crrand: add Perm64

[jbowens]

Add a type Perm64 that implements a pseudorandom permutation of 64 bit integers.

---

This change is 

[RaduBerinde]

Nice!

Reviewable status: 0 of 2 files reviewed, 7 unresolved discussions

---

crrand/perm.go line 21 at r1 (raw file):

import "math/bits"

// MakePerm64 constructs a new Mixer from a 64-bit seed, providing a

[nit] constructs a Perm64

---

crrand/perm.go line 33 at r1 (raw file):

	s1 := bits.RotateLeft64(seed^c0, 13)
	s2 := bits.RotateLeft64(seed^c1, 37)
	s3 := bits.RotateLeft64(seed^(c0^c1), 53)

[nit] unnecessary parens

---

crrand/perm.go line 49 at r1 (raw file):

// Nth returns the nth value in the permutation of the 64-bit values. The return
// value may be passed to Index to recover n. The permutation is pseduorandom.
func (p Perm64) Nth(n uint64) uint64 {

[nit] consider Apply() and Inverse() or maybe At() and IndexOf().

---

crrand/perm.go line 50 at r1 (raw file):

// value may be passed to Index to recover n. The permutation is pseduorandom.
func (p Perm64) Nth(n uint64) uint64 {
	// Use a simple Feistel network with 4 rounds to shuffle data.

If this implementation is inspired from somewhere, can you cite the source?

---

crrand/perm.go line 51 at r1 (raw file):

func (p Perm64) Nth(n uint64) uint64 {
	// Use a simple Feistel network with 4 rounds to shuffle data.

[nit] extra blank line

---

crrand/perm.go line 76 at r1 (raw file):

// ARX-only round function.
func f(x, k uint32) uint32 {

[nit] rename to arx

---

crrand/perm_test.go line 32 at r1 (raw file):

func TestPerm64(t *testing.T) {
	for _, seed := range interestingUint64s {
		mixer := MakePerm64(seed)

[nit] s/mixer/perm

[RaduBerinde]

Reviewable status: 0 of 2 files reviewed, 10 unresolved discussions (waiting on @jbowens)

---

crrand/perm.go line 22 at r2 (raw file):

// MakePerm64 constructs a new Perm64 from a 64-bit seed, providing a
// deterministic, pseduorandom, bijective mapping of 64-bit values X to 64-bit

s/pseduorandom/pseudorandom

---

crrand/perm.go line 42 at r2 (raw file):

}

// A Perm64 provides a deterministic, pseduorandom permutation of 64-bit values.

s/pseduorandom/pseudorandom

---

crrand/perm.go line 48 at r2 (raw file):

// At returns the nth value in the permutation of the 64-bit values. The return
// value may be passed to Index to recover n. The permutation is pseduorandom.

s/pseduorandom/pseudorandom

[RaduBerinde]

Reviewable status: 0 of 2 files reviewed, 12 unresolved discussions (waiting on @jbowens)

---

crrand/perm.go line 25 at r2 (raw file):

// values Y.
func MakePerm64(seed uint64) Perm64 {
	// derive 4 x 32-bit round keys from the 64-bit seed using only ARX ops.

[nit] We're not using ARX here (no Addition).

---

crrand/perm.go line 35 at r2 (raw file):

	s3 := bits.RotateLeft64(seed^c0^c1, 53)

	m.seed[0] = uint32(s0)

[nit] This looks a little strange.. we're only using 32-bits of each value but we're choosing either the low or the high?

[RaduBerinde]

crrand/perm.go line 25 at r2 (raw file):

Previously, RaduBerinde wrote…

[nit] We're not using ARX here (no Addition).

We could just initialize a PCG and use Uint32() on it four times instead of rolling out our own..

[jbowens]

TFTR!

Reviewable status: 0 of 2 files reviewed, 3 unresolved discussions (waiting on @RaduBerinde)

---

crrand/perm.go line 21 at r1 (raw file):

Previously, RaduBerinde wrote…

[nit] constructs a Perm64

Done

---

crrand/perm.go line 49 at r1 (raw file):

Previously, RaduBerinde wrote…

[nit] consider Apply() and Inverse() or maybe At() and IndexOf().

Done

---

crrand/perm.go line 51 at r1 (raw file):

Previously, RaduBerinde wrote…

[nit] extra blank line

Done

---

crrand/perm.go line 76 at r1 (raw file):

Previously, RaduBerinde wrote…

[nit] rename to arx

Done

---

crrand/perm.go line 22 at r2 (raw file):

Previously, RaduBerinde wrote…

s/pseduorandom/pseudorandom

Done

---

crrand/perm.go line 25 at r2 (raw file):

Previously, RaduBerinde wrote…

We could just initialize a PCG and use Uint32() on it four times instead of rolling out our own..

Good idea—done

---

crrand/perm.go line 35 at r2 (raw file):

Previously, RaduBerinde wrote…

[nit] This looks a little strange.. we're only using 32-bits of each value but we're choosing either the low or the high?

Done

---

crrand/perm.go line 42 at r2 (raw file):

Previously, RaduBerinde wrote…

s/pseduorandom/pseudorandom

Done

---

crrand/perm_test.go line 32 at r1 (raw file):

Previously, RaduBerinde wrote…

[nit] s/mixer/perm

Done

[RaduBerinde]

I don't think you pushed the latest changes? (or github has a hiccup)