musikr.pipeline: shuffle songs to extract

This helps avoid the entire tag parsing flow from getting blocked up
by several tracks that are blocking trying to write a single cover.

musikr/src/main/java/org/oxycblt/musikr/pipeline/ExtractStep.kt

@@ -89,6 +89,7 @@ private class ExtractStepImpl(
 
         val fds =
             uncachedSongs
+                .shuffle()
                 .mapNotNull {
                     wrap(it) { file ->
                         withContext(Dispatchers.IO) {

musikr/src/main/java/org/oxycblt/musikr/pipeline/FlowUtil.kt

@@ -20,9 +20,12 @@ package org.oxycblt.musikr.pipeline
 
 import kotlinx.coroutines.channels.Channel
 import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.asFlow
+import kotlinx.coroutines.flow.emitAll
 import kotlinx.coroutines.flow.flow
 import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.flow.receiveAsFlow
+import kotlinx.coroutines.flow.toList
 import kotlinx.coroutines.flow.withIndex
 
 internal sealed interface Divert<L, R> {
@@ -79,3 +82,15 @@ internal fun <T> Flow<T>.distribute(n: Int): DistributedFlow<T> {
     val hotFlows = posChannels.map { it.receiveAsFlow() }.toTypedArray()
     return DistributedFlow(managerFlow, hotFlows)
 }
+
+internal fun <T> Flow<T>.shuffle() = flow {
+    // As far as I'm aware, the only way to get a truly normal distribution
+    // on a flow is by evaluating it. I tried a bunch of different strategies
+    // on lazily shuffling a flow and it simply doesn't produce a good enough
+    // distribution since you need to emit late stuff early and early stuff
+    // late. It's best to just eval and re-emit.
+    val output = mutableListOf<T>()
+    toList(output)
+    output.shuffle()
+    emitAll(output.asFlow())
+}