> On Jun 26, 2017, at 14:05 , Philippe Hausler <phaus...@apple.com> wrote:
>
>
>
>> On Jun 26, 2017, at 1:47 PM, Roderick Mann <rm...@latencyzero.com> wrote:
>>
>>>
>>> On Jun 26, 2017, at 10:20 , Charles Srstka <cocoa...@charlessoft.com> wrote:
>>>
>>> Rats, I was hoping that one of the reasons about being so explicit what
>>> we’re going to access and where with bindMemory() and friends would be to
>>> take care of these sorts of issues.
>>>
>>> In that case, the simplest way to do it is probably just this:
>>>
>>> let crc = (UInt16(myData[myData.endIndex]) << 8) |
>>> UInt16(myData[myData.endIndex - 1])
>>
>> By the way, self.endIndex == self.count, so shouldn't these both have an
>> additional 1 subtracted?
>>
>> That's what I'm seeing, and what the docs show. What's the point of
>> endIndex? Completeness?
>
> If the data was sliced.
>
> e.g.
>
> let d = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
> let slice = d[2..<4]
>
> slice.endIndex != slice.count
Ohhh, yeah, gotcha. Now I know something new about slices. Thanks (and thanks,
Charles, for your expanded explanation)!
>
>>
>>>
>>> (or the reverse, depending on the endianness of the source data)
>>>
>>> Charles
>>>
>>>> On Jun 26, 2017, at 12:05 PM, Philippe Hausler via swift-users
>>>> <swift-users@swift.org> wrote:
>>>>
>>>> Data.copyBytes will do that under the hood
>>>>
>>>> var crc: UInt16 = 0
>>>> let amountCopied = withUnsafeMutablePointer(to: &crc) { data.copyBytes(to:
>>>> UnsafeMutableBufferPointer(start: $0, count: 1)) }
>>>> if amountCopied == MemoryLayout<UInt16>.size {
>>>> // we have a full crc
>>>> }
>>>>
>>>> That will probably do what you want; plus it will allow you to do it from
>>>> a given range of bytes.
>>>>
>>>>
>>>>> On Jun 26, 2017, at 9:57 AM, Joe Groff via swift-users
>>>>> <swift-users@swift.org> wrote:
>>>>>
>>>>>
>>>>>> On Jun 26, 2017, at 1:55 AM, Daniel Vollmer via swift-users
>>>>>> <swift-users@swift.org> wrote:
>>>>>>
>>>>>> Hi Rick,
>>>>>>
>>>>>>> On 26. Jun 2017, at 02:37, Rick Mann via swift-users
>>>>>>> <swift-users@swift.org> wrote:
>>>>>>
>>>>>> [snip]
>>>>>>
>>>>>>> I'd also like to avoid unnecessary copying of the data. All of it is
>>>>>>> immutable for the purposes of this problem.
>>>>>>>
>>>>>>> How can I get the UInt16 that starts at byte X in a Data? Same goes for
>>>>>>> Double or Int32 or whatever.
>>>>>>
>>>>>> I’m not sure what Swift’s stance on this is, but not all platforms allow
>>>>>> misaligned memory accesses (such as your attempt to access a UInt16 that
>>>>>> lies at an odd memory address).
>>>>>
>>>>> Unaligned memory accesses are not currently allowed by the language
>>>>> semantics, regardless of the underlying ISA. You should use memcpy if you
>>>>> need to load potentially-unaligned values out of raw memory.
>>>>>
>>>>> -Joe
>>>>> _______________________________________________
>>>>> swift-users mailing list
>>>>> swift-users@swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-users
>>>>
>>>
>>
>>
>> --
>> Rick Mann
>> rm...@latencyzero.com
>
--
Rick Mann
rm...@latencyzero.com
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