Make the peek method behaviour in the path selector deterministic #598 (#600)

Author: CaelmBleidd

utbot-framework/src/main/kotlin/org/utbot/engine/selectors/nurs/NonUniformRandomSearch.kt

@@ -58,6 +58,13 @@ abstract class NonUniformRandomSearch(
 
  private val randomGen: Random? = seed?.let { Random(seed) }
 
+ // We use this value to avoid non-deterministic behaviour of the
+ // peek method. Without it, we might have different states for
+ // a sequence of the `peek` calls.
+ // Now we remember it at the first `peek` call and use it
+ // until a first `poll` call. The first call should reset it back to null.
+ private var lastTakenRandomValue: Double? = null
+
  override fun update() {
  executionQueue.updateAll()
  }
@@ -75,15 +82,22 @@ abstract class NonUniformRandomSearch(
  * with probability executionState.asWeight.weight / sumWeights
  */
  override fun peekImpl(): ExecutionState? {
- val rand = (randomGen?.nextDouble() ?: 1.0) * sumWeights
+ if (lastTakenRandomValue == null) {
+ lastTakenRandomValue = randomGen?.nextDouble()
+ }
+
+ val rand = (lastTakenRandomValue ?: 1.0) * sumWeights
 
  return executionQueue.findLeftest(rand)?.first
  }
 
  override fun removeImpl(state: ExecutionState): Boolean = executionQueue.remove(state)
 
  override fun pollImpl(): ExecutionState? =
- peekImpl()?.also { remove(it) }
+ peekImpl()?.also {
+ remove(it)
+ lastTakenRandomValue = null
+ }
 
  override fun isEmpty() =
  executionQueue.isEmpty()