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Revert "math/rand: make Perm match Shuffle"

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This reverts CL 55972.

Reason for revert: this changes Perm's behavior unnecessarily.
I asked for this change originally but I now regret it.
Reverting so that I don't have to justify it in Go 1.10 release notes.

Edited to keep the change to rand_test.go, which seems to have
been mostly unrelated.

Fixes #22744.

Change-Id: If8bb1bcde3ced0db2fdcd0aa65ab128613686c66
Reviewed-on: https://go-review.googlesource.com/78195
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Russ Cox 2017-11-16 02:29:04 +00:00
parent 02298ae11a
commit d9a198c7d0
3 changed files with 27 additions and 35 deletions
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8
src/math/rand/example_test.go
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@ -94,7 +94,7 @@ func Example_rand() {
// Intn(10) 1 2 5 // Intn(10) 1 2 5
// Int31n(10) 4 7 8 // Int31n(10) 4 7 8
// Int63n(10) 7 6 3 // Int63n(10) 7 6 3
// Perm [0 1 4 2 3] [0 4 3 1 2] [1 2 3 0 4] // Perm [1 4 2 3 0] [4 2 1 3 0] [1 2 4 0 3]
} }
func ExamplePerm() { func ExamplePerm() {
@ -115,7 +115,7 @@ func ExampleShuffle() {
fmt.Println(words) fmt.Println(words)
// Output: // Output:
// [my of the mouth corners from ink runs] // [mouth my the of runs corners from ink]
} }
func ExampleShuffle_slicesInUnison() { func ExampleShuffle_slicesInUnison() {
@ -132,8 +132,8 @@ func ExampleShuffle_slicesInUnison() {
// Output: // Output:
// C: 3 // C: 3
// D: 4
// A: 1
// E: 5 // E: 5
// B: 2 // B: 2
// A: 1
// D: 4
} }

26
src/math/rand/rand.go
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@ -213,24 +213,16 @@ again:
// Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n). // Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n).
func (r *Rand) Perm(n int) []int { func (r *Rand) Perm(n int) []int {
m := make([]int, n) m := make([]int, n)
for i := range m { // In the following loop, the iteration when i=0 always swaps m[0] with m[0].
m[i] = i // A change to remove this useless iteration is to assign 1 to i in the init
// statement. But Perm also effects r. Making this change will affect
// the final state of r. So this change can't be made for compatibility
// reasons for Go 1.
for i := 0; i < n; i++ {
j := r.Intn(i + 1)
m[i] = m[j]
m[j] = i
} }
// The code that follows is equivalent to calling
// r.Shuffle(n, func(i, j int) { m[i], m[j] = m[j], m[i] })
// but with the swap function inlined.
// This inlining provides a 10-15% speed-up.
i := n - 1
for ; i > 1<<31-1-1; i-- {
j := int(r.Int63n(int64(i + 1)))
m[i], m[j] = m[j], m[i]
}
for ; i > 0; i-- {
j := int(r.int31n(int32(i + 1)))
m[i], m[j] = m[j], m[i]
}
return m return m
} }

28
src/math/rand/regress_test.go
View File

@ -323,24 +323,24 @@ var regressGolden = []interface{}{
float64(-0.5987943422687668), // NormFloat64() float64(-0.5987943422687668), // NormFloat64()
[]int{}, // Perm(0) []int{}, // Perm(0)
[]int{0}, // Perm(1) []int{0}, // Perm(1)
[]int{2, 3, 1, 0, 4}, // Perm(5) []int{0, 4, 1, 3, 2}, // Perm(5)
[]int{5, 6, 0, 4, 3, 1, 7, 2}, // Perm(8) []int{3, 1, 0, 4, 7, 5, 2, 6}, // Perm(8)
[]int{2, 4, 0, 5, 7, 3, 1, 6, 8}, // Perm(9) []int{5, 0, 3, 6, 7, 4, 2, 1, 8}, // Perm(9)
[]int{6, 0, 4, 2, 5, 1, 9, 8, 3, 7}, // Perm(10) []int{4, 5, 0, 2, 6, 9, 3, 1, 8, 7}, // Perm(10)
[]int{7, 11, 12, 14, 0, 15, 2, 5, 9, 3, 8, 13, 4, 1, 6, 10}, // Perm(16) []int{14, 2, 0, 8, 3, 5, 13, 12, 1, 4, 6, 7, 11, 9, 15, 10}, // Perm(16)
[]int{}, // Perm(0) []int{}, // Perm(0)
[]int{0}, // Perm(1) []int{0}, // Perm(1)
[]int{2, 1, 0, 4, 3}, // Perm(5) []int{3, 0, 1, 2, 4}, // Perm(5)
[]int{4, 2, 1, 6, 0, 5, 3, 7}, // Perm(8) []int{5, 1, 2, 0, 4, 7, 3, 6}, // Perm(8)
[]int{7, 3, 1, 2, 8, 5, 4, 6, 0}, // Perm(9) []int{4, 0, 6, 8, 1, 5, 2, 7, 3}, // Perm(9)
[]int{3, 0, 7, 4, 8, 9, 5, 6, 1, 2}, // Perm(10) []int{8, 6, 1, 7, 5, 4, 3, 2, 9, 0}, // Perm(10)
[]int{0, 1, 8, 14, 9, 5, 4, 13, 7, 12, 10, 3, 15, 6, 11, 2}, // Perm(16) []int{0, 3, 13, 2, 15, 4, 10, 1, 8, 14, 7, 6, 12, 9, 5, 11}, // Perm(16)
[]int{}, // Perm(0) []int{}, // Perm(0)
[]int{0}, // Perm(1) []int{0}, // Perm(1)
[]int{2, 1, 4, 3, 0}, // Perm(5) []int{0, 4, 2, 1, 3}, // Perm(5)
[]int{4, 0, 7, 5, 1, 6, 2, 3}, // Perm(8) []int{2, 1, 7, 0, 6, 3, 4, 5}, // Perm(8)
[]int{6, 5, 3, 4, 7, 1, 0, 8, 2}, // Perm(9) []int{8, 7, 5, 3, 4, 6, 0, 1, 2}, // Perm(9)
[]int{1, 7, 6, 3, 2, 9, 0, 5, 4, 8}, // Perm(10) []int{1, 0, 2, 5, 7, 6, 9, 8, 3, 4}, // Perm(10)
[]byte{0x1}, // Read([0]) []byte{0x1}, // Read([0])
[]byte{0x94, 0xfd, 0xc2, 0xfa, 0x2f, 0xfc, 0xc0}, // Read([0 0 0 0 0 0 0]) []byte{0x94, 0xfd, 0xc2, 0xfa, 0x2f, 0xfc, 0xc0}, // Read([0 0 0 0 0 0 0])
[]byte{0x41, 0xd3, 0xff, 0x12, 0x4, 0x5b, 0x73, 0xc8}, // Read([0 0 0 0 0 0 0 0]) []byte{0x41, 0xd3, 0xff, 0x12, 0x4, 0x5b, 0x73, 0xc8}, // Read([0 0 0 0 0 0 0 0])