Algorithms and Data Structures

N.Wirth. Algorithms and Data Structures. Oberon version 158
1. A word is considered as any sequence of letters and digits starting with a letter.
2. Since words may be of widely different lengths, the actual characters are stored in an array buffer, and
the tree nodes contain the index of the key's first character.
3. It is desirable that the line numbers be printed in ascending order in the cross-reference index.
Therefore, the item lists must be generated in the same order as they are scanned upon printing. This
requirement suggests the use of two pointers in each word node, one referring to the first, and one referring
to the last item on the list. We assume the existence of global writer W, and a variable representing the
current line number in the text.
CONST WordLen = 32; (* ADenS443_CrossRef *)
TYPE Word = ARRAY WordLen OF CHAR;
Item = POINTER TO RECORD)
lno: INTEGER; next: Item
END;
Node = POINTER TO RECORD
key: Word;
first, last: Item; (*list*)
left, right: Node (*tree*)
END;
VAR line: INTEGER;
PROCEDURE search (VAR w: Node; VAR a: Word);
VAR q: Item;
BEGIN
IF w = NIL THEN (*word not in tree; new entry, insert*)
NEW(w); NEW(q); q.lno := line;
COPY(a, w.key); w.first := q; w.last := q; w.left := NIL; w.right := NIL
ELSIF w.key < a THEN search(w.right, a)
ELSIF w.key > a THEN search(w.left, a)
ELSE (*old entry*)
NEW(q); q.lno := line; w.last.next := q; w.last := q
END
END search;
PROCEDURE Tabulate (w: Node);
VAR m: INTEGER; item: Item;
BEGIN
IF w # NIL THEN
Tabulate(w.left);
Texts.WriteString(W, w.key); Texts.Write(W, TAB); item := w.first; m := 0;
REPEAT
IF m = 10 THEN
Texts.WriteLn(W); Texts.Write(W, TAB); m := 0
END;
INC(m); Texts.WriteInt(W, item.lno, 6); item := item.next
UNTIL item = NIL;
Texts.WriteLn(W); Tabulate(w.right)
END
END Tabulate;
PROCEDURE CrossRef (VAR R: Texts.Reader);
VAR root: Node; (*uses global writer W*)