settings: make the preamp easier to set exactly

The slider alone is awkward for the two things people actually do with a
preamp: zero it, and nudge it by a known amount.

- Minus and plus buttons flank the slider, stepping one grid notch (0.5
  dB) and disabling at the ends.
- A Reset button sits under it, beside the current value, disabled when
  the value is already zero.
- Both go through the same snapping the slider does, so a button cannot
  land off the grid.

app/src/main/kotlin/com/subtracks/ui/settings/ReplayGainControls.kt

@@ -17,6 +17,13 @@ internal val REPLAY_GAIN_PREAMP_STEPS =
 // storing it.
 internal fun snapReplayGainPreampDb(value: Float): Float = (value / REPLAY_GAIN_PREAMP_STEP_DB).roundToInt() * REPLAY_GAIN_PREAMP_STEP_DB
 
+internal fun stepReplayGainPreampDb(
+    value: Float,
+    steps: Int,
+): Float =
+    snapReplayGainPreampDb(value + steps * REPLAY_GAIN_PREAMP_STEP_DB)
+        .coerceIn(MIN_REPLAY_GAIN_PREAMP_DB, MAX_REPLAY_GAIN_PREAMP_DB)
+
 internal fun replayGainModeLabel(
     resources: Resources,
     mode: ReplayGainMode,

app/src/main/kotlin/com/subtracks/ui/settings/SettingsScreen.kt

@@ -28,6 +28,7 @@ import androidx.compose.material.icons.rounded.Edit
 import androidx.compose.material.icons.rounded.Favorite
 import androidx.compose.material.icons.rounded.Info
 import androidx.compose.material.icons.rounded.Language
+import androidx.compose.material.icons.rounded.Remove
 import androidx.compose.material.icons.rounded.Share
 import androidx.compose.material3.AlertDialog
 import androidx.compose.material3.Button
@@ -779,13 +780,44 @@ private fun ReplayGainPreampDialog(
                     style = MaterialTheme.typography.titleSmall,
                     color = MaterialTheme.colorScheme.primary,
                 )
-                Slider(
-                    value = draft,
-                    onValueChange = { draft = snapReplayGainPreampDb(it) },
-                    valueRange = MIN_REPLAY_GAIN_PREAMP_DB..MAX_REPLAY_GAIN_PREAMP_DB,
-                    steps = REPLAY_GAIN_PREAMP_STEPS,
-                )
-                Text(replayGainPreampLabel(resources, draft))
+                Row(verticalAlignment = Alignment.CenterVertically) {
+                    IconButton(
+                        onClick = { draft = stepReplayGainPreampDb(draft, -1) },
+                        enabled = draft > MIN_REPLAY_GAIN_PREAMP_DB,
+                    ) {
+                        Icon(
+                            Icons.Rounded.Remove,
+                            contentDescription = stringResource(R.string.settings_replay_gain_preamp_decrease),
+                        )
+                    }
+                    Slider(
+                        value = draft,
+                        onValueChange = { draft = snapReplayGainPreampDb(it) },
+                        valueRange = MIN_REPLAY_GAIN_PREAMP_DB..MAX_REPLAY_GAIN_PREAMP_DB,
+                        steps = REPLAY_GAIN_PREAMP_STEPS,
+                        modifier = Modifier.weight(1f),
+                    )
+                    IconButton(
+                        onClick = { draft = stepReplayGainPreampDb(draft, 1) },
+                        enabled = draft < MAX_REPLAY_GAIN_PREAMP_DB,
+                    ) {
+                        Icon(
+                            Icons.Rounded.Add,
+                            contentDescription = stringResource(R.string.settings_replay_gain_preamp_increase),
+                        )
+                    }
+                }
+                Row(
+                    verticalAlignment = Alignment.CenterVertically,
+                    modifier = Modifier.fillMaxWidth(),
+                ) {
+                    Text(replayGainPreampLabel(resources, draft))
+                    Spacer(Modifier.weight(1f))
+                    TextButton(
+                        onClick = { draft = 0f },
+                        enabled = draft != 0f,
+                    ) { Text(stringResource(R.string.settings_replay_gain_preamp_reset)) }
+                }
             }
         },
         confirmButton = {

app/src/main/res/values/strings.xml

@@ -77,7 +77,10 @@
     <string name="settings_replay_gain_mode_album">Album gain</string>
     <string name="settings_replay_gain_mode_off">Off</string>
     <string name="settings_replay_gain_mode_track">Track gain</string>
+    <string name="settings_replay_gain_preamp_decrease">Lower preamp</string>
+    <string name="settings_replay_gain_preamp_increase">Raise preamp</string>
     <string name="settings_replay_gain_preamp_prompt">Adjust level on top of the replay gain</string>
+    <string name="settings_replay_gain_preamp_reset">Reset</string>
     <string name="settings_replay_gain_preamp_title">Preamp</string>
     <string name="settings_replay_gain_preamp_value">%1$s dB</string>
     <string name="settings_replay_gain_prompt">Normalise loudness using the gain from the server</string>

app/src/test/kotlin/com/subtracks/ui/settings/ReplayGainControlsTest.kt

@@ -34,6 +34,14 @@ class ReplayGainControlsTest {
         assertEquals(59, REPLAY_GAIN_PREAMP_STEPS)
     }
 
+    @Test
+    fun thePreampStepsAlongTheGridAndStopsAtTheEnds() {
+        assertEquals(-1.5f, stepReplayGainPreampDb(-1f, -1), 0f)
+        assertEquals(-0.5f, stepReplayGainPreampDb(-1f, 1), 0f)
+        assertEquals(MIN_REPLAY_GAIN_PREAMP_DB, stepReplayGainPreampDb(MIN_REPLAY_GAIN_PREAMP_DB, -1), 0f)
+        assertEquals(MAX_REPLAY_GAIN_PREAMP_DB, stepReplayGainPreampDb(MAX_REPLAY_GAIN_PREAMP_DB, 1), 0f)
+    }
+
     @Test
     fun thePreampSnapsBackOntoTheHalfDecibelGrid() {
         assertEquals(0f, snapReplayGainPreampDb(-0f), 0f)