package dehub

import (
	"bytes"
	"dehub/accessctl"
	"dehub/fs"
	"dehub/sigcred"
	"dehub/typeobj"
	"encoding"
	"encoding/base64"
	"errors"
	"fmt"
	"time"

	"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.
type CommitInterface interface {
	// MessageHead returns the head of the commit message (i.e. the first line).
	MessageHead() (string, error)

	// Hash returns the raw hash which Signifiers can sign to accredit this
	// commit. The tree objects given describe the filesystem state of the
	// parent commit, and the filesystem state of this commit.
	//
	// This method should _not_ change any fields on the commit.
	Hash(parent, this *object.Tree) ([]byte, error)

	// GetHash returns the signable Hash embedded in the commit, which should
	// hopefully correspond to the Commit's Credentials.
	GetHash() []byte
}

// 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"`

	// 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"`
}

// 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()
	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)
	}
	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.GetHash())
	if err != nil {
		return commit, fmt.Errorf("could not accredit change commit: %w", err)
	}
	commit.Credentials = append(commit.Credentials, cred)
	return commit, nil
}

// Commit uses the given TextMarshaler to create a git commit object (with the
// specified accountID as the author) and commits it to the current HEAD,
// returning the hash of the commit.
func (r *Repo) Commit(m encoding.TextMarshaler, accountID string) (GitCommit, error) {
	msgB, err := m.MarshalText()
	if err != nil {
		return GitCommit{}, fmt.Errorf("encoding %T to message string: %v", m, err)
	}

	w, err := r.GitRepo.Worktree()
	if err != nil {
		return GitCommit{}, fmt.Errorf("getting git worktree: %w", err)
	}
	h, err := w.Commit(string(msgB), &git.CommitOptions{
		Author: &object.Signature{
			Name: accountID,
			When: time.Now(),
		},
	})
	if err != nil {
		return GitCommit{}, fmt.Errorf("committing to git worktree: %w", err)
	}

	gc, err := r.GetGitCommit(h)
	if err != nil {
		return GitCommit{}, fmt.Errorf("retrieving fresh commit %q back from git: %w", h, err)
	}
	return gc, 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 {
			any = true
			break
		}
	}
	return any, nil
}

type verificationCtx struct {
	commit                 *object.Commit
	commitTree, parentTree *object.Tree
}

// non-gophers gonna hate on this method, but I say it's fine
func (r *Repo) verificationCtx(h plumbing.Hash) (vctx verificationCtx, err error) {
	if vctx.commit, err = r.GitRepo.CommitObject(h); err != nil {
		return vctx, fmt.Errorf("retrieving commit object: %w", err)

	} else if vctx.commitTree, err = r.GitRepo.TreeObject(vctx.commit.TreeHash); err != nil {
		return vctx, fmt.Errorf("retrieving commit tree object %q: %w",
			vctx.commit.TreeHash, err)

	} else if parent, err := vctx.commit.Parent(0); err != nil {
		return vctx, fmt.Errorf("retrieving commit parent: %w", err)

	} else if vctx.parentTree, err = r.GitRepo.TreeObject(parent.TreeHash); err != nil {
		return vctx, fmt.Errorf("retrieving commit parent tree object %q: %w",
			parent.Hash, err)
	}

	return vctx, nil
}

func (r *Repo) assertAccessControls(
	acl []accessctl.AccessControl,
	commit Commit, vctx verificationCtx, branch plumbing.ReferenceName,
) (err error) {
	filesChanged, err := calcDiff(vctx.parentTree, vctx.commitTree)
	if err != nil {
		return fmt.Errorf("calculating diff from tree %q to tree %q: %w",
			vctx.parentTree.Hash, vctx.commitTree.Hash, err)

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

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

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

	return accessctl.AssertCanCommit(acl, accessctl.CommitRequest{
		Type:         commitType,
		Branch:       branch.Short(),
		Credentials:  commit.Credentials,
		FilesChanged: pathsChanged,
	})
}

// 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 {

	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.
		var parentTree *object.Tree
		if i > 0 && gitCommits[i-1].GitCommit.Hash == gitCommit.GitCommit.ParentHashes[0] {
			parentTree = gitCommits[i-1].GitTree
		}

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

// if parentTree is nil then it will be inferred.
func (r *Repo) verifyCommit(branch plumbing.ReferenceName, gitCommit GitCommit, parentTree *object.Tree) error {
	isRoot := gitCommit.Root()

	if parentTree == nil {
		if isRoot {
			parentTree = new(object.Tree)
		} else if parentCommit, err := gitCommit.GitCommit.Parent(0); err != nil {
			return fmt.Errorf("getting parent commit %q: %w",
				gitCommit.GitCommit.ParentHashes[0], err)
		} else if parentTree, err = r.GitRepo.TreeObject(parentCommit.TreeHash); err != nil {
			return fmt.Errorf("getting parent tree object %q: %w",
				parentCommit.TreeHash, err)
		}
	}

	vctx := verificationCtx{
		commit:     gitCommit.GitCommit,
		commitTree: gitCommit.GitTree,
		parentTree: parentTree,
	}

	var sigFS fs.FS
	if isRoot {
		sigFS = fs.FromTree(vctx.commitTree)
	} else {
		sigFS = fs.FromTree(vctx.parentTree)
	}

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

	err = r.assertAccessControls(cfg.AccessControls, gitCommit.Commit, vctx, branch)
	if err != nil {
		return fmt.Errorf("enforcing access controls: %w", err)
	}

	changeHash := gitCommit.Interface.GetHash()
	expectedChangeHash, err := gitCommit.Interface.Hash(vctx.parentTree, vctx.commitTree)
	if err != nil {
		return fmt.Errorf("calculating expected change hash: %w", err)
	} else if !bytes.Equal(changeHash, expectedChangeHash) {
		return fmt.Errorf("malformed change_hash in commit body, is %s but should be %s",
			base64.StdEncoding.EncodeToString(expectedChangeHash),
			base64.StdEncoding.EncodeToString(changeHash))
	}

	for _, cred := range gitCommit.Commit.Credentials {
		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, expectedChangeHash, cred); err != nil {
			return fmt.Errorf("verifying credential %+v: %w", cred, err)
		}
	}

	return nil
}