Arduino Program C

For this project an Arduino programm is needed which measures the frequency of a speed sensor is measured and forwarded until a certain frequency. For a higher frequency manipulated values must be given to output. Program should be automatically in manipulation mode but should be possible to disable and enable by getting double input high low hig within 2 seconds.
pin7 = input enable disable
Pin 8 = input sensor
Pin3 = Output

additional inforamtion and calculations are done and will be provided.