Put idfMap calculation behind a memoization layer.

Memoization checks that the card count hasn't changed by 10% or more since the last time we ran.

This leaves a "good enough" idfMap in most cases.

Part of #694.

src/nlp.ts

@@ -1605,8 +1605,23 @@ type IDFMap = {
 	maxIDF: number
 };
 
-//TODO: put this behind memoization logic that cheats if cards hasn't changed length by much.
+let memoizedIDFMap: IDFMap = {idf: {}, maxIDF: 0};
+let memoizedIDMapCardCount = 0;
+let memoizedIDFMapNgramSize = 0;
+
 const idfMapForCards = (cards : ProcessedCards, ngramSize: number) : IDFMap => {
+	const cardCount = Object.keys(cards).length;
+	//Check if the card count is greater than or equal to card count and within 10% of the last time we calculated the idf map
+	const cardCountCloseEnough  = cardCount >= memoizedIDMapCardCount && cardCount <= memoizedIDMapCardCount * 1.1;
+	if (cardCountCloseEnough && ngramSize == memoizedIDFMapNgramSize) return memoizedIDFMap;
+	const result = calcIDFMapForCards(cards, ngramSize);
+	memoizedIDFMap = result;
+	memoizedIDFMapNgramSize = ngramSize;
+	memoizedIDMapCardCount = cardCount;
+	return result;
+};
+
+const calcIDFMapForCards = (cards : ProcessedCards, ngramSize: number) : IDFMap => {
 	if (!cards || Object.keys(cards).length == 0) return {idf: {}, maxIDF: 0};
 
 	//only consider cards that have a body, even if we were provided a set that included others