define expectedsize for decompressor (#14)

Author: bicycle1885

src/decompression.jl

@@ -79,3 +79,17 @@ function TranscodingStreams.process(codec::ZstdDecompressor, input::Memory, outp
         return Δin, Δout, code == 0 ? :end : :ok
     end
 end
+
+function TranscodingStreams.expectedsize(codec::ZstdDecompressor, input::Memory)
+    ret = find_decompressed_size(input.ptr, input.size)
+    if ret == ZSTD_CONTENTSIZE_ERROR
+        # something is bad, but we ignore it here
+        return Int(input.size)
+    elseif ret == ZSTD_CONTENTSIZE_UNKNOWN
+        # random guess
+        return Int(input.size * 2)
+    else
+        # exact size
+        return Int(ret)
+    end
+end

src/libzstd.jl

@@ -103,3 +103,14 @@ end
 function free!(dstream::DStream)
     return ccall((:ZSTD_freeDStream, libzstd), Csize_t, (Ptr{Cvoid},), dstream.ptr)
 end
+
+
+# Misc. functions
+# ---------------
+
+const ZSTD_CONTENTSIZE_UNKNOWN = Culonglong(0) - 1
+const ZSTD_CONTENTSIZE_ERROR   = Culonglong(0) - 2
+
+function find_decompressed_size(src::Ptr, size::Integer)
+    return ccall((:ZSTD_findDecompressedSize, libzstd), Culonglong, (Ptr{Cvoid}, Csize_t), src, size)
+end