Tap tempo by tapping the BPM readout (v0.15.0)

Tapping the "### BPM" label sets the tempo from the average interval of recent
taps (up to 8, refining as you go); a pause over 2 s starts a fresh count. The
−/+ nudge buttons are unchanged. Purely UI-side — a small remembered list of tap
timestamps, no model/engine changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/build.gradle.kts

@@ -22,8 +22,8 @@ android {
         // (android.media.midi) and AAudio low-latency audio we rely on.
         minSdk = 26
         targetSdk = 34
-        versionCode = 38
-        versionName = "0.14.0"
+        versionCode = 39
+        versionName = "0.15.0"
 
         // We provide our own instrumentation runner if/when tests are added.
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"

app/src/main/java/space/rcmd/android/sizzle/ui/tracker/TrackerScreen.kt

@@ -4,6 +4,7 @@
 package space.rcmd.android.sizzle.ui.tracker
 
 import androidx.compose.foundation.background
+import androidx.compose.foundation.gestures.detectTapGestures
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
@@ -19,8 +20,10 @@ import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
+import androidx.compose.runtime.remember
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.ui.text.font.FontFamily
 import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.unit.dp
@@ -147,14 +150,31 @@ private fun TransportButtons(vm: AppViewModel) {
     // Fixed width so the button doesn't resize as the label toggles.
     RetroButton(if (isPlaying) "❚❚ PAUSE" else "▶ PLAY",
         active = isPlaying, width = 112.dp, onClick = vm::playPause)
-    // Tempo nudger: tap -/+ around a fixed-width readout.
+    // Tempo nudger: tap -/+ around a fixed-width readout. Tapping the readout itself
+    // is TAP TEMPO — two or more taps in time set the tempo from the average interval.
+    val tapTimes = remember { ArrayList<Long>() }
     Row(verticalAlignment = Alignment.CenterVertically) {
         RepeatButton("-", onStep = { vm.setTempo(project.tempoBpm - 1) })
         Text(
             "${project.tempoBpm.toInt()} BPM",
             color = c.text, fontFamily = FontFamily.Monospace, fontSize = 12.sp,
             textAlign = TextAlign.Center, maxLines = 1,
-            modifier = Modifier.width(72.dp).padding(vertical = 6.dp),
+            modifier = Modifier
+                .width(72.dp)
+                .pointerInput(Unit) {
+                    detectTapGestures {
+                        val now = System.currentTimeMillis()
+                        // A long pause starts a fresh count rather than averaging across it.
+                        if (tapTimes.isNotEmpty() && now - tapTimes.last() > TAP_RESET_MS) tapTimes.clear()
+                        tapTimes.add(now)
+                        while (tapTimes.size > TAP_MAX) tapTimes.removeAt(0)
+                        if (tapTimes.size >= 2) {
+                            val span = tapTimes.last() - tapTimes.first()
+                            if (span > 0) vm.setTempo(60_000f * (tapTimes.size - 1) / span)
+                        }
+                    }
+                }
+                .padding(vertical = 6.dp),
         )
         RepeatButton("+", onStep = { vm.setTempo(project.tempoBpm + 1) })
     }
@@ -224,3 +244,8 @@ private fun EditButtons(vm: AppViewModel) {
     // Swap the bottom half between the arranger and the punch-in keyboard.
     RetroButton("⌨ KBD", active = vm.punchInMode, onClick = vm::togglePunchIn)
 }
+
+// Tap-tempo tuning: a gap longer than this starts a fresh count; keep at most this
+// many recent taps in the averaging window.
+private const val TAP_RESET_MS = 2000L
+private const val TAP_MAX = 8