max_vals%=1000
count%=0
DIM values%(max_vals%)
REM ---------------------------------------------------------------------------
REM Main program
REM Purpose: load the configured file, sort its values, save the result, report time.
REM Parameters: None. Returns: no value.
REM Variables used: values%, count%, elapsed! and the called procedure state.
REM ---------------------------------------------------------------------------
PRINT "***HEAPSORT GFA BASIC***"
load_input_file("INPUT.TXT")
elapsed!=@heap_sort
save_output_file("OUTPUT.TXT")
PRINT "***RUN COMPLETE***"
PRINT "HEAPSORT TOOK ";elapsed!;" SECONDS."
REM ---------------------------------------------------------------------------
REM save_output_file
REM Purpose: write values%(0..count%-1), one value per line.
REM Parameters: file_spec$ - output file path.
REM Returns: no value.
REM Variables used: v% - array traversal index; #1 - output file channel.
REM ---------------------------------------------------------------------------
PROCEDURE save_output_file(file_spec$)
  PRINT "SAVING OUTPUT FILE...";
  OPEN "o",#1,file_spec$
  FOR v%=0 TO count%-1
    PRINT #1,STR$(values%(v%))
  NEXT v%
  CLOSE #1
  PRINT "DONE."
RETURN
REM ---------------------------------------------------------------------------
REM load_input_file
REM Purpose: read integer lines into values% and update count%.
REM Parameters: file_spec$ - input file path.
REM Returns: no value.
REM Variables used: line$ - current line; count% - number of values loaded.
REM ---------------------------------------------------------------------------
PROCEDURE load_input_file(file_spec$)
  PRINT "LOADING INPUT FILE...";
  DIM line$(80)
  count%=0
  OPEN "i",#1,file_spec$
  DO WHILE NOT EOF(#1)
    INPUT #1,line$
    IF line$<>"" THEN
      values%(count%)=VAL(line$)
      count%=count%+1
    ENDIF
  LOOP
  CLOSE #1
  PRINT "DONE."
RETURN
REM ---------------------------------------------------------------------------
REM sift_down
REM Purpose: restore the max-heap property below root% in values%.
REM Parameters: root% - zero-based root; last% - exclusive heap end.
REM Returns: no value; mutates values%.
REM Variables used: child%, temp% - child index and swap storage.
REM ---------------------------------------------------------------------------
PROCEDURE sift_down(root%,last%)
  DO WHILE 2*root%+1<last%
    child%=2*root%+1
    IF child%+1<last% AND values%(child%)<values%(child%+1) THEN
      child%=child%+1
    ENDIF
    EXIT IF values%(root%)>=values%(child%)
    temp%=values%(root%)
    values%(root%)=values%(child%)
    values%(child%)=temp%
    root%=child%
  LOOP
RETURN
REM ---------------------------------------------------------------------------
REM heap_sort
REM Purpose: sort values% in place and measure elapsed time.
REM Parameters: None; uses global values% and count%.
REM Returns: elapsed duration in seconds.
REM Variables used: start%, end%, temp% - heap state; start_time!, end_time! - ticks.
REM ---------------------------------------------------------------------------
FUNCTION heap_sort
  PRINT "SORTING ELEMENTS...";
  start_time!=TIMER
  FOR start%=count%/2-1 TO 0 STEP -1
    sift_down(start%,count%)
  NEXT start%
  FOR end%=count%-1 TO 1 STEP -1
    temp%=values%(0)
    values%(0)=values%(end%)
    values%(end%)=temp%
    sift_down(0,end%)
  NEXT end%
  end_time!=TIMER
  PRINT "DONE."
  RETURN (end_time!-start_time!)/200
ENDFUNC