gui: name the other copy of a duplicate, and offer to keep this one instead

3 files changed, 54 insertions, 0 deletions

diff --git a/internal/engine/facts.go b/internal/engine/facts.go
index 641f1ef..ce05df4 100644
--- a/internal/engine/facts.go
+++ b/internal/engine/facts.go
@@ -158,6 +158,10 @@ type facts struct {
 	// could not read), so (matched) is unknown while none has matched.
 	undecided bool
 
+	// dupOriginal is the file a (duplicate) test last matched against,
+	// absolute, for a front end that offers to act on the other copy.
+	dupOriginal string
+
 	contentDone bool            // extraction was attempted
 	contentErr  error           // why it failed
 	partialErr  error           // extract.ErrPartial: a keyword not found may be in the unread part
@@ -290,9 +294,17 @@ func (f *facts) Duplicate(dirs []string) (string, bool, error) {
 	if !isDup {
 		return "", false, nil
 	}
+	// The absolute path is kept for a front end that has to act on the
+	// other copy - the window offers to keep this one instead - while the
+	// reason text stays as it reads best (plan 21).
+	f.dupOriginal = orig
 	return displayOriginal(orig, root), true, nil
 }
 
+// DuplicateOriginal is the file the last (duplicate) test matched against,
+// absolute; "" when none did.
+func (f *facts) DuplicateOriginal() string { return f.dupOriginal }
+
 // displayOriginal reports orig relative to root when it lies inside root,
 // else home-abbreviated (xdg.Abbrev), as every other user-visible path is.
 func displayOriginal(orig, root string) string {
diff --git a/internal/engine/match.go b/internal/engine/match.go
index 59cea42..25c05e5 100644
--- a/internal/engine/match.go
+++ b/internal/engine/match.go
@@ -46,6 +46,13 @@ type FileMatch struct {
 	// directory's rules use, or that check failed. Only set for a file some
 	// matching rule would delete.
 	NoDelete string
+
+	// DuplicateOf is the file a (duplicate) test matched this one against,
+	// absolute; "" when none did. The reason text says the same thing the
+	// way it reads best - relative to the directory when it is inside it -
+	// which leaves a front end no way to act on the other copy, or even to
+	// say where it is (his report, 2026-09-17).
+	DuplicateOf string
 }
 
 // Result is everything Match found in one directory.
@@ -190,6 +197,7 @@ func evalFile(run *matchRun, file scan.File) FileMatch {
 	if len(run.d.DupScopes) > 0 && deletes(fm.Rules) {
 		fm.NoDelete = noDelete(f, run.d.DupScopes)
 	}
+	fm.DuplicateOf = f.DuplicateOriginal()
 	return fm
 }
 
diff --git a/internal/engine/match_test.go b/internal/engine/match_test.go
index 63bb54a..224f881 100644
--- a/internal/engine/match_test.go
+++ b/internal/engine/match_test.go
@@ -460,3 +460,37 @@ func TestRuleDuplicateWarningShortensThePath(t *testing.T) {
 		t.Errorf("no shortened rule warning for a.txt: %+v %+v", r.Matched, r.Unmatched)
 	}
 }
+
+// TestFileMatchNamesTheDuplicate: a file a (duplicate) test matched comes
+// back with the other copy's absolute path, so a front end can say where it
+// is and act on it - the reason text alone says "duplicate of NAME" for a
+// copy inside the directory, which reads as no place at all (plan 21).
+func TestFileMatchNamesTheDuplicate(t *testing.T) {
+	e, d, _ := fixture(t)
+	res, err := e.Match(context.Background(), d)
+	if err != nil {
+		t.Fatal(err)
+	}
+	found := false
+	for _, fm := range res.Matched {
+		if fm.File.Rel != "report (1).pdf" && fm.File.Rel != "report.pdf" {
+			continue
+		}
+		if fm.DuplicateOf == "" {
+			continue
+		}
+		found = true
+		if !filepath.IsAbs(fm.DuplicateOf) {
+			t.Errorf("%s: DuplicateOf = %q, want an absolute path", fm.File.Rel, fm.DuplicateOf)
+		}
+		if fm.DuplicateOf == fm.File.Path {
+			t.Errorf("%s: reported as a duplicate of itself", fm.File.Rel)
+		}
+		if _, err := os.Stat(fm.DuplicateOf); err != nil {
+			t.Errorf("%s: DuplicateOf does not exist: %v", fm.File.Rel, err)
+		}
+	}
+	if !found {
+		t.Fatal("neither copy came back with the file it duplicates")
+	}
+}