flate: Add limited window compression

[klauspost]

Adds a medium compressor that can operate with limited window size.

Exposed in gzip outside deflate for now.

Example sizes:

=== RUN   TestFileWindow/32
    gzip_test.go:349: size: 82504 bytes
=== RUN   TestFileWindow/64
    gzip_test.go:349: size: 75350 bytes
=== RUN   TestFileWindow/128
    gzip_test.go:349: size: 70668 bytes
=== RUN   TestFileWindow/256
    gzip_test.go:349: size: 69276 bytes
=== RUN   TestFileWindow/512
    gzip_test.go:349: size: 68327 bytes
=== RUN   TestFileWindow/1024
    gzip_test.go:349: size: 67876 bytes
=== RUN   TestFileWindow/2048
    gzip_test.go:349: size: 40900 bytes
=== RUN   TestFileWindow/4096
    gzip_test.go:349: size: 38684 bytes
=== RUN   TestFileWindow/8192
    gzip_test.go:349: size: 36263 bytes
=== RUN   TestFileWindow/16384
    gzip_test.go:349: size: 35434 bytes
=== RUN   TestFileWindow/32768
    gzip_test.go:349: size: 34654 bytes
--- PASS: TestFileWindow (0.03s)

Limited testing done.

[klauspost]

@rojer - are you able to test if this does what you need?

[rojer]

not immediately, but will do

[rojer]

ok, i've run tests and happy to report that it works.
i didn't go as far as integrating it into our app, but i did the next closes thing: we use miniz in embedded applications. i patched tinfl_decompress_mem_to_callback to take dict size as well, coupled it with flate.NewWriterWindow and ran compress/decompress tests for all window sizes, from 32 to 32768. it works.
https://github.com/rojer/compress/commit/5a2c99d7f2c68bbb45e2257cc046ea44107131f3 is the test, i don't know if you'd like to adopt it for the library or not, but this is what i used:

$ go test -v -asan ./flate/miniz/...
=== RUN   Test_miniz_inflate
--- PASS: Test_miniz_inflate (0.10s)
PASS
ok      github.com/klauspost/compress/flate/miniz       0.107s

[klauspost]

@rojer Great! I will merge it then.

It seems like the next version will be a feature release, so it may take a bit to get out.

[rojer]

thank you @klauspost , much appreciated!