15.5.116 crunch

crunch, array, byte_slice, out

crunch(array, byte_slice, out)

Tries to compress array and writes the result in array out, which must be predefined and large enough to contain the result. byte_slice specifies how many bits per number are passed to out without compression. If byte_slice is too small or too large, then less efficient compression or even expansion may occur. Try three less than the array’s entropy for byte_slice.

Presently, only compression of byte, word, and (on some platforms) long arrays is supported. Associated system variables are:

!crunch_slice

the last specified byte_slice.

!crunch_bits

the number of bits the last compressed array needed.

!crunch_bpp

the number of bits per compressed element for the last compressed array.

If out has insufficient room to hold the result, then !crunch_bits and !crunch_bpp are set to 0. The subroutine form then also generates an error; the function form does not, but returns a 0. If there is sufficient room, then the function form returns a 1. If other errors occur (e.g., when array is not a numerical array), then both forms generate error states.

Reverse: decrunch

See also: !crunch_bits, !crunch_bpp, !crunch_slice