From 4b04dfed2ac93a673807f2fa9ca020b1d3ed2815 Mon Sep 17 00:00:00 2001
From: Virgil <virgil@lethean.io>
Date: Thu, 2 Apr 2026 09:07:10 +0000
Subject: [PATCH] fix(grammar): dedupe merged signal lists

Co-Authored-By: Virgil <virgil@lethean.io>
---
 grammar.go     | 37 +++++++++++++++++++++++++------------
 loader_test.go | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+), 12 deletions(-)

diff --git a/grammar.go b/grammar.go
index 38a6609..184bf2d 100644
--- a/grammar.go
+++ b/grammar.go
@@ -122,18 +122,10 @@ func mergeSignalData(dst *SignalData, src SignalData) {
 	if dst == nil {
 		return
 	}
-	if len(src.NounDeterminers) > 0 {
-		dst.NounDeterminers = append(dst.NounDeterminers, src.NounDeterminers...)
-	}
-	if len(src.VerbAuxiliaries) > 0 {
-		dst.VerbAuxiliaries = append(dst.VerbAuxiliaries, src.VerbAuxiliaries...)
-	}
-	if len(src.VerbInfinitive) > 0 {
-		dst.VerbInfinitive = append(dst.VerbInfinitive, src.VerbInfinitive...)
-	}
-	if len(src.VerbNegation) > 0 {
-		dst.VerbNegation = append(dst.VerbNegation, src.VerbNegation...)
-	}
+	dst.NounDeterminers = appendUniqueStrings(dst.NounDeterminers, src.NounDeterminers...)
+	dst.VerbAuxiliaries = appendUniqueStrings(dst.VerbAuxiliaries, src.VerbAuxiliaries...)
+	dst.VerbInfinitive = appendUniqueStrings(dst.VerbInfinitive, src.VerbInfinitive...)
+	dst.VerbNegation = appendUniqueStrings(dst.VerbNegation, src.VerbNegation...)
 	if len(src.Priors) == 0 {
 		return
 	}
@@ -148,6 +140,27 @@ func mergeSignalData(dst *SignalData, src SignalData) {
 	}
 }
 
+func appendUniqueStrings(dst []string, values ...string) []string {
+	if len(values) == 0 {
+		return dst
+	}
+	seen := make(map[string]struct{}, len(dst))
+	for _, value := range dst {
+		seen[value] = struct{}{}
+	}
+	for _, value := range values {
+		if value == "" {
+			continue
+		}
+		if _, ok := seen[value]; ok {
+			continue
+		}
+		seen[value] = struct{}{}
+		dst = append(dst, value)
+	}
+	return dst
+}
+
 func grammarDataHasContent(data *GrammarData) bool {
 	if data == nil {
 		return false
diff --git a/loader_test.go b/loader_test.go
index eeb949a..ec20244 100644
--- a/loader_test.go
+++ b/loader_test.go
@@ -454,6 +454,49 @@ func TestMergeGrammarData(t *testing.T) {
 	}
 }
 
+func TestMergeGrammarData_DeduplicatesSignals(t *testing.T) {
+	const lang = "zy"
+	original := GetGrammarData(lang)
+	t.Cleanup(func() {
+		SetGrammarData(lang, original)
+	})
+
+	SetGrammarData(lang, &GrammarData{
+		Signals: SignalData{
+			NounDeterminers: []string{"the", "a"},
+			VerbAuxiliaries: []string{"will"},
+			VerbInfinitive:  []string{"to"},
+			VerbNegation:    []string{"not"},
+		},
+	})
+
+	MergeGrammarData(lang, &GrammarData{
+		Signals: SignalData{
+			NounDeterminers: []string{"a", "some"},
+			VerbAuxiliaries: []string{"will", "can"},
+			VerbInfinitive:  []string{"to", "de"},
+			VerbNegation:    []string{"not", "never"},
+		},
+	})
+
+	data := GetGrammarData(lang)
+	if data == nil {
+		t.Fatal("GetGrammarData returned nil")
+	}
+	if got, want := data.Signals.NounDeterminers, []string{"the", "a", "some"}; !slices.Equal(got, want) {
+		t.Fatalf("NounDeterminers = %v, want %v", got, want)
+	}
+	if got, want := data.Signals.VerbAuxiliaries, []string{"will", "can"}; !slices.Equal(got, want) {
+		t.Fatalf("VerbAuxiliaries = %v, want %v", got, want)
+	}
+	if got, want := data.Signals.VerbInfinitive, []string{"to", "de"}; !slices.Equal(got, want) {
+		t.Fatalf("VerbInfinitive = %v, want %v", got, want)
+	}
+	if got, want := data.Signals.VerbNegation, []string{"not", "never"}; !slices.Equal(got, want) {
+		t.Fatalf("VerbNegation = %v, want %v", got, want)
+	}
+}
+
 func TestGrammarDataLanguageTagNormalisation(t *testing.T) {
 	const rawLang = "tl_PH"
 	const canonicalLang = "tl-PH"