This mod fixes a bug in Slay the Spire that makes seeded runs non-deterministic.

Given the same starting conditions (seed, ascension level, unlocks, etc.), you should be able to reproduce any Slay the Spire run by performing the same actions in the same order.

However, there exists a bug with card discovery (specifically from a potion or the Discovery card) where one of the random number generators (RNGs) is called an arbitrary number of times. For the rest of the floor, any random behavior that relies on that RNG will be non-deterministic.

This is likely a bug since the Foreign Influence card performs similar behavior, but without this issue. For more details, see the comparison below.

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In the example below, we start a floor by opening an attack potion and then a power potion. In a vanilla Slay the Spire run, the cards offered by the power potion are non-deterministic. If we save the run and continue, we'll usually get different cards.

Attempt	Vanilla	w/ Determinism Fix
1
2
3

Under the hood, these potions use a DiscoveryAction to generate cards. The cards are generated within the update method:

// DiscoveryAction.java
public void update() {
      ArrayList generatedCards;
      if (this.returnColorless) {
         generatedCards = this.generateColorlessCardChoices();
      } else {
         generatedCards = this.generateCardChoices(this.cardType);
      }

      if (this.duration == Settings.ACTION_DUR_FAST) {
         AbstractDungeon.cardRewardScreen.customCombatOpen(generatedCards, CardRewardScreen.TEXT[1], this.cardType != null);
         this.tickDuration();
      } else {
         // ...
      }
  }

The update method is called (roughly) every "tick" of the game loop. Every time that happens, one of the generate[...]CardChoices methods is called which then advances the underlying RNG (cardRandomRNG) at least three times.

Compare that with Foreign Influence. This card has nearly identical behavior, but a different implementation via ForeignInfluenceAction:

// ForeignInfluenceAction.java
public void update() {
	if (this.duration == Settings.ACTION_DUR_FAST) {
		AbstractDungeon.cardRewardScreen.customCombatOpen(this.generateCardChoices(), CardRewardScreen.TEXT[1], true);
		this.tickDuration();
	} else {
		// ...
	}
}

The logic within the this.duration == Settings.ACTION_DUR_FAST if statement is only called once (since this.duration is immediately decremented by tickDuration), so generateCardChoices is only called once.

Returning to our example, using the attack potion performs a DiscoveryAction which calls cardRandomRNG an arbitrary number of times. At this point, performing any actions that rely on cardRandomRNG will be non-deterministic (generally any random actions that arise from playing cards, such as which enemies are hit by Bouncing Flask). The cards generated by a power potion rely on cardRandomRNG, so they are non-deterministic.

Note the attack potion in this example always generates the same cards. This is because the card reward screen is opened with whichever cards are generated on the first call to update. The subsequent calls to the generate[...]CardChoices methods are ignored, but they still advance cardRandomRNG (which is the problem!). This mod fixes this issue by preventing each DiscoveryAction from calling the generate[...]CardChoices methods more than once.

The developers are no longer working on Slay the Spire, but this bug could be fixed upstream by updating DiscoveryAction to match ForeignInfluenceAction:

// DiscoveryAction.java
public void update() {
-     ArrayList generatedCards;
-     if (this.returnColorless) {
-        generatedCards = this.generateColorlessCardChoices();
-     } else {
-        generatedCards = this.generateCardChoices(this.cardType);
-     }
-
      if (this.duration == Settings.ACTION_DUR_FAST) {
+        ArrayList generatedCards;
+        if (this.returnColorless) {
+           generatedCards = this.generateColorlessCardChoices();
+        } else {
+           generatedCards = this.generateCardChoices(this.cardType);
+        }
+
         AbstractDungeon.cardRewardScreen.customCombatOpen(generatedCards, CardRewardScreen.TEXT[1], this.cardType != null);
         this.tickDuration();
      } else {
         // ...
      }
  }