add cancelRerollDice() as helper function

Cancels out 10s and 1s, for use in rerollNeeded()

dice.ts

@@ -90,7 +90,7 @@ class D10 implements Dice {
 	 */
 }
 
-class Asphodice extends D10 {
+export class Asphodice extends D10 {
 	readonly type: string = "asphodice";
 	readonly passCrit: number = 10;
 	readonly failCrit: number = 1;
@@ -250,15 +250,50 @@ class Asphodice extends D10 {
 	}
 
 	/**
-	 * Roll an Asphodie or Asphodice
+	 * 'Cancel out' reroll dice
 	 *
-	 * @param numToRoll
-	 * @returns a {@link DiceResult} with additional properties:
-	 * - reroll: boolean - whether we rerolled
-	 * - olddice: Array - original roll
-	 * - dice: Array - final outcome
-	 * - balance: number - +/- of outcomes (successes/failures)
+	 * @remarks
+	 *
+	 * Helper function.
+	 *
+	 * **In the context of deciding a reroll only**, the reroll dice can
+	 * 'cancel' each other out. The dice themselves remain intact for the
+	 * purposes of determining the final outcome.
+	 *
+	 * @returns Filtered dice
 	 */
+	cancelRerollDice (resultDice: Array<number>): Array<number> {
+		// Naive approach for now:
+		// - count 10s and 1s
+		// - remove all 10s and 1s from input array
+		// - re-add 10s/1s after 'cancelling'
+		let tens = 0;
+		let ones = 0;
+
+		// count
+		for (let die of resultDice) {
+			if (die === 10) {
+				tens++;
+			} else if (die === 1) {
+				ones++;
+			}
+		}
+
+		// remove any reroll dice
+		let outputDice = resultDice.filter( die => (die != 1 && die != 10) );
+
+		let balance = tens - ones;
+
+		if (balance < 0) {
+			// add balance of ones
+			outputDice.push(...Array(Math.abs(balance)).fill(1));
+		} else if (balance > 0) {
+			// add balance of tens
+			outputDice.push(...Array(Math.abs(balance)).fill(10));
+		}
+
+		return outputDice;
+	}
 
 	/**
 	 * Determines if a reroll is needed
@@ -297,11 +332,22 @@ class Asphodice extends D10 {
 			return false;
 		}
 
+
 		// We should re-roll
 		return true;
 
 	}
 
+	/**
+	 * Roll an Asphodie or Asphodice
+	 *
+	 * @param numToRoll
+	 * @returns a {@link DiceResult} with additional properties:
+	 * - reroll: boolean - whether we rerolled
+	 * - olddice: Array - original roll
+	 * - dice: Array - final outcome
+	 * - balance: number - +/- of outcomes (successes/failures)
+	 */
 	roll (numToRoll: number): DiceResult {
 		let results: DiceResult = { total: 0, dice: [] };