1 files changed, 249 insertions, 0 deletions
diff --git a/services/gitdiff/git_diff_tree.go b/services/gitdiff/git_diff_tree.go
new file mode 100644
index 0000000000..8039de145d
--- /dev/null
+++ b/services/gitdiff/git_diff_tree.go
@@ -0,0 +1,249 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gitdiff
+
+import (
+	"bufio"
+	"context"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+
+	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/log"
+)
+
+type DiffTree struct {
+	Files []*DiffTreeRecord
+}
+
+type DiffTreeRecord struct {
+	// Status is one of 'added', 'deleted', 'modified', 'renamed', 'copied', 'typechanged', 'unmerged', 'unknown'
+	Status string
+
+	// For renames and copies, the percentage of similarity between the source and target of the move/rename.
+	Score uint8
+
+	HeadPath   string
+	BasePath   string
+	HeadMode   git.EntryMode
+	BaseMode   git.EntryMode
+	HeadBlobID string
+	BaseBlobID string
+}
+
+// GetDiffTree returns the list of path of the files that have changed between the two commits.
+// If useMergeBase is true, the diff will be calculated using the merge base of the two commits.
+// This is the same behavior as using a three-dot diff in git diff.
+func GetDiffTree(ctx context.Context, gitRepo *git.Repository, useMergeBase bool, baseSha, headSha string) (*DiffTree, error) {
+	gitDiffTreeRecords, err := runGitDiffTree(ctx, gitRepo, useMergeBase, baseSha, headSha)
+	if err != nil {
+		return nil, err
+	}
+
+	return &DiffTree{
+		Files: gitDiffTreeRecords,
+	}, nil
+}
+
+func runGitDiffTree(ctx context.Context, gitRepo *git.Repository, useMergeBase bool, baseSha, headSha string) ([]*DiffTreeRecord, error) {
+	useMergeBase, baseCommitID, headCommitID, err := validateGitDiffTreeArguments(gitRepo, useMergeBase, baseSha, headSha)
+	if err != nil {
+		return nil, err
+	}
+
+	cmd := git.NewCommand(ctx, "diff-tree", "--raw", "-r", "--find-renames", "--root")
+	if useMergeBase {
+		cmd.AddArguments("--merge-base")
+	}
+	cmd.AddDynamicArguments(baseCommitID, headCommitID)
+	stdout, _, runErr := cmd.RunStdString(&git.RunOpts{Dir: gitRepo.Path})
+	if runErr != nil {
+		log.Warn("git diff-tree: %v", runErr)
+		return nil, runErr
+	}
+
+	return parseGitDiffTree(strings.NewReader(stdout))
+}
+
+func validateGitDiffTreeArguments(gitRepo *git.Repository, useMergeBase bool, baseSha, headSha string) (shouldUseMergeBase bool, resolvedBaseSha, resolvedHeadSha string, err error) {
+	// if the head is empty its an error
+	if headSha == "" {
+		return false, "", "", fmt.Errorf("headSha is empty")
+	}
+
+	// if the head commit doesn't exist its and error
+	headCommit, err := gitRepo.GetCommit(headSha)
+	if err != nil {
+		return false, "", "", fmt.Errorf("failed to get commit headSha: %v", err)
+	}
+	headCommitID := headCommit.ID.String()
+
+	// if the base is empty we should use the parent of the head commit
+	if baseSha == "" {
+		// if the headCommit has no parent we should use an empty commit
+		// this can happen when we are generating a diff against an orphaned commit
+		if headCommit.ParentCount() == 0 {
+			objectFormat, err := gitRepo.GetObjectFormat()
+			if err != nil {
+				return false, "", "", err
+			}
+
+			// We set use merge base to false because we have no base commit
+			return false, objectFormat.EmptyTree().String(), headCommitID, nil
+		}
+
+		baseCommit, err := headCommit.Parent(0)
+		if err != nil {
+			return false, "", "", fmt.Errorf("baseSha is '', attempted to use parent of commit %s, got error: %v", headCommit.ID.String(), err)
+		}
+		return useMergeBase, baseCommit.ID.String(), headCommitID, nil
+	}
+
+	// try and get the base commit
+	baseCommit, err := gitRepo.GetCommit(baseSha)
+	// propagate the error if we couldn't get the base commit
+	if err != nil {
+		return useMergeBase, "", "", fmt.Errorf("failed to get base commit %s: %v", baseSha, err)
+	}
+
+	return useMergeBase, baseCommit.ID.String(), headCommit.ID.String(), nil
+}
+
+func parseGitDiffTree(gitOutput io.Reader) ([]*DiffTreeRecord, error) {
+	/*
+		The output of `git diff-tree --raw -r --find-renames` is of the form:
+
+		:<old_mode> <new_mode> <old_sha> <new_sha> <status>\t<path>
+
+		or for renames:
+
+		:<old_mode> <new_mode> <old_sha> <new_sha> <status>\t<old_path>\t<new_path>
+
+		See: <https://git-scm.com/docs/git-diff-tree#_raw_output_format> for more details
+	*/
+	results := make([]*DiffTreeRecord, 0)
+
+	lines := bufio.NewScanner(gitOutput)
+	for lines.Scan() {
+		line := lines.Text()
+
+		if len(line) == 0 {
+			continue
+		}
+
+		record, err := parseGitDiffTreeLine(line)
+		if err != nil {
+			return nil, err
+		}
+
+		results = append(results, record)
+	}
+
+	if err := lines.Err(); err != nil {
+		return nil, err
+	}
+
+	return results, nil
+}
+
+func parseGitDiffTreeLine(line string) (*DiffTreeRecord, error) {
+	line = strings.TrimPrefix(line, ":")
+	splitSections := strings.SplitN(line, "\t", 2)
+	if len(splitSections) < 2 {
+		return nil, fmt.Errorf("unparsable output for diff-tree --raw: `%s`)", line)
+	}
+
+	fields := strings.Fields(splitSections[0])
+	if len(fields) < 5 {
+		return nil, fmt.Errorf("unparsable output for diff-tree --raw: `%s`, expected 5 space delimited values got %d)", line, len(fields))
+	}
+
+	baseMode, err := git.ParseEntryMode(fields[0])
+	if err != nil {
+		return nil, err
+	}
+
+	headMode, err := git.ParseEntryMode(fields[1])
+	if err != nil {
+		return nil, err
+	}
+
+	baseBlobID := fields[2]
+	headBlobID := fields[3]
+
+	status, score, err := statusFromLetter(fields[4])
+	if err != nil {
+		return nil, fmt.Errorf("unparsable output for diff-tree --raw: %s, error: %s", line, err)
+	}
+
+	filePaths := strings.Split(splitSections[1], "\t")
+
+	var headPath, basePath string
+	if status == "renamed" {
+		if len(filePaths) != 2 {
+			return nil, fmt.Errorf("unparsable output for diff-tree --raw: `%s`, expected 2 paths found %d", line, len(filePaths))
+		}
+		basePath = filePaths[0]
+		headPath = filePaths[1]
+	} else {
+		basePath = filePaths[0]
+		headPath = filePaths[0]
+	}
+
+	return &DiffTreeRecord{
+		Status:     status,
+		Score:      score,
+		BaseMode:   baseMode,
+		HeadMode:   headMode,
+		BaseBlobID: baseBlobID,
+		HeadBlobID: headBlobID,
+		BasePath:   basePath,
+		HeadPath:   headPath,
+	}, nil
+}
+
+func statusFromLetter(rawStatus string) (status string, score uint8, err error) {
+	if len(rawStatus) < 1 {
+		return "", 0, fmt.Errorf("empty status letter")
+	}
+	switch rawStatus[0] {
+	case 'A':
+		return "added", 0, nil
+	case 'D':
+		return "deleted", 0, nil
+	case 'M':
+		return "modified", 0, nil
+	case 'R':
+		score, err = tryParseStatusScore(rawStatus)
+		return "renamed", score, err
+	case 'C':
+		score, err = tryParseStatusScore(rawStatus)
+		return "copied", score, err
+	case 'T':
+		return "typechanged", 0, nil
+	case 'U':
+		return "unmerged", 0, nil
+	case 'X':
+		return "unknown", 0, nil
+	default:
+		return "", 0, fmt.Errorf("unknown status letter: '%s'", rawStatus)
+	}
+}
+
+func tryParseStatusScore(rawStatus string) (uint8, error) {
+	if len(rawStatus) < 2 {
+		return 0, fmt.Errorf("status score missing")
+	}
+
+	score, err := strconv.ParseUint(rawStatus[1:], 10, 8)
+	if err != nil {
+		return 0, fmt.Errorf("failed to parse status score: %w", err)
+	} else if score > 100 {
+		return 0, fmt.Errorf("status score out of range: %d", score)
+	}
+
+	return uint8(score), nil
+}