A new snapshot is always created after new event

src/main/kotlin/eventDemo/libs/event/projection/ProjectionSnapshotRepositoryInMemory.kt

@@ -15,10 +15,20 @@ import kotlin.time.Duration
 import kotlin.time.Duration.Companion.minutes
 
 data class SnapshotConfig(
+  /**
+   * Keep snapshot when is on the head of the queue cache
+   */
   val maxSnapshotCacheSize: Int = 20,
+  /**
+   * Keep snapshot when is newer of
+   *
+   *     snapshot.date > now + maxSnapshotCacheTtl
+   */
   val maxSnapshotCacheTtl: Duration = 10.minutes,
   /**
-   * Only create [snapshots][Projection] every [X][modulo] [events][Event]
+   * Keep snapshot when version is this modulo
+   *
+   *     snapshot.lastVersion % modulo == 1
    */
   val modulo: Int = 10,
 )
@@ -42,14 +52,12 @@ class ProjectionSnapshotRepositoryInMemory<E : Event<ID>, P : Projection<ID>, ID
    * 6. remove old one
    */
   fun applyAndPutToCache(event: E) {
-    if ((event.version % snapshotCacheConfig.modulo) == 1) {
     getUntil(event)
       .also {
         save(it)
         removeOldSnapshot(it.aggregateId)
       }
   }
-  }
 
   /**
    * Build the list of all [Projections][Projection]
@@ -100,46 +108,65 @@ class ProjectionSnapshotRepositoryInMemory<E : Event<ID>, P : Projection<ID>, ID
   }
 
   /**
-   * Remove the oldest snapshot.
+   * Remove the oldest [snapshot][P] of the [queue][projectionsSnapshot].
    *
    * The rules are pass in the controller.
    */
   private fun removeOldSnapshot(aggregateId: ID) {
     projectionsSnapshot[aggregateId]?.let { queue ->
-      // never remove the last one
-      val theLastOne = getLastSnapshot(aggregateId)
-      removeByDate(queue, theLastOne)
-      removeBySize(queue, theLastOne)
-    }
-  }
-
-  private fun removeBySize(
-    queue: ConcurrentLinkedQueue<Pair<P, Instant>>,
-    theLastOne: Pair<P, Instant>?,
-  ) {
-    // Remove if size exceeds the limit
-    val size = queue.size
-    if (size > snapshotCacheConfig.maxSnapshotCacheSize) {
-      val numberToRemove = size - snapshotCacheConfig.maxSnapshotCacheSize
-      if (numberToRemove > 0) {
       queue
-          .sortedBy { it.first.lastEventVersion }
-          .take(numberToRemove)
-          .let { it - theLastOne }
+        .excludeFirstAndLast()
+        .excludeTheHeadBySize()
+        .excludeNewerByDate()
+        .excludeByModulo()
         .forEach { queue.remove(it) }
     }
   }
+
+  /**
+   * Return a new list without the first and last snapshot.
+   *
+   * Exclude from deletion the first and the last.
+   */
+  private fun FilteredList<P>.excludeFirstAndLast(): FilteredList<P> =
+    sortedBy { it.first.lastEventVersion }
+      .drop(1)
+      .dropLast(1)
+
+  /**
+   * Return a new list of event filtered by the version modulo.
+   *
+   * Exclude from deletion 1 element out of 10 (if modulo 10 in [config][snapshotCacheConfig]).
+   */
+  private fun FilteredList<P>.excludeByModulo(): FilteredList<P> =
+    filter { (it.first.lastEventVersion % snapshotCacheConfig.modulo) != 1 }
+
+  /**
+   * Return a new list of event filtered by the maximum size.
+   *
+   * Exclude from removal all [snapshot][projectionsSnapshot] that in the head of the queue.
+   */
+  private fun FilteredList<P>.excludeTheHeadBySize(): FilteredList<P> {
+    // filter if size exceeds the limit
+    if (size > snapshotCacheConfig.maxSnapshotCacheSize) {
+      val numberToRemove = size - snapshotCacheConfig.maxSnapshotCacheSize
+      if (numberToRemove > 0) {
+        return sortedBy { it.first.lastEventVersion }
+          .takeLast(numberToRemove)
+      }
+    }
+    return this
   }
 
-  private fun removeByDate(
-    queue: ConcurrentLinkedQueue<Pair<P, Instant>>,
-    theLastOne: Pair<P, Instant>?,
-  ) {
-    // remove the oldest by time
+  /**
+   * Return a new list of event filtered by the maximum date.
+   *
+   * Exclude from removal all [snapshot][projectionsSnapshot] that newer of the date (in [config][SnapshotConfig]).
+   */
+  private fun FilteredList<P>.excludeNewerByDate(): FilteredList<P> {
     val now = Clock.System.now()
     val deadLine = now - snapshotCacheConfig.maxSnapshotCacheTtl
-    val toRemove = queue.filter { deadLine > it.second }
-    (toRemove - theLastOne).forEach { queue.remove(it) }
+    return filter { deadLine < it.second }
   }
 
   /**
@@ -204,3 +231,5 @@ class ProjectionSnapshotRepositoryInMemory<E : Event<ID>, P : Projection<ID>, ID
     }
   }
 }
+
+private typealias FilteredList<P> = Collection<Pair<P, Instant>>