minor refactor of replaceInvalidRune() in bridge.go

prometheus/graphite/bridge.go

@@ -307,8 +307,13 @@ func replaceInvalidRune(c rune) rune {
 	if c == ' ' {
 		return '.'
 	}
-	// TODO: Apply De Morgan's law to the condition. Make sure to test the condition first.
-	if !((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_' || c == ':' || c == '-' || (c >= '0' && c <= '9')) { //nolint:staticcheck
+	// !((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_' || c == ':' || c == '-' || (c >= '0' && c <= '9'))
+	if (c < 'a' || c > 'z') &&
+		(c < 'A' || c > 'Z') &&
+		c != '_' &&
+		c != ':' &&
+		c != '-' &&
+		(c < '0' || c > '9') {
 		return '_'
 	}
 	return c

prometheus/graphite/bridge_test.go

@@ -468,3 +468,33 @@ func ExampleBridge() {
 	// Start pushing metrics to Graphite in the Run() loop.
 	b.Run(ctx)
 }
+
+func TestReplaceInvalidRune(t *testing.T) {
+	tests := []struct {
+		in   rune
+		want rune
+	}{
+		{' ', '.'},
+
+		{'a', 'a'},
+		{'B', 'B'},
+		{'0', '0'},
+		{'9', '9'},
+		{'_', '_'},
+		{':', ':'},
+		{'-', '-'},
+
+		{'#', '_'},
+		{'$', '_'},
+		{'@', '_'},
+		{'!', '_'},
+		{'~', '_'},
+	}
+
+	for _, tt := range tests {
+		got := replaceInvalidRune(tt.in)
+		if got != tt.want {
+			t.Fatalf("replaceInvalidRune(%q) = %q; want %q", tt.in, got, tt.want)
+		}
+	}
+}