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@oprypin
Created Dec 3, 2021
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private def overwrite_memory(src : T, dst : T) : Nil forall T
src = src.as(T*).as(UInt8*)
dst = dst.as(T*).as(UInt8*)
src.copy_to(dst, instance_sizeof(T))
end
abstract class Capture
abstract def transform(bytes : Bytes) : Bytes
@done = Channel(Nil).new
@stream : IO::FileDescriptor? = nil
@original_stream : IO::FileDescriptor? = nil
def tap_output_stream(stream : IO::FileDescriptor) : Nil
raise "Already captured." if @stream
@stream = stream
@original_stream = original_stream = stream.dup
reader, writer = IO.pipe
overwrite_memory(src: writer, dst: stream)
spawn do
buf = Bytes.new(original_stream.buffer_size)
loop do
size = reader.read(buf)
break if size == 0
original_stream.write(transform(buf[0, size]))
end
original_stream.flush
@done.send(nil)
end
end
def untap : Nil
if (stream = @stream)
stream.close
@done.receive
if (original_stream = @original_stream)
overwrite_memory(src: original_stream, dst: stream)
@original_stream = nil
end
@stream = nil
end
end
def self.tap_output_stream(stream : IO::FileDescriptor) : Nil
capture = new
capture.tap_output_stream(stream)
begin
yield
ensure
capture.untap
end
end
end
class Rot13Capture < Capture
def transform(buf : Bytes) : Bytes
buf.map! do |c|
case c
when 'a'.ord..'z'.ord
(c - 'a'.ord + 13) % 26 + 'a'.ord
when 'A'.ord..'Z'.ord
(c - 'A'.ord + 13) % 26 + 'A'.ord
else c
end
end
end
end
puts "Before"
Rot13Capture.tap_output_stream(STDOUT) do
puts "Hello, World!"
end
puts "After"
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