Add a general shuffler and deck shuffling.

cardsim/sliceutil.go

@@ -2,6 +2,7 @@ package cardsim
 
 import (
 	"fmt"
+	"math/rand"
 )
 
 // InsertInto inserts one or more items into a slice at an arbitrary index.
@@ -98,3 +99,30 @@ func DeleteNFrom[T any](slice []T, loc, n int) []T {
 	copy(slice[loc:], slice[loc+n:])
 	return slice[:len(slice)-n]
 }
+
+// ShuffleAll shuffles everything in slice, using the provided rand.Rand.
+// If no rand.Rand is provided, this uses the default source.
+func ShuffleAll[T any](slice []T, r *rand.Rand) {
+	shuffle := rand.Shuffle
+	if r != nil {
+		shuffle = r.Shuffle
+	}
+	shuffle(len(slice), func(i, j int) {
+		slice[i], slice[j] = slice[j], slice[i]
+	})
+}
+
+// ShufflePart shuffles the `n` elements of `slice` starting at `loc`
+// in-place, using the provided rand.Rand. If the range of items to
+// shuffle is not entirely within `slice`, this panics.
+//
+// If no rand.Rand is provided, this uses the default source.
+func ShufflePart[T any](slice []T, r *rand.Rand, loc, n int) {
+	if loc < 0 || loc+n > len(slice) {
+		panic(fmt.Sprintf("can't shuffle %d elements from a %d-element slice at location %d", n, len(slice), loc))
+	}
+	if n < 1 {
+		return
+	}
+	ShuffleAll(slice[loc:loc+n], r)
+}