Structures | Signatures | Identifiers

Signature PACK_WORD

Byte-level packaging of word values.

The PackWord<N>Big and PackWord<N>Little structures provide facilities for packing and unpacking N-bit word elements into Word8 vectors. This mechanism allows word values to be transmitted in binary format over networks. The PackWord<N>Big structures perform big-endian packing and unpacking, while the PackWord<N>Little structures perform little-endian packing and unpacking. 
structure PackWord32Little : PACK_WORD
structure PackWord32Big : PACK_WORD

signature PACK_WORD = 
  sig
    val bytesPerElem : int
    val isBigEndian : bool
    val subVec  : Word8Vector.vector * int -> LargeWord.word
    val subVecX : Word8Vector.vector * int -> LargeWord.word
    val subArr  : Word8Array.array * int -> LargeWord.word
    val subArrX : Word8Array.array * int -> LargeWord.word
    val update : Word8Array.array * int * LargeWord.word -> unit
  end
[bytesPerElem]
The number of bytes per element. Most implementations will provide several structures with values of bytesPerElem that are small powers of two (e.g., 1, 2, 4, and 8, corresponding to N of 8, 16, 32, 64, respectively).

[isBigEndian]
True if the structure implements a big-endian view of the data (most-significant byte first). Otherwise, the structure implements a little-endian view (least-significant byte first).

[subVec (vec, i)]
[subVecX (vec, i)]
These extract the subvector 
   vec[bytesPerElem*i..bytesPerElem*(i+1)-1]
of the vector vec and convert it into a word according to the endianness of the structure. The subVecX version extends the sign bit (most significant bit) when converting the subvector to a word. The functions raise the Subscript exception if i < 0 or if Word8Vector.length vec < bytesPerElem * (i + 1).

[subArr (arr, i)]
[subArrX (arr, i)]
These extract the subarray 
   arr[bytesPerElem*i..bytesPerElem*(i+1)-1]
of the array arr and convert it into a word according to the endianness of the structure. The subArrX version extends the sign bit (most significant bit) when converting the subarray into a word. The functions raise the Subscript exception if i < 0 or if Word8Array.length arr < bytesPerElem * (i+1).

[update (arr, i, w)]
stores the bytesPerElem low-order bytes of the word w into the bytes bytesPerElem*i through bytesPerElem*(i+1)-1 of the array arr, according to the structure's endianness. It raises the Subscript exception if i < 0 or if Word8Array.length arr < bytesPerElem * (i+1).

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