rds: support explicit text clearing and symbol bootstrap

internal/rds/encoder.go

@@ -94,8 +94,17 @@ type Encoder struct {
 
 	// Live-updatable text — written by control API, read at group boundaries.
 	// Zero-contention: atomic swap, checked once per RDS group (~88ms at 228kHz).
-	livePS atomic.Value // string
-	liveRT atomic.Value // string
+	// pendingText.set distinguishes "no pending update" from "update to empty string"
+	// so that PS/RT can be explicitly cleared via UpdateText.
+	livePS atomic.Value // pendingText
+	liveRT atomic.Value // pendingText
+}
+
+// pendingText carries a pending text update for PS or RT.
+// set=false means no update is pending; set=true means apply val (even if empty).
+type pendingText struct {
+	val string
+	set bool
 }
 
 func NewEncoder(cfg RDSConfig) (*Encoder, error) {
@@ -163,16 +172,29 @@ func (e *Encoder) Reset() {
 
 // UpdateText hot-swaps PS and/or RT. Thread-safe — called from HTTP handlers,
 // applied at the next RDS group boundary by the DSP goroutine.
-// Pass empty string to leave a field unchanged.
+//
+// Pass empty string to leave a field unchanged. To explicitly clear a field
+// (set PS to 8 spaces, or RT to empty), use ClearPS/ClearRT instead.
 func (e *Encoder) UpdateText(ps, rt string) {
 	if ps != "" {
-		e.livePS.Store(normalizePS(ps))
+		e.livePS.Store(pendingText{val: normalizePS(ps), set: true})
 	}
 	if rt != "" {
-		e.liveRT.Store(normalizeRT(rt))
+		e.liveRT.Store(pendingText{val: normalizeRT(rt), set: true})
 	}
 }
 
+// ClearPS resets the Program Service name to 8 spaces at the next group boundary.
+func (e *Encoder) ClearPS() {
+	e.livePS.Store(pendingText{val: normalizePS(""), set: true})
+}
+
+// ClearRT resets RadioText to an empty string at the next group boundary.
+// Per RDS spec, an empty RT causes receivers to clear their display.
+func (e *Encoder) ClearRT() {
+	e.liveRT.Store(pendingText{val: "", set: true})
+}
+
 // NextSample returns the next RDS subcarrier sample at the configured rate.
 // Uses the internal free-running 57 kHz carrier. Prefer NextSampleWithCarrier
 // for phase-locked operation in a stereo MPX chain.
@@ -192,15 +214,15 @@ func (e *Encoder) NextSampleWithCarrier(carrier float64) float64 {
 			// Apply live text updates at group boundaries (~88ms at 228kHz).
 			// Atomics are consumed (cleared) after reading to prevent
 			// re-applying the same text every group and toggling A/B flag.
-			if ps, ok := e.livePS.Load().(string); ok && ps != "" {
-				e.scheduler.cfg.PS = ps
-				e.livePS.Store("") // consumed
+			if pt, ok := e.livePS.Load().(pendingText); ok && pt.set {
+				e.scheduler.cfg.PS = pt.val
+				e.livePS.Store(pendingText{}) // consumed
 			}
-			if rt, ok := e.liveRT.Load().(string); ok && rt != "" {
-				e.scheduler.cfg.RT = rt
+			if pt, ok := e.liveRT.Load().(pendingText); ok && pt.set {
+				e.scheduler.cfg.RT = pt.val
 				e.scheduler.rtIdx = 0 // restart RT transmission for new text
 				e.scheduler.rtABFlag = !e.scheduler.rtABFlag // toggle A/B per RDS spec
-				e.liveRT.Store("") // consumed
+				e.liveRT.Store(pendingText{}) // consumed
 			}
 			e.getRDSGroup()
 			e.bitPos = 0
@@ -240,12 +262,27 @@ func (e *Encoder) Generate(n int) []float64 {
 	out := make([]float64, n); for i := range out { out[i] = e.NextSample() }; return out
 }
 func (e *Encoder) Symbol() float64 {
-	if e.bitPos >= bitsPerGroup { return -1 }
-	sym := 1.0; if e.bitBuffer[e.bitPos] == 0 { sym = -1.0 }
+	// Populate the bit buffer on first call (bitPos starts at bitsPerGroup
+	// after NewEncoder/Reset, so the guard below would return -1 immediately
+	// without this bootstrap step).
+	if e.bitPos >= bitsPerGroup {
+		e.getRDSGroup()
+		e.bitPos = 0
+	}
+	sym := 1.0
+	if e.bitBuffer[e.bitPos] == 0 {
+		sym = -1.0
+	}
 	e.sampleCount++
-	if e.sampleCount >= e.spb { e.sampleCount = 0; e.bitPos++
-		if e.bitPos >= bitsPerGroup { e.getRDSGroup(); e.bitPos = 0 }
-	}; return sym
+	if e.sampleCount >= e.spb {
+		e.sampleCount = 0
+		e.bitPos++
+		if e.bitPos >= bitsPerGroup {
+			e.getRDSGroup()
+			e.bitPos = 0
+		}
+	}
+	return sym
 }
 
 func (e *Encoder) getRDSGroup() {