Implement readavailable and bytesavailable (#53)

These are used notably by the fallback `write(::IO, ::IO)` implementation.
Their absence makes `CSV.File(open(f, enc"..."))` fail with recent CSV.jl versions.

Author: nalimilan

src/StringEncodings.jl

@@ -6,8 +6,9 @@ using Libiconv_jll
 
 using Base.Libc: errno, strerror, E2BIG, EINVAL, EILSEQ
 
-import Base: close, eachline, eof, flush, isreadable, iswritable,
-             open, read, readbytes!, readline, readlines, readuntil, show, write
+import Base: bytesavailable, close, eachline, eof, flush, isreadable, iswritable,
+             open, read, readavailable, readbytes!, readline, readlines,
+             readuntil, show, write
 
 export StringEncoder, StringDecoder, encode, decode, encodings
 export StringEncodingError, OutputBufferError, IConvError
@@ -339,6 +340,18 @@ function read(s::StringDecoder, ::Type{UInt8})
     eof(s) ? throw(EOFError()) : s.outbuf[s.skip+=1]
 end
 
+bytesavailable(s::StringDecoder) =
+    Int(BUFSIZE - s.outbytesleft[] - s.skip)
+
+function readavailable(s::StringDecoder)
+    s.cd == C_NULL && throw(ArgumentError("cannot read from closed StringDecoder"))
+    eof(s) # Load more data into buffer if it is empty
+    ob = s.outbytesleft[]
+    res = s.outbuf[(s.skip+1):(BUFSIZE - ob)]
+    s.skip = BUFSIZE - ob
+    return res
+end
+
 isreadable(s::StringDecoder) = s.cd != C_NULL && isreadable(s.stream)
 iswritable(s::StringDecoder) = false
 

test/runtests.jl

@@ -246,6 +246,30 @@ end
     end
 end
 
+@testset "readavailable and bytesavailable" begin
+    s = "a string チャネルパートナーの選択"^10
+    b = IOBuffer(encode(s, "UTF-16LE"))
+    p = StringDecoder(b, "UTF-16LE")
+    @test read(p, String) == s
+
+    b = IOBuffer(encode(s, "UTF-16LE"))
+    p = StringDecoder(b, "UTF-16LE")
+    @test bytesavailable(p) == 0
+    @test read(p, UInt8) === 0x61
+    @test bytesavailable(p) == 197
+    @test String(readavailable(p)) == s[2:196]
+    @test bytesavailable(p) == 0
+    @test String(readavailable(p)) == s[199:394]
+    @test bytesavailable(p) == 0
+    @test String(readavailable(p)) == s[397:end]
+    @test isempty(readavailable(p))
+    @test bytesavailable(p) == 0
+    @test eof(p)
+    close(p)
+    @test bytesavailable(p) == 0
+    @test_throws ArgumentError readavailable(p)
+end
+
 
 ## Test encodings support
 b = IOBuffer()