Fix build changed count always showing total packages (#46)

* Restore build change detection lost during Composer v1 removal

The p/-based content-addressed change detection was removed when Composer
v1 support was dropped, leaving changedPkgs incrementing unconditionally.
This caused the admin builds page to show all packages as changed every
build. Restore detection using byte comparison of p2/ files against the
previous build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix gofmt formatting

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: retlehs

cmd/wppackages/cmd/build.go

@@ -5,6 +5,7 @@ import (
 	"path/filepath"
 	"time"
 
+	"github.com/roots/wp-packages/internal/deploy"
 	"github.com/roots/wp-packages/internal/repository"
 	"github.com/spf13/cobra"
 )
@@ -24,13 +25,21 @@ func runBuild(cmd *cobra.Command, args []string) error {
 		output = filepath.Join("storage", "repository", "builds")
 	}
 
+	// Resolve previous build for change detection
+	repoDir := filepath.Dir(output) // storage/repository
+	prevBuildDir := ""
+	if id, err := deploy.CurrentBuildID(repoDir); err == nil && id != "" {
+		prevBuildDir = deploy.BuildDirFromID(repoDir, id)
+	}
+
 	result, err := repository.Build(cmd.Context(), application.DB, repository.BuildOpts{
-		OutputDir:   output,
-		AppURL:      application.Config.AppURL,
-		Force:       force,
-		PackageName: pkg,
-		BuildID:     pipelineBuildID,
-		Logger:      application.Logger,
+		OutputDir:        output,
+		AppURL:           application.Config.AppURL,
+		Force:            force,
+		PackageName:      pkg,
+		BuildID:          pipelineBuildID,
+		PreviousBuildDir: prevBuildDir,
+		Logger:           application.Logger,
 	})
 	if err != nil {
 		return err

internal/repository/builder.go

@@ -1,6 +1,7 @@
 package repository
 
 import (
+	"bytes"
 	"context"
 	"database/sql"
 	"encoding/json"
@@ -16,13 +17,14 @@ import (
 
 // BuildOpts configures a repository build.
 type BuildOpts struct {
-	OutputDir    string // base output dir (e.g. storage/repository/builds)
-	AppURL       string // absolute app URL for notify-batch
-	Force        bool
-	PackageName  string   // optional: build single package
-	PackageNames []string // optional: build only these slugs
-	BuildID      string   // optional: pre-generated build ID (used by pipeline)
-	Logger       *slog.Logger
+	OutputDir        string // base output dir (e.g. storage/repository/builds)
+	AppURL           string // absolute app URL for notify-batch
+	Force            bool
+	PackageName      string   // optional: build single package
+	PackageNames     []string // optional: build only these slugs
+	BuildID          string   // optional: pre-generated build ID (used by pipeline)
+	PreviousBuildDir string   // optional: compare against previous build to count changes
+	Logger           *slog.Logger
 }
 
 // BuildResult holds build metadata for manifest.json and the builds table.
@@ -175,13 +177,23 @@ func Build(ctx context.Context, db *sql.DB, opts BuildOpts) (*BuildResult, error
 			return nil, fmt.Errorf("hashing %s: %w", composerName, err)
 		}
 
-		p2File := filepath.Join(buildDir, "p2", composerName+".json")
+		p2Rel := filepath.Join("p2", composerName+".json")
+		p2File := filepath.Join(buildDir, p2Rel)
 		if err := os.WriteFile(p2File, data, 0644); err != nil {
 			return nil, fmt.Errorf("writing %s: %w", p2File, err)
 		}
-		changedPkgs++
 		artifactCount++
 
+		// Compare against previous build to determine if this package changed
+		if opts.PreviousBuildDir != "" {
+			prevData, err := os.ReadFile(filepath.Join(opts.PreviousBuildDir, p2Rel))
+			if err != nil || !bytes.Equal(prevData, data) {
+				changedPkgs++
+			}
+		} else {
+			changedPkgs++
+		}
+
 		if totalPkgs%500 == 0 {
 			opts.Logger.Info("build progress", "packages", totalPkgs)
 		}