move Identity to CodecIdentity

Author: bicycle1885

docs/src/index.md

@@ -185,6 +185,33 @@ compressed = take!(buf)
 close(stream)
 ```
 
+### Use an identity (no-op) codec
+
+Sometimes, the `Identity` codec, which does nothing, may be useful. The
+following example creates a decompression stream based on the extension of a
+filepath:
+```julia
+using CodecZlib
+using CodecBzip2
+using TranscodingStreams
+using TranscodingStreams.CodecIdentity
+
+function makestream(filepath)
+    if endswith(filepath, ".gz")
+        codec = GzipDecompression()
+    elseif endswith(filepath, ".bz2")
+        codec = Bzip2Decompression()
+    else
+        codec = Identity()
+    end
+    return TranscodingStream(codec, open(filepath))
+end
+
+makestream("data.txt.gz")
+makestream("data.txt.bz2")
+makestream("data.txt")
+```
+
 ### Transcode data in one shot
 
 TranscodingStreams.jl extends the `transcode` function to transcode a data

src/identity.jl

@@ -1,10 +1,39 @@
 # Identity Codec
 # ==============
 
-struct Identity <: Codec end
+module CodecIdentity
 
-function process(::Identity, input::Memory, output::Memory)
+export
+    Identity,
+    IdentityStream
+
+import TranscodingStreams:
+    TranscodingStreams,
+    TranscodingStream,
+    Memory
+
+"""
+    Identity()
+
+Create an identity (no-op) codec.
+"""
+struct Identity <: TranscodingStreams.Codec end
+
+const IdentityStream{S} = TranscodingStream{Identity,S}
+
+"""
+    IdentityStream(stream::IO)
+
+Create an identity (no-op) codec.
+"""
+function IdentityStream(stream::IO)
+    return TranscodingStream(Identity(), stream)
+end
+
+function TranscodingStreams.process(::Identity, input::Memory, output::Memory)
     n = Int(min(input.size, output.size))
     unsafe_copy!(output.ptr, input.ptr, n)
     return n, n, ifelse(input.size == 0, :end, :ok)
 end
+
+end

test/runtests.jl

@@ -1,9 +1,8 @@
 using TranscodingStreams
+using TranscodingStreams.CodecIdentity
 using Base.Test
 
 @testset "Identity Codec" begin
-    Identity = TranscodingStreams.Identity
-
     source = IOBuffer("")
     stream = TranscodingStream(Identity(), source)
     @test eof(stream)
@@ -84,6 +83,9 @@ using Base.Test
     @test transcode(Identity(), b"") == b""
     @test transcode(Identity(), b"foo") == b"foo"
     TranscodingStreams.test_roundtrip_transcode(Identity, Identity)
+
+    TranscodingStreams.test_roundtrip_read(IdentityStream, IdentityStream)
+    TranscodingStreams.test_roundtrip_write(IdentityStream, IdentityStream)
 end
 
 installed = keys(Pkg.installed())