feat(internal): add ListOr[T any] type

This is a utility type that lets you decode a JSON T or list of T as a
single value. This will be used with Redis Sentinel config so that you
can specify multiple sentinel addresses.

Ref TecharoHQ/botstopper#24

Assisted-by: GLM 4.6 via Claude Code
Signed-off-by: Xe Iaso <me@xeiaso.net>

internal/listor.go

@@ -0,0 +1,39 @@
+package internal
+
+import (
+	"encoding/json"
+)
+
+// ListOr[T any] is a slice that can contain either a single T or multiple T values.
+// During JSON unmarshaling, it checks if the first character is '[' to determine
+// whether to treat the JSON as an array or a single value.
+type ListOr[T any] []T
+
+func (lo *ListOr[T]) UnmarshalJSON(data []byte) error {
+	if len(data) == 0 {
+		return nil
+	}
+
+	// Check if first non-whitespace character is '['
+	firstChar := data[0]
+	for i := 0; i < len(data); i++ {
+		if data[i] != ' ' && data[i] != '\t' && data[i] != '\n' && data[i] != '\r' {
+			firstChar = data[i]
+			break
+		}
+	}
+
+	if firstChar == '[' {
+		// It's an array, unmarshal directly
+		return json.Unmarshal(data, (*[]T)(lo))
+	} else {
+		// It's a single value, unmarshal as a single item in a slice
+		var single T
+		if err := json.Unmarshal(data, &single); err != nil {
+			return err
+		}
+		*lo = ListOr[T]{single}
+	}
+
+	return nil
+}

internal/listor_test.go

@@ -0,0 +1,79 @@
+package internal
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestListOr_UnmarshalJSON(t *testing.T) {
+	t.Run("single value should be unmarshaled as single item", func(t *testing.T) {
+		var lo ListOr[string]
+
+		err := json.Unmarshal([]byte(`"hello"`), &lo)
+		if err != nil {
+			t.Fatalf("Failed to unmarshal single string: %v", err)
+		}
+
+		if len(lo) != 1 {
+			t.Fatalf("Expected 1 item, got %d", len(lo))
+		}
+
+		if lo[0] != "hello" {
+			t.Errorf("Expected 'hello', got %q", lo[0])
+		}
+	})
+
+	t.Run("array should be unmarshaled as multiple items", func(t *testing.T) {
+		var lo ListOr[string]
+
+		err := json.Unmarshal([]byte(`["hello", "world"]`), &lo)
+		if err != nil {
+			t.Fatalf("Failed to unmarshal array: %v", err)
+		}
+
+		if len(lo) != 2 {
+			t.Fatalf("Expected 2 items, got %d", len(lo))
+		}
+
+		if lo[0] != "hello" {
+			t.Errorf("Expected 'hello', got %q", lo[0])
+		}
+		if lo[1] != "world" {
+			t.Errorf("Expected 'world', got %q", lo[1])
+		}
+	})
+
+	t.Run("single number should be unmarshaled as single item", func(t *testing.T) {
+		var lo ListOr[int]
+
+		err := json.Unmarshal([]byte(`42`), &lo)
+		if err != nil {
+			t.Fatalf("Failed to unmarshal single number: %v", err)
+		}
+
+		if len(lo) != 1 {
+			t.Fatalf("Expected 1 item, got %d", len(lo))
+		}
+
+		if lo[0] != 42 {
+			t.Errorf("Expected 42, got %d", lo[0])
+		}
+	})
+
+	t.Run("array of numbers should be unmarshaled as multiple items", func(t *testing.T) {
+		var lo ListOr[int]
+
+		err := json.Unmarshal([]byte(`[1, 2, 3]`), &lo)
+		if err != nil {
+			t.Fatalf("Failed to unmarshal number array: %v", err)
+		}
+
+		if len(lo) != 3 {
+			t.Fatalf("Expected 3 items, got %d", len(lo))
+		}
+
+		if lo[0] != 1 || lo[1] != 2 || lo[2] != 3 {
+			t.Errorf("Expected [1, 2, 3], got %v", lo)
+		}
+	})
+}