{>>>>KEY_SEARCH<<<<}

FUNCTION KEY_SEARCH(VAR KEYS     : KEY_FILE;
                    VAR KEY_REF  : INTEGER;
                        MATCH_IT : STRING80) : BOOLEAN;

VAR HI,LO,MID   : INTEGER;
    REC_COUNT   : INTEGER;
    SEARCH_REC  : KEY_REC;
    FOUND       : BOOLEAN;
    COLLIDED    : BOOLEAN;

BEGIN
  KEY_REF := 0;
  REC_COUNT := FILESIZE(KEYS);
  HI := REC_COUNT;
  LO := 0;
  KEY_SEARCH := FALSE; FOUND := FALSE; COLLIDED := FALSE;

  MID := (LO + HI) DIV 2;    { Calc first midpoint }

  IF REC_COUNT > 0 THEN      { Don't search if file is empty }
    REPEAT
      SEEK(KEYS,MID);
      READ(KEYS,SEARCH_REC);

           { Was there a collision between MID & LO or MID & HI? }
      IF (LO = MID) OR (HI = MID) THEN COLLIDED := TRUE;
      IF MATCH_IT = SEARCH_REC.KEY THEN  { Found it! }
        BEGIN
          FOUND := TRUE;             { Set found flag... }
          KEY_SEARCH := TRUE;        { ...and function value... }
          KEY_REF := SEARCH_REC.REF  { ...and key into data file }
        END
      ELSE                  { No luck...divide and try again... }
        BEGIN
          IF MATCH_IT > SEARCH_REC.KEY THEN LO := MID
            ELSE HI := MID;             { Halve the field }
          MID := (LO + HI) DIV 2;       { Recalc the midpoint }
          KEY_REF := MID                { Save MID in KEY_REF }
        END
    UNTIL COLLIDED OR FOUND
  END;