package dehub

import (
	"bytes"
	"errors"
	"fmt"
	"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"
	"dehub.dev/src/dehub.git/yamlutil"

	"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"
)

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

	// Fingerprint returns the raw fingerprint which can be signed when
	// accrediting this payload. The ChangedFile objects given describe the file
	// changes between the parent commit and this commit.
	//
	// If this method returns nil it means that the payload has no fingerprint
	// in-and-of-itself.
	Fingerprint([]ChangedFile) ([]byte, error)
}

// PayloadCommon describes the fields common to all Payloads.
type PayloadCommon struct {
	Fingerprint yamlutil.Blob             `yaml:"fingerprint"`
	Credentials []sigcred.CredentialUnion `yaml:"credentials"`

	// LegacyChangeHash is no longer used, use Fingerprint instead.
	LegacyChangeHash yamlutil.Blob `yaml:"change_hash,omitempty"`
}

func (cc PayloadCommon) 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[:77] + "..."
	}
	return msg
}

// PayloadUnion represents a single Payload of variable type. Only one field
// should be set on a PayloadUnion, unless otherwise noted.
type PayloadUnion struct {
	Change     *PayloadChange     `type:"change,default"`
	Credential *PayloadCredential `type:"credential"`
	Comment    *PayloadComment    `type:"comment"`

	// Common may be set in addition to one of the other fields.
	Common PayloadCommon `yaml:",inline"`
}

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

// UnmarshalYAML implements the yaml.Unmarshaler interface.
func (p *PayloadUnion) UnmarshalYAML(unmarshal func(interface{}) error) error {
	if err := typeobj.UnmarshalYAML(p, unmarshal); err != nil {
		return err
	} else if len(p.Common.LegacyChangeHash) > 0 {
		p.Common.Fingerprint = p.Common.LegacyChangeHash
		p.Common.LegacyChangeHash = nil
	}
	return nil
}

// Payload returns the Payload instance encapsulated by this PayloadUnion.
//
// This will panic if a Payload field is not populated.
func (p PayloadUnion) Payload() Payload {
	el, _, err := typeobj.Element(p)
	if err != nil {
		panic(err)
	}
	return el.(Payload)
}

// Type returns the Payload's type (as would be used in its YAML "type" field).
//
// This will panic if a Payload field is not populated.
func (p PayloadUnion) Type() string {
	_, typeStr, err := typeobj.Element(p)
	if err != nil {
		panic(err)
	}
	return typeStr
}

// MarshalText implements the encoding.TextMarshaler interface by returning the
// form the payload in the git commit message.
func (p PayloadUnion) MarshalText() ([]byte, error) {
	msgHead := abbrevCommitMessage(p.Payload().MessageHead(p.Common))
	msgBodyB, err := yaml.Marshal(p)
	if err != nil {
		return nil, fmt.Errorf("marshaling payload %+v as yaml: %w", p, 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
// payload object which has been encoded into a git commit message.
func (p *PayloadUnion) 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, p); err != nil {
		return fmt.Errorf("unmarshaling commit payload from yaml: %w", err)
	}
	return nil
}

// AccreditPayload returns the given PayloadUnion with an appended Credential
// provided by the given SignifierInterface.
func (proj *Project) AccreditPayload(payUn PayloadUnion, sig sigcred.Signifier) (PayloadUnion, error) {
	headFS, err := proj.headFS()
	if err != nil {
		return payUn, fmt.Errorf("retrieving HEAD fs: %w", err)
	}

	cred, err := sig.Sign(headFS, payUn.Common.Fingerprint)
	if err != nil {
		return payUn, fmt.Errorf("signing fingerprint %q: %w", payUn.Common.Fingerprint, err)
	}
	payUn.Common.Credentials = append(payUn.Common.Credentials, cred)
	return payUn, nil
}

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

// CommitDirect constructs a git commit object and and stores it, returning the
// resulting Commit. This method does not interact with HEAD at all.
func (proj *Project) CommitDirect(params CommitDirectParams) (Commit, error) {
	msgB, err := params.PayloadUnion.MarshalText()
	if err != nil {
		return Commit{}, fmt.Errorf("encoding payload to message string: %w", 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 := proj.GitRepo.Storer.NewEncodedObject()
	if err := commit.Encode(commitObj); err != nil {
		return Commit{}, fmt.Errorf("encoding commit object: %w", err)
	}

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

	return proj.GetCommit(commitHash)
}

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

	headHash, err := proj.ReferenceToHash(headRefName)
	if err != nil {
		return Commit{}, 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(proj.GitRepo)
	if err != nil {
		return Commit{}, fmt.Errorf("getting staged changes: %w", err)
	}

	commit, err := proj.CommitDirect(CommitDirectParams{
		PayloadUnion: payUn,
		Author:       strings.Join(payUn.Common.credIDs(), ", "),
		ParentHash:   headHash,
		GitTree:      stagedTree,
	})
	if err != nil {
		return Commit{}, err
	}

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

	return commit, nil
}

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

	status, err := w.Status()
	if err != nil {
		return false, fmt.Errorf("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 (proj *Project) VerifyCommits(branchName plumbing.ReferenceName, commits []Commit) error {
	// this isn't strictly necessary for this method, but it helps discover bugs
	// in other parts of the code.
	if len(commits) == 0 {
		return errors.New("cannot call VerifyCommits with empty commit slice")
	}

	// First determine the root of the main branch. All commits need to be an
	// ancestor of it. If the main branch has not been created yet then there
	// might not be a root commit yet.
	var rootCommitObj *object.Commit
	mainCommit, err := proj.GetCommitByRevision(plumbing.Revision(MainRefName))
	if errors.Is(err, plumbing.ErrReferenceNotFound) {

		// main branch hasn't been created yet. The commits can only be verified
		// if they are for the main branch and they include the root commit.
		if branchName != MainRefName {
			return fmt.Errorf("cannot verify commits in branch %q when no main branch exists", branchName)
		}

		for _, commit := range commits {
			if commit.Object.NumParents() == 0 {
				rootCommitObj = commit.Object
				break
			}
		}
		if rootCommitObj == nil {
			return errors.New("root commit of main branch cannot be determined")
		}

	} else if err != nil {
		return fmt.Errorf("retrieving commit at HEAD of %q: %w", MainRefName.Short(), err)

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

	// We also need the HEAD of the given branch, if it exists.
	branchCommit, err := proj.GetCommitByRevision(plumbing.Revision(branchName))
	if err != nil && !errors.Is(err, plumbing.ErrReferenceNotFound) {
		return fmt.Errorf("retrieving commit at HEAD of %q: %w", branchName.Short(), err)
	}

	for i, commit := range commits {
		// It's not a requirement that the given Commits are in ancestral order,
		// but usually they are; if the previous commit is the parent of this
		// one we can skip a bunch of work.
		var parentTree *object.Tree
		var isNonFF bool
		if i > 0 && commits[i-1].Hash == commit.Object.ParentHashes[0] {
			parentTree = commits[i-1].TreeObject

		} else if commit.Hash == rootCommitObj.Hash {
			// looking at the root commit, assume it's ok

		} else {
			var err error
			isAncestor := func(older, younger *object.Commit) bool {
				var isAncestor bool
				if err != nil {
					return false
				} else if isAncestor, err = older.IsAncestor(younger); err != nil {
					err = fmt.Errorf("determining if %q is an ancestor of %q: %w",
						younger.Hash, older.Hash, err)
					return false

				}
				return isAncestor
			}

			ancestorOfRoot := isAncestor(rootCommitObj, commit.Object)
			if branchCommit.Hash != plumbing.ZeroHash { // checking if the var was set
				// this could only be a nonFF if the branch actually exists.
				isNonFF = !isAncestor(branchCommit.Object, commit.Object)
			}

			if err != nil {
				return err
			} else if !ancestorOfRoot {
				return fmt.Errorf("commit %q must be direct descendant of root commit of %q (%q)",
					commit.Hash, MainRefName.Short(), rootCommitObj.Hash,
				)
			}
		}

		if err := proj.verifyCommit(branchName, commit, parentTree, isNonFF); err != nil {
			return fmt.Errorf("verifying commit %q: %w", commit.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 (proj *Project) 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 := proj.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 (proj *Project) verifyCommit(
	branchName plumbing.ReferenceName,
	commit Commit,
	parentTree *object.Tree,
	isNonFF bool,
) error {
	if parentTree == nil {
		var err error
		if parentTree, err = proj.parentTree(commit.Object); err != nil {
			return fmt.Errorf("retrieving parent tree of commit: %w", err)
		}
	}

	var sigFS fs.FS
	if commit.Object.NumParents() == 0 {
		sigFS = fs.FromTree(commit.TreeObject)
	} else {
		sigFS = fs.FromTree(parentTree)
	}

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

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

	} else if len(changedFiles) > 0 && commit.Payload.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 := commit.Payload.Type()
	err = accessctl.AssertCanCommit(cfg.AccessControls, accessctl.CommitRequest{
		Type:           commitType,
		Branch:         branchName.Short(),
		Credentials:    commit.Payload.Common.Credentials,
		FilesChanged:   pathsChanged,
		NonFastForward: isNonFF,
	})
	if err != nil {
		return fmt.Errorf("asserting access controls: %w", err)
	}

	// ensure the fingerprint is what it's expected to be
	storedFingerprint := commit.Payload.Common.Fingerprint
	expectedFingerprint, err := commit.Payload.Payload().Fingerprint(changedFiles)
	if err != nil {
		return fmt.Errorf("calculating expected payload fingerprint: %w", err)
	} else if expectedFingerprint == nil {
		// the payload doesn't have a fingerprint of its own, it's just carrying
		// one, so no point in checking if it's "correct".
	} else if !bytes.Equal(storedFingerprint, expectedFingerprint) {
		return fmt.Errorf("unexpected fingerprint in payload, is %q but should be %q",
			storedFingerprint, yamlutil.Blob(expectedFingerprint))
	}

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

	return nil
}

// LastChangeDescription iterates over the given commits in reverse order and
// returns the first change description it comes across. A change description
// may come from a change payload or a credential payload which covers a set of
// changes.
//
// This function will return an error if no given commits contain a change
// description.
func LastChangeDescription(commits []Commit) (string, error) {
	for i := range commits {
		i = len(commits) - 1 - i
		payUn := commits[i].Payload
		if payUn.Change != nil {
			return payUn.Change.Description, nil
		} else if payUn.Credential != nil && payUn.Credential.ChangeDescription != "" {
			return payUn.Credential.ChangeDescription, nil
		}
	}
	return "", errors.New("no commits in range contain a change description")
}

type changeRangeInfo struct {
	changeCommits      []Commit
	authors            map[string]struct{}
	startTree, endTree *object.Tree
	changeDescription  string
}

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

	for _, commit := range commits {
		if commit.Payload.Change != nil {
			info.changeCommits = append(info.changeCommits, commit)
			for _, cred := range commit.Payload.Common.Credentials {
				info.authors[cred.AccountID] = struct{}{}
			}
		}
	}
	if len(info.changeCommits) == 0 {
		return changeRangeInfo{}, errors.New("no change commits found in range")
	}

	// 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 = proj.parentTree(commits[0].Object); err != nil {
		return changeRangeInfo{}, fmt.Errorf("getting tree of parent of %q: %w",
			commits[0].Hash, err)
	} else if info.changeDescription, err = LastChangeDescription(commits); err != nil {
		return changeRangeInfo{}, err
	}

	lastChangeCommit := info.changeCommits[len(info.changeCommits)-1]
	info.endTree = lastChangeCommit.TreeObject
	return info, nil
}

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

	return genChangeFingerprint(nil, descr, changedFiles), nil
}

// VerifyCanSetBranchHEADTo is used to verify that a branch's HEAD can be set to
// the given hash. It verifies any new commits which are being added, and
// handles verifying non-fast-forward commits as well.
//
// If the given hash matches the current HEAD of the branch then this performs
// no further checks and returns nil.
func (proj *Project) VerifyCanSetBranchHEADTo(branchName plumbing.ReferenceName, hash plumbing.Hash) error {
	oldCommitRef, err := proj.GitRepo.Reference(branchName, true)
	if errors.Is(err, plumbing.ErrReferenceNotFound) {
		// if the branch is being created then just pull all of its commits and
		// verify them.
		// TODO optimize this so that it tries to use the merge-base with main,
		// so we're not re-verifying a ton of commits unecessarily
		commits, err := proj.GetCommitRange(plumbing.ZeroHash, hash)
		if err != nil {
			return fmt.Errorf("retrieving %q and all its ancestors: %w", hash, err)
		}
		return proj.VerifyCommits(branchName, commits)

	} else if err != nil {
		return fmt.Errorf("resolving branch reference to a hash: %w", err)

	} else if oldCommitRef.Hash() == hash {
		// if the HEAD is already at the given hash then it must be fine.
		return nil
	}

	oldCommitObj, err := proj.GitRepo.CommitObject(oldCommitRef.Hash())
	if err != nil {
		return fmt.Errorf("retrieving commit object %q: %w", oldCommitRef.Hash(), err)
	}

	newCommit, err := proj.GetCommit(hash)
	if err != nil {
		return fmt.Errorf("retrieving commit %q: %w", hash, err)
	}

	if isAncestor, err := newCommit.Object.IsAncestor(oldCommitObj); err != nil {
		return fmt.Errorf("determining if %q is an ancestor of %q: %w",
			newCommit.Hash, oldCommitObj.Hash, err)
	} else if isAncestor {
		// if the new commit is an ancestor of the old one then the branch is
		// being force-pushed to a previous commit. This is weird to handle
		// using VerifyCommits, so just call verifyCommit directly.
		return proj.verifyCommit(branchName, newCommit, nil, true)
	}

	mbCommits, err := oldCommitObj.MergeBase(newCommit.Object)
	if err != nil {
		return fmt.Errorf("determining merge-base between %q and %q: %w",
			oldCommitObj.Hash, newCommit.Hash, err)
	} else if len(mbCommits) == 0 {
		return fmt.Errorf("%q and %q have no ancestors in common",
			oldCommitObj.Hash, newCommit.Hash)
	} else if len(mbCommits) == 2 {
		return fmt.Errorf("%q and %q have more than one ancestor in common",
			oldCommitObj.Hash, newCommit.Hash)
	}

	commits, err := proj.GetCommitRange(mbCommits[0].Hash, hash)
	if err != nil {
		return fmt.Errorf("retrieving commits %q to %q: %w", mbCommits[0].Hash, hash, err)
	}
	return proj.VerifyCommits(branchName, commits)
}