From 843d9fbb8d31b2735621d02631924155615d97f6 Mon Sep 17 00:00:00 2001
From: Jeff Carr <jcarr@wit.com>
Date: Tue, 23 Sep 2025 14:51:23 -0500
Subject: [PATCH] lookup git PatchId

---
 repo.new.go | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/repo.new.go b/repo.new.go
index 619ff05..cb79bf2 100644
--- a/repo.new.go
+++ b/repo.new.go
@@ -1,9 +1,13 @@
 package gitpb
 
 import (
+	"bytes"
 	"errors"
+	"fmt"
 	"net/url"
+	"os/exec"
 	"path/filepath"
+	"strings"
 
 	"go.wit.com/log"
 )
@@ -131,3 +135,56 @@ func NewRepo(fullpath string) (*Repo, error) {
 	}
 	return &repo, nil
 }
+
+func (repo *Repo) FindPatchId(hash string) (string, error) {
+	if hash == "" {
+		return "", log.Errorf("commit hash blank")
+	}
+
+	// 1. Create the command to get the diff for the commit.
+	//    "git show" is the perfect tool for this.
+	cmdShow := exec.Command("git", "show", hash)
+	cmdShow.Dir = repo.GetFullPath()
+
+	// 2. Create the command to calculate the patch-id from stdin.
+	cmdPipeID := exec.Command("git", "patch-id", "--stable")
+	cmdPipeID.Dir = repo.GetFullPath()
+
+	// 3. Connect the output of "git show" to the input of "git patch-id".
+	//    This is the Go equivalent of the shell pipe `|`.
+	pipe, err := cmdShow.StdoutPipe()
+	if err != nil {
+		return "", fmt.Errorf("failed to create pipe: %w", err)
+	}
+	cmdPipeID.Stdin = pipe
+
+	// 4. We need a buffer to capture the final output from git patch-id.
+	var output bytes.Buffer
+	cmdPipeID.Stdout = &output
+
+	// 5. Start the reading command (patch-id) first.
+	if err := cmdPipeID.Start(); err != nil {
+		return "", fmt.Errorf("failed to start git-patch-id: %w", err)
+	}
+
+	// 6. Run the writing command (show). This will block until it's done.
+	if err := cmdShow.Run(); err != nil {
+		return "", fmt.Errorf("failed to run git-show: %w", err)
+	}
+
+	// 7. Wait for the reading command to finish.
+	if err := cmdPipeID.Wait(); err != nil {
+		return "", fmt.Errorf("failed to wait for git-patch-id: %w", err)
+	}
+
+	fields := strings.Fields(output.String())
+	if len(fields) != 2 {
+		return "", fmt.Errorf("git-patch-id produced empty output")
+	}
+
+	if fields[1] != hash {
+		return "", fmt.Errorf("patchid did not match %s != %v", hash, fields)
+	}
+
+	return fields[0], nil
+}