fix: edge case with ~uint64 weight >= platform maxInt

Exposed during fuzzing, possible  edge (very edge!) case where when
using a Chooser[any, ~uint64], if a single Choice weight exceeds system
maxInt, then when it gets converted to system int type it no longer
triggers the the check for exceeding maxInt in the running total,
resulting in a Chooser that can panic on Pick.

Author: mroth

testdata/fuzz/FuzzNewChooser/a547669aeb7ca0ca

@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("0000000\x8f00000000")

weightedrand.go

@@ -48,11 +48,16 @@ func NewChooser[T any, W integer](choices ...Choice[T, W]) (*Chooser[T, W], erro
 	totals := make([]int, len(choices))
 	runningTotal := 0
 	for i, c := range choices {
-		weight := int(c.Weight)
-		if weight < 0 {
+		if c.Weight < 0 {
 			continue // ignore negative weights, can never be picked
 		}
 
+		// case of single ~uint64 or similar value that exceeds maxInt on its own
+		if uint64(c.Weight) >= maxInt {
+			return nil, errWeightOverflow
+		}
+
+		weight := int(c.Weight) // convert weight to int for internal counter usage
 		if (maxInt - runningTotal) <= weight {
 			return nil, errWeightOverflow
 		}
@@ -68,8 +73,9 @@ func NewChooser[T any, W integer](choices ...Choice[T, W]) (*Chooser[T, W], erro
 }
 
 const (
-	intSize = 32 << (^uint(0) >> 63) // cf. strconv.IntSize
-	maxInt  = 1<<(intSize-1) - 1
+	intSize   = 32 << (^uint(0) >> 63) // cf. strconv.IntSize
+	maxInt    = 1<<(intSize-1) - 1
+	maxUint64 = 1<<64 - 1
 )
 
 // Possible errors returned by NewChooser, preventing the creation of a Chooser

weightedrand_test.go

@@ -81,10 +81,42 @@ func TestNewChooser(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			_, err := NewChooser(tt.cs...)
+			c, err := NewChooser(tt.cs...)
 			if err != tt.wantErr {
 				t.Errorf("NewChooser() error = %v, wantErr %v", err, tt.wantErr)
 			}
+
+			if err == nil { // run a few Picks to make sure there are no panics
+				for i := 0; i < 10; i++ {
+					_ = c.Pick()
+				}
+			}
+		})
+	}
+
+	u64tests := []struct {
+		name    string
+		cs      []Choice[rune, uint64]
+		wantErr error
+	}{
+		{
+			name:    "weight overflow from single uint64 exceeding system maxInt",
+			cs:      []Choice[rune, uint64]{{Item: 'a', Weight: maxInt + 1}},
+			wantErr: errWeightOverflow,
+		},
+	}
+	for _, tt := range u64tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c, err := NewChooser(tt.cs...)
+			if err != tt.wantErr {
+				t.Errorf("NewChooser() error = %v, wantErr %v", err, tt.wantErr)
+			}
+
+			if err == nil { // run a few Picks to make sure there are no panics
+				for i := 0; i < 10; i++ {
+					_ = c.Pick()
+				}
+			}
 		})
 	}
 }