, err)
			} else if err := sig.Verify(sigFS, expectedCommitHash, cred); err != nil {
				return fmt.Errorf("verifying credential %+v: %w", cred, err)
			}
		}
	}

	return nil
}

type changeRangeInfo struct {
	changeCommits      []GitCommit
	authors            map[string]struct{}
	msg                string
	startTree, endTree *object.Tree
	changeHash         []byte
}

// changeRangeInfo returns various pieces of information about a range of
// commits' changes.
func (r *Repo) changeRangeInfo(commits []GitCommit) (changeRangeInfo, error) {
	info := changeRangeInfo{
		authors: map[string]struct{}{},
	}

	for _, commit := range commits {
		if _, ok := commit.Interface.(*CommitChange); ok {
			info.changeCommits = append(info.changeCommits, commit)
			for _, cred := range commit.Commit.Common.Credentials {
				info.authors[cred.AccountID] = struct{}{}
			}
		}
	}
	if len(info.changeCommits) == 0 {
		return changeRangeInfo{}, errors.New("no change commits found")
	}

	// startTree has to be the tree of the parent of the first commit, which
	// isn't included in commits. Determine it the hard way.
	var err error
	if info.startTree, err = r.parentTree(commits[0].GitCommit); err != nil {
		return changeRangeInfo{}, fmt.Errorf("getting tree of parent of %q: %w",
			commits[0].GitCommit.Hash, err)
	}

	lastChangeCommit := info.changeCommits[len(info.changeCommits)-1]
	info.msg = lastChangeCommit.Commit.Change.Message
	info.endTree = lastChangeCommit.GitTree

	changedFiles, err := ChangedFilesBetweenTrees(info.startTree, info.endTree)
	if err != nil {
		return changeRangeInfo{}, fmt.Errorf("calculating diff of commit trees %q and %q: %w",
			info.startTree.Hash, info.endTree.Hash, err)
	}

	info.changeHash = genChangeHash(nil, info.msg, changedFiles)
	return info, nil
}