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math/rand: document whether NewSource always implements Source64 #44488

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dsnet opened this issue Feb 22, 2021 · 5 comments
Closed

math/rand: document whether NewSource always implements Source64 #44488

dsnet opened this issue Feb 22, 2021 · 5 comments
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Documentation FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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@dsnet
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dsnet commented Feb 22, 2021

The concrete type returned by NewSource happens to implement Source64. However, this is not documented in any way. Is this always guaranteed?
@ulikunitz
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I looked into the source code of rng.go and since Int63() calls Uint64(), NewSource is always returning a Source64 implementation.

// Int63 returns a non-negative pseudo-random 63-bit integer as an int64.
func (rng *rngSource) Int63() int64 {
	return int64(rng.Uint64() & rngMask)
}

@dsnet
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dsnet commented Feb 22, 2021

I believe the snippet you actually wanted to reference is:

go/src/math/rand/rng.go

Lines 237 to 252 in 87e984a

// Uint64 returns a non-negative pseudo-random 64-bit integer as an uint64.
func (rng *rngSource) Uint64() uint64 {
rng.tap--
if rng.tap < 0 {
rng.tap += rngLen
}
rng.feed--
if rng.feed < 0 {
rng.feed += rngLen
}
x := rng.vec[rng.feed] + rng.vec[rng.tap]
rng.vec[rng.feed] = x
return uint64(x)
}

I'm aware that it current returns something that implements Source64 and has done so for many years. The question is whether this should be a documented guarantee (which I think it should be).

@ianlancetaylor
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I think we should document it.

@cagedmantis cagedmantis added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Feb 23, 2021
@cagedmantis cagedmantis added this to the Backlog milestone Feb 23, 2021
@gopherbot
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Change https://golang.org/cl/352689 mentions this issue: math/rand: update NewSource document

@gopherbot
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Change https://go.dev/cl/425974 mentions this issue: math/rand: document that Source returned by NewSource implements Source64

@golang golang locked and limited conversation to collaborators Sep 2, 2023
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