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// To understand:
// high(x) is length of array x - 1
// Fmax(x) is 2 element vector addition
// Fmul(x) is 2 element vector addition
// inc(i, l2) is i += l2;
procedure ComplexFFT(dir: integer; var x: array of complex);
var i,i1,i2,j,k,l,l1,l2,n: integer;
    t1,u,c: complex;
    m: integer;
begin
 
  for m := 0 to 16 do
    if (1 shl m) >= high(x) then break;
  // calculate the number of points
  n := 1;
  for i := 0 to m-1 do
    n := n shl 1;
  // do the bit reversal
 
  i2 := n shr 1;
  j := 0;
 
  for i := 0 to n-2 do
  begin
    if i < j then ComplexSwap(x[i],x[j]);
    k := i2;
    while k <= j do
    begin
      j := j - k;
      k := k shr 1;
    end;
    inc(j,k);
  end;
 
  // compute the FFT
  c := Makefloat(-1,0);
  l2 := 1;
  for l := 0 to m-1 do
  begin
    l1 := l2;
    l2 := l2 shl 1;
    u := Makefloat(1,0);
    for j := 0 to l1-1 do
    begin
      i := j;
      while i < n do
      begin
        i1 := i + l1;
        t1 := Complexmul(u,x[i1]);
        x[i1] := Fsub(x[i],t1);
        x[i] := Fadd(x[i],t1);
        inc(i,l2);
      end;
      u := Complexmul(u,c);
    end;
    c[1] := sqrt((1.0-c[0])*0.5);
    if dir = 1 then c[1] := -c[1];
    c[0] := sqrt((1.0+c[0])*0.5);
  end;
  // scaling for forward transformation
  if dir = 1 then
  begin
    for i := 0 to n-1 do
    begin
      x[i] := Fmul(x[i],1/n);
    end;
  end;
end;

Free Pascal Compiler version 2.2.0 [2009/11/16] for i386
Copyright (c) 1993-2007 by Florian Klaempfl
Target OS: Linux for i386
Compiling prog.pas
prog.pas(6,59) Error: Identifier not found "complex"
prog.pas(8,20) Error: Identifier not found "complex"
prog.pas(8,20) Error: Error in type definition
prog.pas(25,30) Error: Identifier not found "ComplexSwap"
prog.pas(25,41) Error: Illegal expression
prog.pas(36,17) Error: Identifier not found "Makefloat"
prog.pas(42,19) Error: Identifier not found "Makefloat"
prog.pas(49,25) Error: Identifier not found "Complexmul"
prog.pas(50,22) Error: Identifier not found "Fsub"
prog.pas(51,21) Error: Identifier not found "Fadd"
prog.pas(54,22) Error: Identifier not found "Complexmul"
prog.pas(56,7) Error: Illegal qualifier
prog.pas(56,25) Error: Illegal qualifier
prog.pas(57,23) Error: Illegal qualifier
prog.pas(57,32) Error: Illegal qualifier
prog.pas(58,7) Error: Illegal qualifier
prog.pas(58,25) Error: Illegal qualifier
prog.pas(65,19) Error: Identifier not found "Fmul"
prog.pas(68,4) Fatal: Syntax error, "BEGIN" expected but "end of file" found
Fatal: Compilation aborted
Error: /usr/bin/ppc386 returned an error exitcode (normal if you did not specify a source file to be compiled)

Standard output is empty