Algorithms and Data Structures

N.Wirth. Algorithms and Data Structures. Oberon version 111
procedure TryNextMove. On a board that is infinite in all directions, each position x, y has a number
candidate moves u, v, which at this point there is no need to specify (see fig. 3.7). The predicate to choose
an acceptable move can be expressed as the logical conjunction of the conditions that the new field lies on
the board, i.e. 0 ≤ u < n and 0 ≤ v < n, and that it had not been visited previously, i.e., h uv = 0. One further
detail must not be overlooked: A variable h uv does exist only if both u and v lie within the index range 0 ...
n-1. Consequently, the Boolean expression, substituted for acceptable in the general schema, is valid only
if its first four constituent terms are true. It is therefore relevant that the term h uv = 0 appears last. As a
result, the selection of the next acceptable candidate move is expressed by the familiar scheme of linear
search (only formulated now in terms of the repeat loop instead of while, which in this case is possible and
convenient). To signal that there are now further candidate moves, the variable eos can be used. Let us
formulate the operation as a procedure Next, explicitly specifying the relevant variables as parameters:
PROCEDURE Next (VAR eos: BOOLEAN; VAR u, v: INTEGER);
BEGIN
(*~eos*)
REPEAT select the next candidate move u, v
UNTIL (no more candidates) OR
((0