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############################################################################
#
# File: bitstrm.icn
#
# Subject: Procedures to read and write strings of bits in files
#
# Author: Robert J. Alexander
#
# Date: August 14, 1996
#
############################################################################
#
# This file is in the public domain.
#
############################################################################
#
# Procedures for reading and writing integer values made up of an
# arbitrary number of bits, stored without regard to character
# boundaries.
#
############################################################################
#
# Usage of BitStreamWrite, by example:
#
# record bit_value(value, nbits)
# ...
# BitStreamWrite() #initialize
# while value := get_new_value() do # loop to output values
# BitStreamWrite(outfile, value.nbits, value.value)
# BitStreamWrite(outfile) # output any buffered bits
#
# Note the interesting effect that BitStreamWrite(outproc), as well as
# outputting the complete string, pads the output to an even character
# boundary. This can be dune during construction of a bit string if
# the effect is desired.
#
# The "value" argument defaults to zero.
#
############################################################################
#
# Usage of BitStreamRead, by example:
#
# BitStreamRead()
# while value := BitStreamRead(infile, nbits) do
# # do something with value
#
# BitStringRead fails when too few bits remain to satisfy a request.
#
############################################################################
#
# See also: bitstr.icn
#
############################################################################
procedure BitStreamWrite(outfile,bits,value,outproc)
local outvalue
static buffer,bufferbits
#
# Initialize.
#
initial {
buffer := bufferbits := 0
}
/outproc := writes
#
# If this is "close" call, flush buffer and reinitialize.
#
if /value then {
outvalue := &null
if bufferbits > 0 then
outproc(outfile,char(outvalue := ishift(buffer,8 - bufferbits)))
buffer := bufferbits := 0
return outvalue
}
#
# Merge new value into buffer.
#
buffer := ior(ishift(buffer,bits),value)
bufferbits +:= bits
#
# Output bits.
#
while bufferbits >= 8 do {
outproc(outfile,char(outvalue := ishift(buffer,8 - bufferbits)))
buffer := ixor(buffer,ishift(outvalue,bufferbits - 8))
bufferbits -:= 8
}
return outvalue
end
procedure BitStreamRead(infile,bits,inproc)
local value
static buffer,bufferbits
#
# Initialize.
#
initial {
buffer := bufferbits := 0
}
#
# Reinitialize if called with no arguments.
#
if /infile then {
buffer := bufferbits := 0
return
}
#
# Read in more data if necessary.
#
/inproc := reads
while bufferbits < bits do {
buffer := ior(ishift(buffer,8),ord(inproc(infile))) | fail
bufferbits +:= 8
}
#
# Extract value from buffer and return.
#
value := ishift(buffer,bits - bufferbits)
buffer := ixor(buffer,ishift(value,bufferbits - bits))
bufferbits -:= bits
return value
end
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