feat: method to set randomness source

The internal mechanics of this are a bit inelegant, since unfortunately
the global randomness source is not exported, necessitating these nil
check methods instead.

The API here needs some user feedback. I believe the majority case will
want to set this once and not on a per-call basis (cf. the deprecated
PickSource method in the previous version), but that needs be validated.

weightedrand.go

@@ -37,6 +37,8 @@ type Chooser[T any, W integer] struct {
 	data   []Choice[T, W]
 	totals []uint64
 	max    uint64
+
+	customRand *rand.Rand
 }
 
 // NewChooser initializes a new Chooser for picking from the provided choices.
@@ -64,7 +66,13 @@ func NewChooser[T any, W integer](choices ...Choice[T, W]) (*Chooser[T, W], erro
 		return nil, errNoValidChoices
 	}
 
-	return &Chooser[T, W]{data: choices, totals: totals, max: runningTotal}, nil
+	return &Chooser[T, W]{data: choices, totals: totals, max: runningTotal, customRand: nil}, nil
+}
+
+// SetRand applies an optional custom randomness source r for the Chooser.  If
+// set to nil nil, global rand will be used.
+func (c *Chooser[T, W]) SetRand(r *rand.Rand) {
+	c.customRand = r
 }
 
 // Possible errors returned by NewChooser, preventing the creation of a Chooser
@@ -82,9 +90,17 @@ var (
 
 // Pick returns a single weighted random Choice.Item from the Chooser.
 //
-// Utilizes global rand as the source of randomness. Safe for concurrent usage.
+// Utilizes global rand as the source of randomness by default, which is safe
+// for concurrent usage. If a custom rand source was set with SetRand, that
+// source will be used instead.
 func (c Chooser[T, W]) Pick() T {
-	r := rand.Uint64N(c.max) + 1
+	var r uint64
+	if c.customRand == nil {
+		r = rand.Uint64N(c.max) + 1
+	} else {
+		r = c.customRand.Uint64N(c.max) + 1
+	}
+
 	i, _ := slices.BinarySearch(c.totals, r)
 	return c.data[i].Item
 }