day7: Explain theory a bit

day7/day7.kt

@@ -87,6 +87,18 @@ fun typeJoker(cards: Hand): Int {
     }
 
     val jokers = groups.remove('J') ?: 0
+
+    // Jokers act as a kind of tie breaker here - similar to the full
+    // scenario, the label we want to add the joker to is the one with
+    // the highest card value *as well as* the highest type.
+    //
+    // So we sort it first by value (descending so the highest type
+    // gets i == 0), and second by the value of the card (so that if
+    // two hands have the same type the one with the higher card value
+    // wins).
+    //
+    // The hands are base 13 numbers, so multiplying the value puts it
+    // one digit ahead of the card value.
     return groups.entries.sortedByDescending { 13 * it.value + cardValueJoker(it.key) }.foldIndexed(
                     0
             ) { i, acc, entry ->