From a23e060f625cd74d25a63a172b4df82f78023daf Mon Sep 17 00:00:00 2001
From: Thibault Duplessis <t@lichess.org>
Date: Tue, 19 Nov 2024 10:58:05 +0100
Subject: [PATCH] glicko calculations can fail

somehow it happens
---
 core/src/main/scala/glicko/glicko.scala           | 15 ++++++++++-----
 .../src/test/scala/glicko/GlickoCalculator.scala  |  2 +-
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/core/src/main/scala/glicko/glicko.scala b/core/src/main/scala/glicko/glicko.scala
index 43fd550aa..beb494bd1 100644
--- a/core/src/main/scala/glicko/glicko.scala
+++ b/core/src/main/scala/glicko/glicko.scala
@@ -2,6 +2,7 @@ package chess
 package glicko
 
 import java.time.Instant
+import scala.util.Try
 
 case class Player(
     rating: Double,
@@ -31,23 +32,27 @@ trait GlickoCalculatorApi:
   // ): List[Player]
 
   // Simpler use case: a single game
-  def computeGame(game: Game, skipDeviationIncrease: Boolean = false): ByColor[Player]
+  def computeGame(game: Game, skipDeviationIncrease: Boolean = false): Try[ByColor[Player]]
 
   /** This is the formula defined in step 6. It is also used for players who have not competed during the rating period.
     */
-  // def previewDeviation(player: Player, ratingPeriodEndDate: Instant, reverse: Boolean): Double
+  def previewDeviation(player: Player, ratingPeriodEndDate: Instant, reverse: Boolean): Double
 
 final class GlickoCalculator(config: Config) extends GlickoCalculatorApi:
 
   private val calculator = chess.glicko.impl.RatingCalculator(config.tau, config.ratingPeriodsPerDay)
 
   // Simpler use case: a single game
-  def computeGame(game: Game, skipDeviationIncrease: Boolean = false): ByColor[Player] =
+  def computeGame(game: Game, skipDeviationIncrease: Boolean = false): Try[ByColor[Player]] =
     val ratings       = game.players.map(conversions.toRating)
     val gameResult    = conversions.toGameResult(ratings, game.outcome)
     val periodResults = impl.GameRatingPeriodResults(List(gameResult))
-    calculator.updateRatings(periodResults, skipDeviationIncrease)
-    ratings.map(conversions.toPlayer)
+    Try:
+      calculator.updateRatings(periodResults, skipDeviationIncrease)
+      ratings.map(conversions.toPlayer)
+
+  def previewDeviation(player: Player, ratingPeriodEndDate: Instant, reverse: Boolean): Double =
+    calculator.previewDeviation(conversions.toRating(player), ratingPeriodEndDate, reverse)
 
   private object conversions:
 
diff --git a/test-kit/src/test/scala/glicko/GlickoCalculator.scala b/test-kit/src/test/scala/glicko/GlickoCalculator.scala
index 91899c4cb..870b29b91 100644
--- a/test-kit/src/test/scala/glicko/GlickoCalculator.scala
+++ b/test-kit/src/test/scala/glicko/GlickoCalculator.scala
@@ -11,7 +11,7 @@ class GlickoCalculatorTest extends ScalaCheckSuite with MunitExtensions:
     )
 
   def computeGame(players: ByColor[Player], outcome: Outcome) =
-    calc.computeGame(Game(players, outcome), skipDeviationIncrease = true).toPair
+    calc.computeGame(Game(players, outcome), skipDeviationIncrease = true).get.toPair
 
   {
     val players = ByColor.fill: