> 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
>> <mailto: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
>
>>
>> (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 <mailto:rm...@latencyzero.com>
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