Update PRNG logic

The new algorithm, Mulberry32, doesn't return its internal state as
the generated random number. This means that previousRandom, the value
we keep between generations, should not be the actual previous random
number generated, but the previous seed normally kept inside the
generator. To do that, a new nextSeed() method is added.

Also, the storySeed is no longer fed into the PRNG, but only used on
first initialization.

Author: PiOverFour

src/engine/Story.ts

@@ -1189,15 +1189,15 @@ export class Story extends InkObject {
                 ". The maximum must be larger"
             );
 
-          let resultSeed = this.state.storySeed + this.state.previousRandom;
+          let resultSeed = this.state.previousRandom;
           let random = new PRNG(resultSeed);
 
           let nextRandom = random.next();
           let chosenValue = (nextRandom % randomRange) + minInt.value;
           this.state.PushEvaluationStack(new IntValue(chosenValue));
 
           // Next random number (rather than keeping the Random object around)
-          this.state.previousRandom = nextRandom;
+          this.state.previousRandom = random.nextSeed();
           break;
         }
 
@@ -1212,8 +1212,7 @@ export class Story extends InkObject {
             return throwNullException("minInt.value");
           }
 
-          this.state.storySeed = seed.value;
-          this.state.previousRandom = 0;
+          this.state.previousRandom = seed.value;
 
           this.state.PushEvaluationStack(new Void());
           break;
@@ -1349,7 +1348,7 @@ export class Story extends InkObject {
             newList = new InkList();
           } else {
             // Generate a random index for the element to take
-            let resultSeed = this.state.storySeed + this.state.previousRandom;
+            let resultSeed = this.state.previousRandom;
             let random = new PRNG(resultSeed);
 
             let nextRandom = random.next();
@@ -1377,7 +1376,7 @@ export class Story extends InkObject {
             newList = new InkList(randomItem.Key.originName, this);
             newList.Add(randomItem.Key, randomItem.Value);
 
-            this.state.previousRandom = nextRandom;
+            this.state.previousRandom = random.nextSeed();
           }
 
           this.state.PushEvaluationStack(new ListValue(newList));
@@ -2092,7 +2091,9 @@ export class Story extends InkObject {
     }
 
     for (let i = 0; i <= iterationIndex; ++i) {
-      let chosen = random.next() % unpickedIndices.length;
+      let nextRandom = random.next();
+      let chosen = nextRandom % unpickedIndices.length;
+      random.nextSeed();
       let chosenIndex = unpickedIndices[chosen];
       unpickedIndices.splice(chosen, 1);