subcoding/gen/rand.go

package gen

import "math/rand"

// Generate a random uint64 with an even distribution of bits.Len64()
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.Intn(65)
}

// Like RandBiasedUint64() but always returns < n
func RandBiasedUint64n(n uint64) uint64 {
	for {
		ret := RandBiasedUint64()
		if ret < n {
			return ret
		}
	}
}

// Generate a random int64 with an even distribution of the tuple
// {sign, bits.Len64(math.Abs())}
func RandBiasedInt64() uint64 {
	shift := rand.Intn(127)
	// [0,62]: positive
	// [63,63]: zero
	// [64,126]: negative
	if shift < 64 {
		return uint64(int64((rand.Uint64() | 0x8000000000000000) >> (shift + 1)))
	} else {
		return uint64(int64((rand.Uint64()|0x8000000000000000)>>(shift-63)) * -1)
	}
}

// Mixture of RandBiasedUint64() and RandBiasedInt64(), with probability shifted
// toward more unsigned values.
func RandBiasedUSint64() uint64 {
	if rand.Intn(2) == 0 {
		return RandBiasedUint64()
	} else {
		return RandBiasedInt64()
	}
}