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math/rand: add a comment for the i=0 iteration
Fixes #13215 Change-Id: I126117d42e7c1e69cbc7fad0760e225b03ed15bd Reviewed-on: https://go-review.googlesource.com/16852 Reviewed-by: Keith Randall <khr@golang.org>
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@ -148,6 +148,11 @@ func (r *Rand) Float32() float32 {
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// Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n).
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func (r *Rand) Perm(n int) []int {
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m := make([]int, n)
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// In the following loop, the iteration when i=0 always swaps m[0] with m[0].
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// A change to remove this useless iteration is to assign 1 to i in the init
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// statement. But Perm also effects r. Making this change will affect
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// the final state of r. So this change can't be made for compatibility
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// reasons for Go 1.
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for i := 0; i < n; i++ {
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j := r.Intn(i + 1)
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m[i] = m[j]
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