fix(blast-radius): order the impact list by count then path so identical runs render identically

scanner/git.go

@@ -272,21 +272,27 @@ func AnalyzeImpactFromAnalyses(changedFiles []FileInfo, analyses []FileAnalysis)
 		}
 	}
 
-	// Build impact info
-	var impacts []ImpactInfo
+	// Usage count descending, then full path ascending. usageCounts is a map,
+	// and a sort on count alone left equal counts in map order, so the diff
+	// impact footer and blast-radius JSON reordered between identical runs
+	// and an output budget then showed a different subset each time. The
+	// tie-break is the full path, not the rendered basename, so two changed
+	// files with the same name in different directories still order the same.
+	paths := make([]string, 0, len(usageCounts))
 	for file, count := range usageCounts {
 		if count > 0 {
-			impacts = append(impacts, ImpactInfo{
-				File:   filepath.Base(file),
-				UsedBy: count,
-			})
+			paths = append(paths, file)
 		}
 	}
-
-	// Sort by usage count descending
-	sort.Slice(impacts, func(i, j int) bool {
-		return impacts[i].UsedBy > impacts[j].UsedBy
+	sort.Slice(paths, func(i, j int) bool {
+		if usageCounts[paths[i]] != usageCounts[paths[j]] {
+			return usageCounts[paths[i]] > usageCounts[paths[j]]
+		}
+		return paths[i] < paths[j]
 	})
-
+	impacts := make([]ImpactInfo, 0, len(paths))
+	for _, file := range paths {
+		impacts = append(impacts, ImpactInfo{File: filepath.Base(file), UsedBy: usageCounts[file]})
+	}
 	return impacts
 }

scanner/impact_order_test.go

@@ -0,0 +1,44 @@
+package scanner
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+)
+
+// usageCounts is a map, and the sort was by count alone, so changed files with
+// the same importer count came out in map order: the diff impact footer and
+// blast-radius JSON reordered between identical runs, and an output budget
+// then kept a different subset each time.
+func TestAnalyzeImpactOrderIsDeterministicAcrossTies(t *testing.T) {
+	var changed []FileInfo
+	var analyses []FileAnalysis
+	for i := 0; i < 12; i++ {
+		name := fmt.Sprintf("src/c%02d.ts", i)
+		changed = append(changed, FileInfo{Path: name})
+		// every changed file is imported by exactly two others: all tied
+		for j := 0; j < 2; j++ {
+			analyses = append(analyses, FileAnalysis{
+				Path:    fmt.Sprintf("src/user%02d_%d.ts", i, j),
+				Imports: []string{"./" + fmt.Sprintf("c%02d", i)},
+			})
+		}
+	}
+	// one file with more importers must lead
+	analyses = append(analyses, FileAnalysis{Path: "src/extra.ts", Imports: []string{"./c07"}})
+
+	first := AnalyzeImpactFromAnalyses(changed, analyses)
+	if len(first) != 12 || first[0].File != "c07.ts" || first[0].UsedBy != 3 {
+		t.Fatalf("impacts = %+v, want c07.ts (3) first", first)
+	}
+	for i := 2; i < len(first); i++ {
+		if first[i-1].UsedBy == first[i].UsedBy && first[i-1].File > first[i].File {
+			t.Fatalf("tie not broken by path: %s before %s", first[i-1].File, first[i].File)
+		}
+	}
+	for run := 1; run < 8; run++ {
+		if got := AnalyzeImpactFromAnalyses(changed, analyses); !reflect.DeepEqual(got, first) {
+			t.Fatalf("run %d = %+v, want %+v", run, got, first)
+		}
+	}
+}