Template:Prelude/pow mod.a68

COMMENT
  This routine is used in more than one place, and is essentially a
  template that can by used for many different types, eg INT, LONG INT...
USAGE
  MODE POWMODSTRUCT = INT, LONG INT, COMPL, FRAC, MODULAS, MATRIX etc
  PR READ "preclude/pow_mod.a68" PR
END COMMENT

PROC pow mod = (POWMODSTRUCT b,in e, mod)POWMODSTRUCT: (
  POWMODSTRUCT sq := b, e := in e;
  POWMODSTRUCT out:= IF ODD e THEN b ELSE 1 FI;
  e:=e OVER 2;
  WHILE e /= 0 DO
    sq *:= sq %* mod;
    IF ODD e THEN out := out * sq %* mod FI ;
    e:=e OVER 2
  OD;
  out
)