Compress Vector of Short Integers R←X(219⌶)Y

In this section, the term sint_vector is used to refer to a simple integer vector whose items are all in the range ¯128 to 127, that is, they are type 83. For further information see Data Representation (Monadic).

In most cases this I-Beam functionality will be used in conjunction with 220⌶ (Serialise/Deserialise Array). However, it may be possible to pass the raw compressed data to and from other applications.

X specifies the operation to be performed, either compression or decompression, the compression library to be used, and any optional parameters. Y contains the data to be operated on.

Compression

Y must be a sint_vector.

R is a two item vector, each of which is a sint_vector. R[1] describes the compression, and R[2] contains the raw data which is the result of applying the compression library to the input data Y.

X is specified as follows:

X[1]	X[2]	Compression Library
1	n/a	LZ4
2	0 .. 9	zlib
3	0 .. 9	gzip
4	n/a	LZ4 with frames (compresses arrays >2GB)

If LZ4 compression is required, then X must either be a scalar or a one element vector. Otherwise, X[2], if present, specifies the compression level; higher numbers produce better compression, but take longer.

Decompression

R is a sint_vector, containing the output of applying the decompression library to the input data, Y.

If X is a scalar or a one item vector, and has the value 0, then Y must be a vector of two items which is the result of previously calling 219⌶ to compress a sint_vector.

Otherwise, X is a scalar or one or two element vector and Y must be a sint_vector.

The first element of X must be one of the following values.

X[1]	Compression Library
¯1	LZ4
¯2	zlib
¯3	gzip

The second, optional, element of X specifies the length of the uncompressed data. Its presence results in a more efficient use of the compression library.

X may not be a two item vector whose first item has the value 0.

Examples

      sint←{⍵-256×⍵>127}
      utf8←'UTF-8'∘⎕ucs
      str←'empty←⍬'
      ⊣v←sint utf8  str
101 109 112 116 121 ¯30 ¯122 ¯112 ¯30 ¯115 ¯84          
      ⊣comp←1 (219⌶) v
8 ¯55 1 0 0 0 0 11  ¯80 101 109 112 116 121 ¯30 ¯122 ¯112 ¯30 ¯115 ¯84          

      utf8 256| 0(219⌶)comp
empty←⍬
      utf8 256| ¯1(219⌶)2⊃comp
empty←⍬