Verify random battle and capture routines against the original engine

src/game/battle/battle.ts

@@ -296,7 +296,7 @@ export function applyTypeMatchups(
  * native translation. Unsupported AI, switching, contest and item paths are refused. */
 /** PokeBallEffect's wFinalCatchRate for Poké, Great and Ultra Balls, in
  * the original 8- and 16-bit registers. Returns undefined when the
- * formula would divide by zero (a maximum HP of 342). */
+ * formula would divide by zero (e.g. maximum HP of 342 or 683). */
 export function catchRate(input: {
   rate: number;
   ball: number;
@@ -1310,33 +1310,7 @@ export class Battle {
     this.wildMon = 0;
     yield* this.say(BATTLE.ItemUsedText);
 
-    let caught = tutorial || item === C.MASTER_BALL;
-    if (!caught) {
-      const held = this.data.heldEffect(this.player.mon?.item ?? 0);
-      const chance = catchRate({
-        rate: this.enemyBase.catchRate,
-        ball: item,
-        hp: this.enemy.mon.hp,
-        maxHp: this.enemy.mon.maxHp,
-        status: this.enemy.mon.status,
-        held,
-      });
-      if (chance === undefined) throw new UnsupportedBattle("The catch rate divides by zero.");
-      this.finalCatchRate = chance;
-      caught = this.random() <= chance;
-    }
-
-    // BattleAnim_ThrowPokeBall's .Shake loop runs GetPokeBallWobble until
-    // the ball clicks or the Pokémon breaks free.
-    let wobbles = 0;
-    for (;;) {
-      wobbles++;
-      if (wobbles === 4) break;
-      if (caught) continue;
-      let entry = BATTLE.WobbleProbabilities;
-      while (this.reader.byte(entry) < this.finalCatchRate) entry += 2;
-      if (this.random() >= this.reader.byte(entry + 1)) break;
-    }
+    const { caught, wobbles } = this.rollCapture(item, tutorial);
 
     if (!caught) {
       yield* this.say(
@@ -1410,6 +1384,42 @@ export class Battle {
     this.tossItem("balls", item, slot);
     return true;
   }
+  /** PokeBallEffect's decision, before animation and caught-mon loading. */
+  private rollCapture(item: number, tutorial: boolean) {
+    let caught = tutorial || item === C.MASTER_BALL;
+    if (!caught) {
+      const held = this.data.heldEffect(this.player.mon?.item ?? 0);
+      const chance = catchRate({
+        rate: this.enemyBase.catchRate,
+        ball: item,
+        hp: this.enemy.mon.hp,
+        maxHp: this.enemy.mon.maxHp,
+        status: this.enemy.mon.status,
+        held,
+      });
+      if (chance === undefined) throw new UnsupportedBattle("The catch rate divides by zero.");
+      this.finalCatchRate = chance;
+      caught = this.random() <= chance;
+    }
+
+    return { caught, wobbles: this.ballWobbles(caught) };
+  }
+
+  private ballWobbles(caught: boolean): number {
+    // BattleAnim_ThrowPokeBall's .Shake loop runs GetPokeBallWobble until
+    // the ball clicks or the Pokémon breaks free.
+    let wobbles = 0;
+    for (;;) {
+      wobbles++;
+      if (wobbles === 4) break;
+      if (caught) continue;
+      let entry = BATTLE.WobbleProbabilities;
+      while (this.reader.byte(entry) < this.finalCatchRate) entry += 2;
+      if (this.random() >= this.reader.byte(entry + 1)) break;
+    }
+    return wobbles;
+  }
+
   /** AskGiveNicknameText, then NamingScreen (NAME_MON) if accepted: the
    * chosen name, or the species name for a declined or blank one. */
   private *askNickname(name: string): Gen<string> {