{master} counter player biases in daily puzzle selection

modules/common/src/main/Random.scala

@@ -38,6 +38,11 @@ abstract class Random {
     vec.nonEmpty ?? {
       vec lift nextInt(vec.size)
     }
+
+  // odds(1) = 100% true
+  // odds(2) = 50% true
+  // odds(3) = 33% true
+  def odds(n: Int): Boolean = nextInt(n) == 0
 }
 
 object ThreadLocalRandom extends Random {

modules/puzzle/src/main/DailyPuzzle.scala

@@ -7,6 +7,7 @@ import scala.concurrent.duration._
 
 import lila.db.dsl._
 import lila.memo.CacheApi._
+import lila.common.ThreadLocalRandom
 
 final private[puzzle] class DailyPuzzle(
     colls: PuzzleColls,
@@ -26,7 +27,7 @@ final private[puzzle] class DailyPuzzle(
   def get: Fu[Option[DailyPuzzle.WithHtml]] = cache.getUnit
 
   private def find: Fu[Option[DailyPuzzle.WithHtml]] =
-    (findCurrent orElse findNew) recover { case e: Exception =>
+    (findCurrent orElse findNewBiased()) recover { case e: Exception =>
       logger.error("find daily", e)
       none
     } flatMap { _ ?? makeDaily }
@@ -48,6 +49,18 @@ final private[puzzle] class DailyPuzzle(
         .one[Puzzle]
     }
 
+  private def findNewBiased(tries: Int = 0): Fu[Option[Puzzle]] = {
+    def tryAgainMaybe = (tries < 5) ?? findNewBiased(tries + 1)
+    import lila.common.ThreadLocalRandom.odds
+    import PuzzleTheme._
+    findNew flatMap {
+      case None                                             => tryAgainMaybe
+      case Some(p) if p.hasTheme(anastasiaMate) && !odds(3) => tryAgainMaybe dmap (_ orElse p.some)
+      case Some(p) if p.hasTheme(arabianMate) && odds(2)    => tryAgainMaybe dmap (_ orElse p.some)
+      case p                                                => fuccess(p)
+    }
+  }
+
   private def findNew: Fu[Option[Puzzle]] =
     colls
       .path {

modules/puzzle/src/main/Puzzle.scala

@@ -29,6 +29,8 @@ case class Puzzle(
   } err s"Can't apply puzzle $id first move"
 
   def color = fen.color.fold[chess.Color](chess.White)(!_)
+
+  def hasTheme(theme: PuzzleTheme) = themes(theme.key)
 }
 
 object Puzzle {