package dehub

import (
	"encoding/hex"
	"errors"
	"fmt"
	"path/filepath"
	"strings"

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

// Commit wraps a single git commit object, and also contains various fields
// which are parsed out of it, including the payload. It is used as a
// convenience type, in place of having to manually retrieve and parse specific
// information out of commit objects.
type Commit struct {
	Payload PayloadUnion

	Hash       plumbing.Hash
	Object     *object.Commit
	TreeObject *object.Tree
}

// GetCommit retrieves the Commit at the given hash, and all of its sub-data
// which can be pulled out of it.
func (proj *Project) GetCommit(h plumbing.Hash) (c Commit, err error) {
	if c.Object, err = proj.GitRepo.CommitObject(h); err != nil {
		return c, fmt.Errorf("getting git commit object: %w", err)
	} else if c.TreeObject, err = proj.GitRepo.TreeObject(c.Object.TreeHash); err != nil {
		return c, fmt.Errorf("getting git tree object %q: %w",
			c.Object.TreeHash, err)
	} else if err = c.Payload.UnmarshalText([]byte(c.Object.Message)); err != nil {
		return c, fmt.Errorf("decoding commit message: %w", err)
	}
	c.Hash = c.Object.Hash
	return
}

// ErrHeadIsZero is used to indicate that HEAD resolves to the zero hash. An
// example of when this can happen is if the project was just initialized and
// has no commits, or if an orphan branch is checked out.
var ErrHeadIsZero = errors.New("HEAD resolves to the zero hash")

// GetHeadCommit returns the Commit which is currently referenced by HEAD.
// This method may return ErrHeadIsZero if HEAD resolves to the zero hash.
func (proj *Project) GetHeadCommit() (Commit, error) {
	headHash, err := proj.ReferenceToHash(plumbing.HEAD)
	if err != nil {
		return Commit{}, fmt.Errorf("resolving HEAD: %w", err)
	} else if headHash == plumbing.ZeroHash {
		return Commit{}, ErrHeadIsZero
	}

	c, err := proj.GetCommit(headHash)
	if err != nil {
		return Commit{}, fmt.Errorf("getting commit %q: %w", headHash, err)
	}
	return c, nil
}

// GetCommitRange returns an ancestry of Commits, with the first being the
// commit immediately following the given starting hash, and the last being the
// given ending hash.
//
// If start is plumbing.ZeroHash then the root commit will be the starting hash.
func (proj *Project) GetCommitRange(start, end plumbing.Hash) ([]Commit, error) {
	curr, err := proj.GetCommit(end)
	if err != nil {
		return nil, fmt.Errorf("retrieving commit %q: %w", end, err)
	}

	var commits []Commit
	var found bool
	for {
		if found = start != plumbing.ZeroHash && curr.Hash == start; found {
			break
		}

		commits = append(commits, curr)
		numParents := curr.Object.NumParents()
		if numParents == 0 {
			break
		} else if numParents > 1 {
			return nil, fmt.Errorf("commit %q has more than one parent: %+v",
				curr.Hash, curr.Object.ParentHashes)
		}

		parentHash := curr.Object.ParentHashes[0]
		parent, err := proj.GetCommit(parentHash)
		if err != nil {
			return nil, fmt.Errorf("retrieving commit %q: %w", parentHash, err)
		}
		curr = parent
	}
	if !found && start != plumbing.ZeroHash {
		return nil, fmt.Errorf("unable to find commit %q as an ancestor of %q",
			start, end)
	}

	// reverse the commits to be in the expected order
	for l, r := 0, len(commits)-1; l < r; l, r = l+1, r-1 {
		commits[l], commits[r] = commits[r], commits[l]
	}
	return commits, nil
}

var (
	hashStrLen = len(plumbing.ZeroHash.String())
	errNotHex  = errors.New("not a valid hex string")
)

func (proj *Project) findCommitByShortHash(hashStr string) (plumbing.Hash, error) {
	paddedHashStr := hashStr
	if len(hashStr)%2 > 0 {
		paddedHashStr += "0"
	}

	if hashB, err := hex.DecodeString(paddedHashStr); err != nil {
		return plumbing.ZeroHash, errNotHex
	} else if len(hashStr) == hashStrLen {
		var hash plumbing.Hash
		copy(hash[:], hashB)
		return hash, nil
	} else if len(hashStr) < 2 {
		return plumbing.ZeroHash, errors.New("hash string must be 2 characters long or more")
	}

	for i := 2; i < hashStrLen; i++ {
		hashPrefix, hashTail := hashStr[:i], hashStr[i:]
		path := filepath.Join("objects", hashPrefix)
		fileInfos, err := proj.GitDirFS.ReadDir(path)
		if err != nil {
			return plumbing.ZeroHash, fmt.Errorf("listing files in %q: %w", path, err)
		}

		var matchedHash plumbing.Hash
		for _, fileInfo := range fileInfos {
			objFileName := fileInfo.Name()
			if !strings.HasPrefix(objFileName, hashTail) {
				continue
			}

			objHash := plumbing.NewHash(hashPrefix + objFileName)
			obj, err := proj.GitRepo.Storer.EncodedObject(plumbing.AnyObject, objHash)
			if err != nil {
				return plumbing.ZeroHash, fmt.Errorf("reading object %q off disk: %w", objHash, err)
			} else if obj.Type() != plumbing.CommitObject {
				continue

			} else if matchedHash == plumbing.ZeroHash {
				matchedHash = objHash
				continue
			}

			return plumbing.ZeroHash, fmt.Errorf("both %q and %q match", matchedHash, objHash)
		}

		if matchedHash != plumbing.ZeroHash {
			return matchedHash, nil
		}
	}

	return plumbing.ZeroHash, errors.New("failed to find a commit object with a matching prefix")
}

func (proj *Project) resolveRev(rev plumbing.Revision) (plumbing.Hash, error) {
	if rev == plumbing.Revision(plumbing.ZeroHash.String()) {
		return plumbing.ZeroHash, nil
	}

	{
		// pretend the revision is a short hash until proven otherwise
		shortHash := string(rev)
		hash, err := proj.findCommitByShortHash(shortHash)
		if errors.Is(err, errNotHex) {
			// ok, continue
		} else if err != nil {
			return plumbing.ZeroHash, fmt.Errorf("resolving as short hash: %w", err)
		} else {
			// guess it _is_ a short hash, knew it!
			return hash, nil
		}
	}

	h, err := proj.GitRepo.ResolveRevision(rev)
	if err != nil {
		return plumbing.ZeroHash, fmt.Errorf("resolving revision %q: %w", rev, err)
	}
	return *h, nil
}

// GetCommitByRevision resolves the revision and returns the Commit it references.
func (proj *Project) GetCommitByRevision(rev plumbing.Revision) (Commit, error) {
	hash, err := proj.resolveRev(rev)
	if err != nil {
		return Commit{}, err
	}

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

// GetCommitRangeByRevision is like GetCommitRange, first resolving the given
// revisions into hashes before continuing with GetCommitRange's behavior.
func (proj *Project) GetCommitRangeByRevision(startRev, endRev plumbing.Revision) ([]Commit, error) {
	start, err := proj.resolveRev(startRev)
	if err != nil {
		return nil, err
	}

	end, err := proj.resolveRev(endRev)
	if err != nil {
		return nil, err
	}

	return proj.GetCommitRange(start, end)
}