sliceutil: helpers for mid-slice insert/delete.

Manipulating the hand, deck, etc. is going to use these operations a lot.

cardsim/deck.go

@@ -31,8 +31,8 @@ func (d *Deck[C]) Len() int {
 	return len(d.cards)
 }
 
-// Insert puts a card at a specific location in the Deck. The card previously
-// at that location and all locations after are shifted one card later.
+// Insert puts one or more cards at a specific location in the Deck. Cards
+// at that location and all locations after are shifted deeper into the deck.
 // Negative indexes are counted from the bottom of the deck. BottomOfDeck is
 // a sentinel value for the bottommost position; -1 is one card above.
 //
@@ -44,7 +44,7 @@ func (d *Deck[C]) Len() int {
 // WarningTopClamped. Like all warnings, these can be safely ignored and the
 // program is in a well-defined state, but you may want to check for them
 // if you expect some other behavior.
-func (d *Deck[C]) Insert(idx int, card Card[C]) error {
+func (d *Deck[C]) Insert(idx int, card ...Card[C]) error {
 	var errs ErrorCollector
 	// Calculate actual target index.
 	switch {
@@ -60,14 +60,7 @@ func (d *Deck[C]) Insert(idx int, card Card[C]) error {
 		idx += d.Len()
 	}
 	// remaining case: 0 <= idx <= d.Len(), which is a normal forward insert index.
-
-	// Place new card on bottom and "bubble" into position.
-	// Takes O(N) time. If this turns out to be a problem, implement a more
-	// efficient data structure.
-	d.cards = append(d.cards, card)
-	for i := len(d.cards) - 1; i > idx; i-- {
-		d.cards[i], d.cards[i-1] = d.cards[i-1], d.cards[i]
-	}
+	d.cards = InsertInto(d.cards, idx, card...)
 	return errs.Emit()
 }
 

cardsim/sliceutil.go

@@ -0,0 +1,100 @@
+package cardsim
+
+import (
+	"fmt"
+)
+
+// InsertInto inserts one or more items into a slice at an arbitrary index.
+// Items already in the slice past the target position move to later positons.
+//
+// Like `append`, this may move the underlying array and it produces a new
+// slice header (under the hood, it uses `append`). It returns the new slice
+// (the original is in an undefined state and should no longer be used).
+//
+// If loc is negative or more than one past the end of T, Insert panics.
+func InsertInto[T any](slice []T, loc int, elements ...T) []T {
+	if loc < 0 || loc > len(slice) {
+		panic(fmt.Sprintf("can't Insert at location %d in %d-element slice", loc, len(slice)))
+	}
+
+	// is this a no-op?
+	if len(elements) == 0 {
+		return slice
+	}
+	// is this just an append?
+	if loc == len(slice) {
+		return append(slice, elements...)
+	}
+
+	offset := len(elements)
+	oldLen := len(slice)
+	newSize := oldLen + offset
+	if newSize <= cap(slice) {
+		// We can reslice in place.
+		slice = slice[:newSize]
+
+		// Scoot trailing to their new positions.
+		copy(slice[loc+offset:], slice[loc:oldLen])
+
+		// Insert the new elements.
+		copy(slice[loc:], elements)
+
+		return slice
+	}
+
+	// Reallocate. Do the normal thing of doubling the size as a minimum
+	// when increasing space for a dynamic array; this amortizes the
+	// cost of repeatedly reallocating and moving the slice.
+	newCap := cap(slice) * 2
+	if newCap < newSize {
+		newCap = newSize
+	}
+	newSlice := make([]T, newSize, newCap)
+	if loc > 0 {
+		copy(newSlice, slice[0:loc])
+	}
+	copy(newSlice[loc:], elements)
+	copy(newSlice[loc+offset:], slice[loc:])
+	return newSlice
+}
+
+// DeleteFrom deletes an item from a slice at an arbitrary index. Items after it
+// scoot up to close the gap. This returns the modified slice (like Append).
+//
+// If the provided location is not a valid location in the slice, this panics.
+func DeleteFrom[T any](slice []T, loc int) []T {
+	return DeleteNFrom(slice, loc, 1)
+}
+
+// DeleteNFrom deletes N items from a slice at an arbitrary index. Items after
+// it scoot up to close the gap. This returns the modified slice (like Append).
+//
+// If the range of items that would be deleted is not entirely valid within the
+// slice, this panics.
+func DeleteNFrom[T any](slice []T, loc, n int) []T {
+	if loc < 0 || loc+n > len(slice) {
+		panic(fmt.Sprintf("can't delete %d elements from a %d-element slice at location %d", n, len(slice), loc))
+	}
+
+	// Easy cases.
+	if n == 0 {
+		return slice
+	}
+	if loc == 0 {
+		return slice[n:]
+	}
+	if loc+n == len(slice) {
+		return slice[0:loc]
+	}
+
+	// Is it shorter to move up or move down?
+	if len(slice)-loc-n > loc {
+		// Move forward -- the end is big.
+		copy(slice[n:], slice[:loc])
+		return slice[n:]
+	}
+	// Move backward -- the beginnng is big or they're the same size
+	// (and moving backwards preserves more usable append capacity later).
+	copy(slice[loc:], slice[loc+n:])
+	return slice[:len(slice)-n]
+}