add rand package, utilities for math/rand.
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// Package rand contains utilities for interacting with math/rand, including
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// seeding from a random sed.
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package rand
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import (
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"crypto/rand"
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"encoding/binary"
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mrand "math/rand"
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)
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// CryptoUint64 generates a cryptographically-secure 64-bit integer.
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func CryptoUint64() (uint64, error) {
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bs := make([]byte, 8)
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_, err := rand.Read(bs)
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if err != nil {
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return 0, err
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}
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return binary.BigEndian.Uint64(bs), nil
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}
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// Seed initialises the non-cryptographic PRNG with a random,
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// cryptographically secure value. This is done just as a good
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// way to make this random. The returned 64-bit value is the seed.
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func Seed() (uint64, error) {
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seed, err := CryptoUint64()
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if err != nil {
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return 0, err
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}
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// NB: this is permitted.
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mrand.Seed(int64(seed))
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return seed, nil
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}
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// Int is a wrapper for math.Int so only one package needs to be imported.
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func Int() int {
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return mrand.Int()
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}
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// Intn is a wrapper for math.Intn so only one package needs to be imported.
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func Intn(max int) int {
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return mrand.Intn(max)
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}
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// Intn2 returns a random value between min and max, inclusive.
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func Intn2(min, max int) int {
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return Intn(max-min) + min
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}
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package rand
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import (
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"fmt"
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mrand "math/rand"
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"testing"
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)
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func TestCryptoUint64(t *testing.T) {
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n1, err := CryptoUint64()
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if err != nil {
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t.Fatal(err)
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}
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n2, err := CryptoUint64()
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if err != nil {
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t.Fatal(err)
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}
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// This has such a low chance of occurring that it's likely to be
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// indicative of a bad CSPRNG.
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if n1 == n2 {
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t.Fatalf("repeated random uint64s: %d", n1)
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}
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}
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func TestIntn(t *testing.T) {
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expected := []int{3081, 4887, 4847, 1059, 3081}
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mrand.Seed(1)
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for i := 0; i < 5; i++ {
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n := Intn2(1000, 5000)
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if n != expected[i] {
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fmt.Printf("invalid sequence at %d: expected %d, have %d", i, expected[i], n)
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}
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}
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}
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func TestSeed(t *testing.T) {
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seed1, err := Seed()
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if err != nil {
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t.Fatal(err)
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}
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var seed2 uint64
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n1 := Int()
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tries := 0
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for {
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seed2, err = Seed()
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if err != nil {
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t.Fatal(err)
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}
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if seed1 != seed2 {
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break
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}
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tries++
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if tries > 3 {
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t.Fatal("can't generate two unique seeds")
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}
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}
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n2 := Int()
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// Again, this not impossible, merely statistically improbably and a
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// potential canary for RNG issues.
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if n1 == n2 {
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t.Fatalf("repeated integers fresh from two unique seeds: %d/%d -> %d",
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seed1, seed2, n1)
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}
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}
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