types: introduce Commit type

this is a backwards compatible structure that is capable of
deserializing both go-git Commit structs and the existing
types.NiceDiff.Commit struct.

Signed-off-by: oppiliappan <[e-mail]>

types/commit.go

···
+package types
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"maps"
+	"strings"
+
+	"github.com/go-git/go-git/v5/plumbing"
+	"github.com/go-git/go-git/v5/plumbing/object"
+)
+
+type Commit struct {
+	// hash of the commit object.
+	Hash plumbing.Hash `json:"hash,omitempty"`
+
+	// author is the original author of the commit.
+	Author object.Signature `json:"author"`
+
+	// committer is the one performing the commit, might be different from author.
+	Committer object.Signature `json:"committer"`
+
+	// message is the commit message, contains arbitrary text.
+	Message string `json:"message"`
+
+	// treehash is the hash of the root tree of the commit.
+	Tree string `json:"tree"`
+
+	// parents are the hashes of the parent commits of the commit.
+	ParentHashes []plumbing.Hash `json:"parent_hashes,omitempty"`
+
+	// pgpsignature is the pgp signature of the commit.
+	PGPSignature string `json:"pgp_signature,omitempty"`
+
+	// mergetag is the embedded tag object when a merge commit is created by
+	// merging a signed tag.
+	MergeTag string `json:"merge_tag,omitempty"`
+
+	// changeid is a unique identifier for the change (e.g., gerrit change-id).
+	ChangeId string `json:"change_id,omitempty"`
+
+	// extraheaders contains additional headers not captured by other fields.
+	ExtraHeaders map[string][]byte `json:"extra_headers,omitempty"`
+
+	// deprecated: kept for backwards compatibility with old json format.
+	This string `json:"this,omitempty"`
+
+	// deprecated: kept for backwards compatibility with old json format.
+	Parent string `json:"parent,omitempty"`
+}
+
+// types.Commit is an unify two commit structs:
+//   - git.object.Commit from
+//   - types.NiceDiff.commit
+//
+// to do this in backwards compatible fashion, we define the base struct
+// to use the same fields as NiceDiff.Commit, and then we also unmarshal
+// the struct fields from go-git structs, this custom unmarshal makes sense
+// of both representations and unifies them to have maximal data in either
+// form.
+func (c *Commit) UnmarshalJSON(data []byte) error {
+	type Alias Commit
+
+	aux := &struct {
+		*object.Commit
+		*Alias
+	}{
+		Alias: (*Alias)(c),
+	}
+
+	if err := json.Unmarshal(data, aux); err != nil {
+		return err
+	}
+
+	c.FromGoGitCommit(aux.Commit)
+
+	return nil
+}
+
+// fill in as much of Commit as possible from the given go-git commit
+func (c *Commit) FromGoGitCommit(gc *object.Commit) {
+	if gc == nil {
+		return
+	}
+
+	if c.Hash.IsZero() {
+		c.Hash = gc.Hash
+	}
+	if c.This == "" {
+		c.This = gc.Hash.String()
+	}
+	if isEmptySignature(c.Author) {
+		c.Author = gc.Author
+	}
+	if isEmptySignature(c.Committer) {
+		c.Committer = gc.Committer
+	}
+	if c.Message == "" {
+		c.Message = gc.Message
+	}
+	if c.Tree == "" {
+		c.Tree = gc.TreeHash.String()
+	}
+	if c.PGPSignature == "" {
+		c.PGPSignature = gc.PGPSignature
+	}
+	if c.MergeTag == "" {
+		c.MergeTag = gc.MergeTag
+	}
+
+	if len(c.ParentHashes) == 0 {
+		c.ParentHashes = gc.ParentHashes
+	}
+	if c.Parent == "" && len(gc.ParentHashes) > 0 {
+		c.Parent = gc.ParentHashes[0].String()
+	}
+
+	if len(c.ExtraHeaders) == 0 {
+		c.ExtraHeaders = make(map[string][]byte)
+		maps.Copy(c.ExtraHeaders, gc.ExtraHeaders)
+	}
+
+	if c.ChangeId == "" {
+		if v, ok := gc.ExtraHeaders["change-id"]; ok {
+			c.ChangeId = string(v)
+		}
+	}
+}
+
+func isEmptySignature(s object.Signature) bool {
+	return s.Email == "" && s.Name == "" && s.When.IsZero()
+}
+
+// produce a verifiable payload from this commit's metadata
+func (c *Commit) Payload() string {
+	author := bytes.NewBuffer([]byte{})
+	c.Author.Encode(author)
+
+	committer := bytes.NewBuffer([]byte{})
+	c.Committer.Encode(committer)
+
+	payload := strings.Builder{}
+
+	fmt.Fprintf(&payload, "tree %s\n", c.Tree)
+
+	if len(c.ParentHashes) > 0 {
+		for _, p := range c.ParentHashes {
+			fmt.Fprintf(&payload, "parent %s\n", p.String())
+		}
+	} else {
+		// present for backwards compatibility
+		fmt.Fprintf(&payload, "parent %s\n", c.Parent)
+	}
+
+	fmt.Fprintf(&payload, "author %s\n", author.String())
+	fmt.Fprintf(&payload, "committer %s\n", committer.String())
+
+	if c.ChangeId != "" {
+		fmt.Fprintf(&payload, "change-id %s\n", c.ChangeId)
+	} else if v, ok := c.ExtraHeaders["change-id"]; ok {
+		fmt.Fprintf(&payload, "change-id %s\n", string(v))
+	}
+
+	fmt.Fprintf(&payload, "\n%s", c.Message)
+
+	return payload.String()
+}