Revert "math/rand: make Perm match Shuffle"

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>

3 files changed, 27 insertions, 35 deletions

diff --git a/src/math/rand/example_test.go b/src/math/rand/example_test.go
index f50d255209..aa1f2bcc73 100644
--- a/src/math/rand/example_test.go
+++ b/src/math/rand/example_test.go
@@ -94,7 +94,7 @@ func Example_rand() {
 	// Intn(10)    1                   2                   5
 	// Int31n(10)  4                   7                   8
 	// 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() {
@@ -115,7 +115,7 @@ func ExampleShuffle() {
 	fmt.Println(words)
 
 	// Output:
-	// [my of the mouth corners from ink runs]
+	// [mouth my the of runs corners from ink]
 }
 
 func ExampleShuffle_slicesInUnison() {
@@ -132,8 +132,8 @@ func ExampleShuffle_slicesInUnison() {
 
 	// Output:
 	// C: 3
+	// D: 4
+	// A: 1
 	// E: 5
 	// B: 2
-	// A: 1
-	// D: 4
 }
diff --git a/src/math/rand/rand.go b/src/math/rand/rand.go
index 8edb22e1da..957bebdddd 100644
--- a/src/math/rand/rand.go
+++ b/src/math/rand/rand.go
@@ -213,24 +213,16 @@ again:
 // Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n).
 func (r *Rand) Perm(n int) []int {
 	m := make([]int, n)
-	for i := range m {
-		m[i] = i
+	// In the following loop, the iteration when i=0 always swaps m[0] with m[0].
+	// 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
 }
 
diff --git a/src/math/rand/regress_test.go b/src/math/rand/regress_test.go
index 0be2ca1d34..e31e6c5af0 100644
--- a/src/math/rand/regress_test.go
+++ b/src/math/rand/regress_test.go
@@ -323,24 +323,24 @@ var regressGolden = []interface{}{
 	float64(-0.5987943422687668),  // NormFloat64()
 	[]int{},                                                     // Perm(0)
 	[]int{0},                                                    // Perm(1)
-	[]int{2, 3, 1, 0, 4},                                        // Perm(5)
-	[]int{5, 6, 0, 4, 3, 1, 7, 2},                               // Perm(8)
-	[]int{2, 4, 0, 5, 7, 3, 1, 6, 8},                            // Perm(9)
-	[]int{6, 0, 4, 2, 5, 1, 9, 8, 3, 7},                         // Perm(10)
-	[]int{7, 11, 12, 14, 0, 15, 2, 5, 9, 3, 8, 13, 4, 1, 6, 10}, // Perm(16)
+	[]int{0, 4, 1, 3, 2},                                        // Perm(5)
+	[]int{3, 1, 0, 4, 7, 5, 2, 6},                               // Perm(8)
+	[]int{5, 0, 3, 6, 7, 4, 2, 1, 8},                            // Perm(9)
+	[]int{4, 5, 0, 2, 6, 9, 3, 1, 8, 7},                         // Perm(10)
+	[]int{14, 2, 0, 8, 3, 5, 13, 12, 1, 4, 6, 7, 11, 9, 15, 10}, // Perm(16)
 	[]int{},                                                     // Perm(0)
 	[]int{0},                                                    // Perm(1)
-	[]int{2, 1, 0, 4, 3},                                        // Perm(5)
-	[]int{4, 2, 1, 6, 0, 5, 3, 7},                               // Perm(8)
-	[]int{7, 3, 1, 2, 8, 5, 4, 6, 0},                            // Perm(9)
-	[]int{3, 0, 7, 4, 8, 9, 5, 6, 1, 2},                         // Perm(10)
-	[]int{0, 1, 8, 14, 9, 5, 4, 13, 7, 12, 10, 3, 15, 6, 11, 2}, // Perm(16)
+	[]int{3, 0, 1, 2, 4},                                        // Perm(5)
+	[]int{5, 1, 2, 0, 4, 7, 3, 6},                               // Perm(8)
+	[]int{4, 0, 6, 8, 1, 5, 2, 7, 3},                            // Perm(9)
+	[]int{8, 6, 1, 7, 5, 4, 3, 2, 9, 0},                         // Perm(10)
+	[]int{0, 3, 13, 2, 15, 4, 10, 1, 8, 14, 7, 6, 12, 9, 5, 11}, // Perm(16)
 	[]int{},                             // Perm(0)
 	[]int{0},                            // Perm(1)
-	[]int{2, 1, 4, 3, 0},                // Perm(5)
-	[]int{4, 0, 7, 5, 1, 6, 2, 3},       // Perm(8)
-	[]int{6, 5, 3, 4, 7, 1, 0, 8, 2},    // Perm(9)
-	[]int{1, 7, 6, 3, 2, 9, 0, 5, 4, 8}, // Perm(10)
+	[]int{0, 4, 2, 1, 3},                // Perm(5)
+	[]int{2, 1, 7, 0, 6, 3, 4, 5},       // Perm(8)
+	[]int{8, 7, 5, 3, 4, 6, 0, 1, 2},    // Perm(9)
+	[]int{1, 0, 2, 5, 7, 6, 9, 8, 3, 4}, // Perm(10)
 	[]byte{0x1},                         // Read([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])