MODULE overstrike; (*DEE 2014-08-06/2015-12-02*)
  (*Software Tools in Pascal, exercise 2-5.
   * Compare with Systematic Programming: An Introduction, program 10.24,
   * pg. 79, as well as the same program, updated for J&W Pascal, called
   * insert on pg. 61 of the Second Edition.
   *)
  FROM ST IMPORT getc, putc;
  FROM STchars IMPORT BACKSPACE, BLANK, ENDFILE, FF, NEWLINE, PLUS, character;
  FROM STutility IMPORT max;

  (* overstrike: convert backspaces into multiple lines. *)
  PROCEDURE overstrike;
    CONST
      NOSKIP = PLUS;
      SKIP = BLANK; (* Skip to the next line *)
      PGSKIP = 49; (* ANSI page skip, i.e. '1' *)
    VAR
      col, i, newcol: INTEGER;
      c: character;
      fflag: BOOLEAN; (* form feed flag *)
  BEGIN
    col := 1;
    fflag := FALSE;
    REPEAT
      newcol := col;
      WHILE (getc(c) = BACKSPACE) DO (* Eat backspaces. *)
        newcol := max(newcol - 1, 1)
      END;
      IF (newcol < col) THEN
        putc(NEWLINE);
        putc(NOSKIP); (* Start overstrike line. *)
        FOR i := 1 TO newcol - 1 DO
          putc(BLANK)
        END;
        col := newcol
      ELSIF (col = 1) AND (c # ENDFILE) AND ~fflag THEN
        putc(SKIP) (* normal line *)
      (* ELSE middle of line *)
      END;
      IF (c # ENDFILE) THEN
        IF (c = FF) THEN
          putc(NEWLINE); (*Start page break line.*)
          putc(PGSKIP);
          col := 1;
          fflag := TRUE
        ELSE
          putc(c); (* normal character *)
          fflag := FALSE;
          IF (c = NEWLINE) THEN
            col := 1
          ELSE
            INC(col)
          END;
        END
      END
    UNTIL (c = ENDFILE)
  END overstrike;

BEGIN overstrike
END overstrike.