Version 5 of zlib stream

Documentation

• http://www.tcl.tk/man/tcl/TclCmd/zlib.htm#M43

zlib stream mode ?level?

This command, part of zlib, creates a streaming compression or decompression command, allowing greater control over the compression/decompression process. It returns the name of the stream instance command. The mode must be one of compress, decompress, deflate, inflate, gzip, gunzip. The optional level, which is only valid for compressing streams, gives the compression level from 0 (none) to 9 (max).

The returned streamInst command will support the following subcommands:

streamInst add ?option? data

• • Shortcut for a put followed by a get.

streamInst checksum

• • Returns the current checksum of the uncompressed data, calculated using the appropriate algorithm for the stream's mode.

streamInst close

• • Disposes of the streamInst command. Deleting with rename works the same.

streamInst eof

• • Returns whether the end of the input data has been reached.

streamInst finalize

• • Shortcut for “streamInst put -finalize {}”.

streamInst flush

• • Shortcut for “streamInst put -flush {}”.

streamInst fullflush

• • Shortcut for “streamInst put -fullflush {}”.

streamInst get ?count?

• • Return up to count bytes from the stream's internal buffers. If count is unspecified, return as much as is available (without flushing).

streamInst put ?option? data

• • Appends the bytes data to the stream, compressing or decompressing as necessary. The option controls the type of flush done: -flush means to ensure that all data appended to the stream has been processed and made ready for get at some compression performance penalty, -fullflush also makes sure that the compression engine can restart from the point after the flush (at more penalty), and -finalize states that no more data will be written to the stream, causing any trailing bytes required by the format to be written.

streamInst reset

• • Recreates the stream, ready to start afresh. Discards whatever is in the stream's buffers.

Example - streaming over sockets

For simple zlib streaming over sockets like in HTTP, zlib push is sufficient. This breaks down more interactive protocols, as it gives you no way to control when a block is flushed to the receiver. If you want to flush each line, for example, you will need something like the following.

This code simply forces a flush each time $zchan write is called. If that proves insufficient, simply remove the flush flag in method write and call the object's method flush directly.

This code was inspired by an experiment by karll

See zlib manual and http://www.bolet.org/~pornin/deflate-flush.html for more detail on Zlib's flushing modes.

# it appears that $transchan flush doesn't get called any time interesting.
# So each $transchan write needs to flush by itself.
#
# Flushing an already flushed stream is a harmless error {TCL ZLIB BUF}, so we catch it
#
oo::class create zchan {
    variable Stream
    variable Chan
    variable Mode
    constructor {mode} {
        set Stream zlib stream $mode
        #  oo::objdefine self forward stream $Stream
    }

    method initialize {chan mode} {
        set Chan $chan
        set Mode $mode
        if {$mode eq "write"} {
            return {initialize finalize write flush}
        } elseif {$mode eq "read"} {
            return {initialize finalize read drain}
        }
    }
    method finalize {chan} {
        my destroy
    }

    method write {chan data} {
        try {
            $Stream add -flush $data
            # equivalent to:
            #  $Stream put $data
            #  $Stream flush
            #  $Stream get
        } trap {TCL ZLIB BUF} {} {
            return ""
        }
    }
    method flush {chan} {
        try {
            $Stream add -flush {}
            # equivalent to:
            #  $Stream flush
            #  $Stream get
        } trap {TCL ZLIB BUF} {} {
            return ""
        }
    }

    method read {chan data} {
        $Stream add $data
    }
    method drain {chan} {
        $Stream add -finalize {}
    }
}

if 0 {
    lassign chan pipe r w
    chan configure $w -translation binary -buffering none
    chan configure $r -translation binary -blocking 0
    lassign {gzip gunzip} out in
    puts $w "Frumious bandersnatch!"
    puts "read: gets $r"
    chan push $w zchan create gw $out
    chan push $r zchan create gr $in
    puts $w "Vorpal snacks!"
    puts "read: gets $r"
    puts $w "And bric-a-brac!"
    puts "read: gets $r"
    chan pop $w
    chan pop $r
    puts $w "Galumphing back"
    puts "read: gets $r"
}