util/rand: add Shuffle func
Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>
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@ -1,17 +1,17 @@
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package rand
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import (
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"crypto/rand"
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crand "crypto/rand"
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"encoding/binary"
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)
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// Rand is a wrapper around crypto/rand that adds some convenience functions known from math/rand.
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// Rand is a wrapper around crypto/rand that adds some convenience functions known from math/rand
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type Rand struct {
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buf [8]byte
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}
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func (r *Rand) Int31() int32 {
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_, _ = rand.Read(r.buf[:4])
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_, _ = crand.Read(r.buf[:4])
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return int32(binary.BigEndian.Uint32(r.buf[:4]) & ^uint32(1<<31))
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}
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@ -54,11 +54,11 @@ func (r *Rand) Intn(n int) int {
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}
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func (r *Rand) Int63() int64 {
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_, _ = rand.Read(r.buf[:])
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_, _ = crand.Read(r.buf[:])
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return int64(binary.BigEndian.Uint64(r.buf[:]) & ^uint64(1<<63))
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}
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// copied from the standard library math/rand implementation of Int63n
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// Int31n copied from the standard library math/rand implementation of Int31n
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func (r *Rand) Int31n(n int32) int32 {
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if n&(n-1) == 0 { // n is power of two, can mask
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return r.Int31() & (n - 1)
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@ -71,6 +71,7 @@ func (r *Rand) Int31n(n int32) int32 {
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return v % n
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}
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// Int63n copied from the standard library math/rand implementation of Int63n
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func (r *Rand) Int63n(n int64) int64 {
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if n&(n-1) == 0 { // n is power of two, can mask
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return r.Int63() & (n - 1)
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@ -82,3 +83,26 @@ func (r *Rand) Int63n(n int64) int64 {
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}
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return v % n
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}
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// Shuffle copied from the standard library math/rand implementation of Shuffle
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func (r *Rand) Shuffle(n int, swap func(i, j int)) {
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if n < 0 {
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panic("invalid argument to Shuffle")
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}
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// Fisher-Yates shuffle: https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle
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// Shuffle really ought not be called with n that doesn't fit in 32 bits.
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// Not only will it take a very long time, but with 2³¹! possible permutations,
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// there's no way that any PRNG can have a big enough internal state to
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// generate even a minuscule percentage of the possible permutations.
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// Nevertheless, the right API signature accepts an int n, so handle it as best we can.
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i := n - 1
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for ; i > 1<<31-1-1; i-- {
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j := int(r.Int63n(int64(i + 1)))
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swap(i, j)
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}
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for ; i > 0; i-- {
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j := int(r.Int31n(int32(i + 1)))
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swap(i, j)
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}
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}
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