Buffered Intn.

rngbuffer.go

type rngBuffer struct {
	bitsLeft int
	buffer   [32]byte
}

// Intn returns, as an int, a non-negative random number in the half-open interval [0,n).
// It panics if n <= 0.
func (r *rngBuffer) Intn(n int) int {
	if n <= 0 {
		panic("invalid argument to Intn")
	}
	need := bits.Len(uint(n))
	if r.bitsLeft < need {
		_, err := io.ReadFull(crand.Reader, r.buffer[:])
		if err != nil {
			panic(fmt.Sprintf("crypto/rand.Reader returned: %v", err))
		}
		r.bitsLeft = len(r.buffer) * 8
	}
	wIdx := 0
	idx := len(r.buffer) - r.bitsLeft
	r.bitsLeft -= need
	var res uint64
	for need > 0 {
		readIdx := idx / 8
		bitsUsed := idx & 7
		val := r.buffer[readIdx]
		if bitsUsed == 0 {
			res = res | uint64(val)<<wIdx
			need -= 8
			wIdx += 8
			idx += 8
		} else {
			val >>= bitsUsed
			res = res | uint64(val)<<wIdx
			wIdx += 8 - bitsUsed
			idx += 8 - bitsUsed
			need -= 8 - bitsUsed
		}
	}
	return int(res) % n
}