add RandomGen.RollVariation

src/game-lib/Lib/IRandomGen.cs

@@ -13,5 +13,9 @@ public interface IRandomGen
     TValue Pick<TKey, TValue>(IDictionary<TKey, TValue> dict);
     int RandomizeMissionSiteCountdown();
     bool FlipCoin();
-
+
+    (int result, float variationRoll) RollVariation(int baseValue, (int min, int max) range, int precision);
+
+    (int result, float variationRoll) RollVariation(int baseValue, int min, int max, int precision);
+
 }

src/game-lib/Lib/RandomGen.cs

@@ -71,4 +71,18 @@ public virtual int RandomizeMissionSiteCountdown()
         => Roll(Ruleset.FactionMissionSiteCountdownRange);
 
     public bool FlipCoin() => _random.Next(2) == 1;
+
+    public (int result, float variationRoll) RollVariation(int baseValue, (int min, int max) range, int precision)
+        => RollVariation(baseValue, range.min, range.max, precision);
+
+    public (int result, float variationRoll) RollVariation(int baseValue, int min, int max, int precision)
+    {
+        // e.g.       -15 = Roll(-30, 30)       
+        int variationRoll = Roll(min, max);
+        // e.g.   85 =       100 + (-15)
+        int modifier = precision + variationRoll;
+        // e.g. 42 =        50 *       85 / 100
+        int result = baseValue * modifier / precision;
+        return (result, variationRoll: variationRoll / (float)precision);
+    }
 }

src/game-lib/Model/Ruleset.cs

@@ -80,15 +80,15 @@ public static (int difficulty, int baseDifficulty, float variationRoll) RollMiss
         // Note that currently the only ways of increasing agents survivability of difficulty is:
         // - by surviving missions
         // - via training
-        // As such, if difficulty due to turn would grow at least as fast as Ruleset.AgentTrainingCoefficient,
+        // As such, if difficulty per turn would grow at least as fast as Ruleset.AgentTrainingCoefficient,
         // then at some point missions would become impossible, as eventually even the most experienced
         // agents would die, and any new agents would never be able to catch up with mission difficulty.
-        int precision = MissionSiteDifficultyRollPrecision;
         int baseDifficulty = factionPower / MissionSiteDifficultyFactionPowerDivisor;
-        int variationRoll = randomGen.Roll(MissionSiteDifficultyVariationRange);
-        int difficultyModifier = precision + variationRoll;
-        int difficulty = (baseDifficulty * difficultyModifier) / precision;
-        return (difficulty, baseDifficulty, variationRoll: variationRoll/(float)precision);
+        (int difficulty, float variationRoll) = randomGen.RollVariation(
+            baseValue: baseDifficulty,
+            range: MissionSiteDifficultyVariationRange,
+            precision: MissionSiteDifficultyRollPrecision);
+        return (difficulty, baseDifficulty, variationRoll);
     }
 
     public static int RequiredSurvivingAgentsForSuccess(MissionSite site)