如何转换ByteString和Storable Vector？

Question

什么是转换Storable.Vector Word8和严格的最佳方式ByteString？

当然,非常感谢非复制(无操作)方式.

我应该只是unsafeCoerce或者是否有一个库函数(我找不到)？

此外,方法是否相同Unboxed.Vector Word8？

Answer 1

简单unsafeCoerce不起作用,因为数据构造函数的布局不同:

data StorableArray i e = StorableArray !i !i Int !(ForeignPtr e)

与

data ByteString = PS {-# UNPACK #-} !(ForeignPtr Word8) -- payload
                     {-# UNPACK #-} !Int                -- offset
                     {-# UNPACK #-} !Int                -- length

您可以导入Data.Array.Storable.Internals和Data.ByteString.Internal访问原始构造函数,然后构建一个而不复制数据:

> let bs = pack [1,2,3]
> bs
"\SOH\STX\ETX"
> let sa = case bs of (PS ptr 0 n) -> StorableArray 0 (n-1) n ptr
> :t sa
sa :: StorableArray Int GHC.Word.Word8
> Data.Array.MArray.readArray sa 1
2
> Data.Array.MArray.readArray sa 0
1
> Data.Array.MArray.readArray sa 3
*** Exception: Ix{Int}.index: Index (3) out of range ((0,2))

(我删除了相当长的提示Prelude Data.Array.Storable.Internals Data.ByteString.Internal Data.ByteString>).

这不适用Data.Vector.Unboxed,因为这里的数据在Haskell堆上并由GHC运行时管理,而其他两个管理Haskell堆外的数据.

Answer 2

byteStringToVector :: (Storable a) => BS.ByteString -> V.Vector a
byteStringToVector bs = vec where
    vec = V.unsafeFromForeignPtr (castForeignPtr fptr) (scale off) (scale len)
    (fptr, off, len) = BS.toForeignPtr bs
    scale = (`div` sizeOfElem vec)

sizeOfElem vec = sizeOf (undefined `asTypeOf` V.head vec)

http://hackage.haskell.org/packages/archive/spool/0.1/doc/html/Data-Vector-Storable-ByteString.html