caae0917bf
Perm and Shuffle are fundamentally doing the same work. This change makes Perm's algorithm match Shuffle's. In addition to allowing developers to switch more easily between the two methods, it affords a nice speed-up: name old time/op new time/op delta Perm3-8 75.7ns ± 1% 51.8ns ± 1% -31.59% (p=0.000 n=9+8) Perm30-8 610ns ± 1% 405ns ± 1% -33.67% (p=0.000 n=9+9) This change alters the output from Perm, given the same Source and seed. This is a change from Go 1.0 behavior. This necessitates updating the regression test. This also changes the number of calls made to the Source during Perm, which changes the output of the math/rand examples. This also slightly perturbs the output of Perm, nudging it out of the range currently accepted by TestUniformFactorial. However, it is complete unclear that the helpers relied on by TestUniformFactorial are correct. That is #21211. This change updates checkSimilarDistribution to respect closeEnough for standard deviations, which makes the test pass. The whole situation is muddy; see #21211 for details. There is an alternative implementation of Perm that avoids initializing m, which is more similar to the existing implementation, plus some optimizations: func (r *Rand) Perm(n int) []int { m := make([]int, n) max31 := n if n > 1<<31-1-1 { max31 = 1<<31 - 1 - 1 } i := 1 for ; i < max31; i++ { j := r.int31n(int32(i + 1)) m[i] = m[j] m[j] = i } for ; i < n; i++ { j := r.Int63n(int64(i + 1)) m[i] = m[j] m[j] = i } return m } This is a tiny bit faster than the implementation actually used in this change: name old time/op new time/op delta Perm3-8 51.8ns ± 1% 50.3ns ± 1% -2.83% (p=0.000 n=8+9) Perm30-8 405ns ± 1% 394ns ± 1% -2.66% (p=0.000 n=9+8) However, 3% in performance doesn't seem worth having the two algorithms diverge, nor the reduced readability of this alternative. Updates #16213. Change-Id: I11a7441ff8837ee9c241b4c88f7aa905348be781 Reviewed-on: https://go-review.googlesource.com/55972 Run-TryBot: Ian Lance Taylor <iant@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Rob Pike <r@golang.org> |
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