feat: Add tap tempo

Tapping the TAP button records timestamps and calculates BPM from the
average interval between the last few taps (within a 3 second window).
After 2 seconds of no tapping the window resets. BPM is clamped to the
40-200 range and immediately applied to the running service if active.

The TAP button sits to the left of START/STOP in a shared row, taking
1/3 of the width; START/STOP takes the remaining 2/3.

app/src/main/kotlin/net/jeena/pacer/PacerViewModel.kt

@@ -1,18 +1,27 @@
 package net.jeena.pacer
 
 import android.app.Application
+import android.os.SystemClock
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableFloatStateOf
 import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.setValue
 import androidx.lifecycle.AndroidViewModel
+import androidx.lifecycle.viewModelScope
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+import kotlin.math.roundToInt
 
 class PacerViewModel(application: Application) : AndroidViewModel(application) {
 
     private val ctx = application.applicationContext
     private val settings = SettingsRepository(ctx)
 
+    private val tapTimes = mutableListOf<Long>()
+    private var tapResetJob: Job? = null
+
     var bpm by mutableIntStateOf(settings.loadBpm())
         private set
 
@@ -43,4 +52,21 @@ class PacerViewModel(application: Application) : AndroidViewModel(application) {
         settings.saveVolume(newVolume)
         if (isPlaying) ctx.startService(PacerService.updateVolumeIntent(ctx, newVolume))
     }
+
+    fun onTap() {
+        val now = SystemClock.elapsedRealtime()
+        tapTimes.removeAll { now - it > 3000 }
+        tapTimes.add(now)
+
+        if (tapTimes.size >= 2) {
+            val avgInterval = tapTimes.zipWithNext { a, b -> b - a }.average()
+            updateBpm((60000.0 / avgInterval).roundToInt().coerceIn(40, 200))
+        }
+
+        tapResetJob?.cancel()
+        tapResetJob = viewModelScope.launch {
+            delay(2000)
+            tapTimes.clear()
+        }
+    }
 }

app/src/main/kotlin/net/jeena/pacer/ui/PacerScreen.kt

@@ -10,6 +10,8 @@ import androidx.compose.foundation.background
 import androidx.compose.foundation.border
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.width
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.PaddingValues
 import androidx.compose.foundation.layout.Row
@@ -125,7 +127,8 @@ fun PacerScreen(viewModel: PacerViewModel, modifier: Modifier = Modifier) {
                 dotScale     = dotScale.value,
                 onBpmChange  = { viewModel.updateBpm(it) },
                 onVolChange  = { viewModel.updateVolume(it) },
-                onToggle     = { viewModel.togglePlayback() }
+                onToggle     = { viewModel.togglePlayback() },
+                onTap        = { viewModel.onTap() }
             )
         } else {
             PortraitLayout(
@@ -139,7 +142,8 @@ fun PacerScreen(viewModel: PacerViewModel, modifier: Modifier = Modifier) {
                 dotScale     = dotScale.value,
                 onBpmChange  = { viewModel.updateBpm(it) },
                 onVolChange  = { viewModel.updateVolume(it) },
-                onToggle     = { viewModel.togglePlayback() }
+                onToggle     = { viewModel.togglePlayback() },
+                onTap        = { viewModel.onTap() }
             )
         }
 
@@ -181,6 +185,7 @@ private fun PortraitLayout(
     onBpmChange: (Int)   -> Unit,
     onVolChange: (Float) -> Unit,
     onToggle:    ()      -> Unit,
+    onTap:       ()      -> Unit,
 ) {
     Column(
         modifier = Modifier
@@ -195,7 +200,7 @@ private fun PortraitLayout(
         PulseRing(rippleScale, rippleAlpha, dotScale, size = 80.dp)
         BpmControls(bpm, onBpmChange)
         VolumeControl(volume, onVolChange)
-        StartStopButton(isPlaying, onToggle, Modifier.fillMaxWidth())
+        BottomButtons(isPlaying, onToggle, onTap)
     }
 }
 
@@ -212,6 +217,7 @@ private fun LandscapeLayout(
     onBpmChange: (Int)   -> Unit,
     onVolChange: (Float) -> Unit,
     onToggle:    ()      -> Unit,
+    onTap:       ()      -> Unit,
 ) {
     Row(
         modifier = Modifier
@@ -242,7 +248,7 @@ private fun LandscapeLayout(
         ) {
             BpmControls(bpm, onBpmChange)
             VolumeControl(volume, onVolChange)
-            StartStopButton(isPlaying, onToggle, Modifier.fillMaxWidth())
+            BottomButtons(isPlaying, onToggle, onTap)
         }
     }
 }
@@ -416,24 +422,45 @@ private fun VolumeControl(volume: Float, onVolChange: (Float) -> Unit) {
 }
 
 @Composable
-private fun StartStopButton(isPlaying: Boolean, onToggle: () -> Unit, modifier: Modifier = Modifier) {
+private fun BottomButtons(isPlaying: Boolean, onToggle: () -> Unit, onTap: () -> Unit) {
-    OutlinedButton(
+    Row(modifier = Modifier.fillMaxWidth(), horizontalArrangement = Arrangement.spacedBy(8.dp)) {
-        onClick = onToggle,
+        OutlinedButton(
-        modifier = modifier,
+            onClick = onTap,
-        shape = RectangleShape,
+            modifier = Modifier.weight(1f),
-        border = BorderStroke(2.dp, if (isPlaying) ACCENT else FG),
+            shape = RectangleShape,
-        colors = ButtonDefaults.outlinedButtonColors(
+            border = BorderStroke(2.dp, DIM),
-            contentColor = if (isPlaying) BG else FG,
+            colors = ButtonDefaults.outlinedButtonColors(
-            containerColor = if (isPlaying) ACCENT else Color.Transparent
+                contentColor = FG,
-        )
+                containerColor = Color.Transparent
-    ) {
+            )
-        Text(
+        ) {
-            text = if (isPlaying) "STOP" else "START",
+            Text(
-            fontSize = 18.sp,
+                text = "TAP",
-            fontWeight = FontWeight.Bold,
+                fontSize = 18.sp,
-            fontFamily = JetBrainsMono,
+                fontWeight = FontWeight.Bold,
-            letterSpacing = 3.sp,
+                fontFamily = JetBrainsMono,
-            modifier = Modifier.padding(vertical = 6.dp)
+                letterSpacing = 3.sp,
-        )
+                modifier = Modifier.padding(vertical = 6.dp)
+            )
+        }
+        OutlinedButton(
+            onClick = onToggle,
+            modifier = Modifier.weight(2f),
+            shape = RectangleShape,
+            border = BorderStroke(2.dp, if (isPlaying) ACCENT else FG),
+            colors = ButtonDefaults.outlinedButtonColors(
+                contentColor = if (isPlaying) BG else FG,
+                containerColor = if (isPlaying) ACCENT else Color.Transparent
+            )
+        ) {
+            Text(
+                text = if (isPlaying) "STOP" else "START",
+                fontSize = 18.sp,
+                fontWeight = FontWeight.Bold,
+                fontFamily = JetBrainsMono,
+                letterSpacing = 3.sp,
+                modifier = Modifier.padding(vertical = 6.dp)
+            )
+        }
     }
 }