Signed biased distribution generator

gen/rand.go

@@ -6,5 +6,19 @@ import "math/rand"
 func RandBiasedUint64() uint64 {
 	// The shift-right by up to 64 (shifting it to 0) makes up for randomness
         // lost by setting the high bit.
-	return (rand.Uint64() | 0x8000000000000000) >> rand.Int31n(65)
+	return (rand.Uint64() | 0x8000000000000000) >> rand.Intn(65)
+}
+
+// Generate a random int64 with an even distribution of the tuple
+// {sign, bits.Len64(math.Abs())}
+func RandBiasedInt64() int64 {
+	shift := rand.Intn(127)
+	// [0,62]: positive
+	// [63,63]: zero
+	// [64,126]: negative
+	if shift < 64 {
+		return int64((rand.Uint64() | 0x8000000000000000) >> (shift + 1))
+	} else {
+		return int64((rand.Uint64() | 0x8000000000000000) >> (shift - 63)) * -1
+	}
 }