From b8c0e5603a0c0f18b847487a63064f7e210bba40 Mon Sep 17 00:00:00 2001
From: Kistaro Windrider <kistaro@gmail.com>
Date: Sat, 1 Apr 2023 19:13:42 -0700
Subject: [PATCH] Add a general shuffler and deck shuffling.

---
 cardsim/deck.go      | 44 +++++++++++++++++++++++++++++++++++++++++++-
 cardsim/sliceutil.go | 28 ++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+), 1 deletion(-)

diff --git a/cardsim/deck.go b/cardsim/deck.go
index e1fa3a4..cca7381 100644
--- a/cardsim/deck.go
+++ b/cardsim/deck.go
@@ -23,7 +23,7 @@ const (
 // The Deck stores cards yet-to-be-dealt.
 type Deck[C StatsCollection] struct {
 	cards []Card[C]
-	rand  rand.Rand
+	rand  *rand.Rand
 }
 
 // Len returns the number of cards in the Deck.
@@ -196,3 +196,45 @@ func (d *Deck[C]) InsertRandomRange(loFrac, hiFrac float64, card Card[C]) error
 	errs.Add(d.Insert(slot, card))
 	return errs.Emit()
 }
+
+// Shuffle completely shuffles the deck. If the deck has one or fewer cards,
+// this returns WarningTooFewCards since nothing can be shuffled.
+func (d *Deck[C]) Shuffle() error {
+	if len(d.cards) < 2 {
+		return WarningTooFewCards
+	}
+	ShuffleAll(d.cards, d.rand)
+	return nil
+}
+
+// ShufflePart shuffles the `n` cards of the deck starting at `loc`.
+// If the provided range doesn't fit in the deck, this returns
+// WarningTopClamped and/or WarningBottomClamped. If the eventual range
+// of cards to be shuffled (after any off-the-end issues are corrected)
+// is one or less, this returns WarningTooFewCards since nothing can
+// be shuffled.
+func (d *Deck[C]) ShufflePart(loc, n int) error {
+	if n < 2 {
+		// Nothing to do.
+		return WarningTooFewCards
+	}
+
+	var errs ErrorCollector
+	if loc < 0 {
+		errs.Add(Warningf("%w: loc was %d", WarningTopClamped, loc))
+		loc = 0
+	}
+	if loc+n > d.Len() {
+		errs.Add(Warningf("%w: deck size %d does not have %d cards at and after location %d",
+			WarningBottomClamped, len(d.cards), n, loc))
+		n = d.Len() - loc
+		// Now is there anything to do?
+		if n < 2 {
+			errs.Add(WarningTooFewCards)
+			return errs.Emit()
+		}
+	}
+
+	ShufflePart(d.cards, d.rand, loc, n)
+	return nil
+}
diff --git a/cardsim/sliceutil.go b/cardsim/sliceutil.go
index 1c3e881..e64207e 100644
--- a/cardsim/sliceutil.go
+++ b/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)
+}