Benchmark: timing 1000000 iterations of interp, manual...
  interp:  4 wallclock secs ( 3.68 usr +  0.05 sys =  3.73 CPU) @ 268096.51/s
  manual:  3 wallclock secs ( 3.79 usr + -0.13 sys =  3.66 CPU) @ 273224.04/s