add TOKEN_END

Author: bicycle1885

README.md

@@ -26,12 +26,22 @@ The following snippet is an example of using CodecZlib.jl, which exports
 `TranscodingStream{GzipDecompression,S}` where `S<:IO`:
 
 ```julia
+# Read lines from a gzip-compressed file.
 using CodecZlib
 stream = GzipDecompressionStream(open("data.gzip"))
 for line in eachline(stream)
     # do something...
 end
 close(stream)
+
+# Compress a string using zstd.
+using CodecZstd
+using TranscodingStreams
+buf = IOBuffer()
+stream = ZstdCompressionStream(buf)
+write(stream, "foobarbaz"^100, TranscodingStreams.TOKEN_END)
+compressed = take!(buf)
+close(stream)
 ```
 
 [travisci-img]: https://travis-ci.org/bicycle1885/TranscodingStreams.jl.svg?branch=master

src/stream.jl

@@ -239,6 +239,23 @@ function Base.flush(stream::TranscodingStream)
     flush(stream.stream)
 end
 
+# A singleton type of end token.
+struct EndToken end
+
+"""
+A special token indicating the end of data.
+
+`TOKEN_END` may be written to a transcoding stream like `write(stream,
+TOKEN_END)`, which will terminate the current transcoding block.
+"""
+const TOKEN_END = EndToken()
+
+function Base.write(stream::TranscodingStream, ::EndToken)
+    changestate!(stream, :write)
+    processall(stream)
+    return 0
+end
+
 
 # Buffering
 # ---------