Crystal injecting/replacing IO PoC

injecting_io.cr

struct Slice(T)
  def []?(start, count)
    if start + count >= size
      count = -1
    end

    if count < 0
      count = size - start + (count + 1)
    end

    self[start, count]
  end
end

class InjectingIO
  include IO

  getter? closed = false
  @remaining_token : Bytes?

  def initialize(@io : IO, @token : Bytes, @replacement : Bytes)
  end

  def read(slice : Bytes)
    check_open

    @io.read(slice)
  end

  def write(slice : Bytes)
    check_open

    remaining_token = @remaining_token
    if remaining_token
      if slice[0, remaining_token.size]? == remaining_token
        @io.write(@replacement)
        @io.write(slice[remaining_token.size, -1]?)
        @remaining_token = nil
        return
      elsif slice.size < remaining_token.size
        if slice == remaining_token[0, slice.size]?
          @remaining_token = remaining_token[slice.size, -1]?
          return
        else
          @io.write(@token[0, @token.size - remaining_token.size])
          @remaining_token = nil
        end
      else
        @io.write(@token[0, @token.size - remaining_token.size])
        @remaining_token = nil
      end
    end

    first_half = slice
    second_half = nil
    slice.each_index do |i|
      candidate = slice[i, @token.size]?
      if candidate == @token
        first_half = slice[0, i]?
        second_half = slice[i + @token.size, -1]?
        break
      elsif candidate.size < @token.size && candidate == @token[0, candidate.size]?
        @remaining_token = @token[candidate.size, -1]?
        first_half = slice[0, i]?
        break
      end
    end

    @io.write(first_half)

    if second_half
      @io.write(@replacement)
      @io.write(second_half)
    end
  end

  def close
    @io.close
    @closed = true
  end
end

io = InjectingIO.new(STDOUT, "foo".to_slice, "bar".to_slice)

io.puts "Hello World"
io.puts "Hello foo"
io.print "fo"
io.puts "o is cool"
io.print "f"
io.puts "oo is cooler"
io.print "f"
io.print "o"
io.puts "o is the coolest"
io.print "foo"
io.puts " is okay too"
io.print "fo"
io.puts " stays"
io.close