Algorithms and Data Structures

N.Wirth. Algorithms and Data Structures. Oberon version 32
Nc = 128; (*size of consumer block*)
N = 1024; (*buffer size, common multiple of Np and Nc*)
VAR ne, nf: INTEGER;
in, out: INTEGER;
nonfull: Signals.Signal; (*ne >= 0*)
nonempty: Signals.Signal; (*nf >= 0*)
buf: ARRAY N OF CHAR;
PROCEDURE deposit (VAR x: ARRAY OF CHAR);
BEGIN
ne := ne - Np;
IF ne < 0 THEN Signals.Wait(nonfull) END;
FOR i := 0 TO Np-1 DO buf[in] := x[i]; INC(in) END;
IF in = N THEN in := 0 END;
nf := nf + Np;
IF nf >= 0 THEN Signals.Send(nonempty) END
END deposit;
PROCEDURE fetch (VAR x: ARRAY OF CHAR);
BEGIN
nf := nf - Nc;
IF nf < 0 THEN Signals.Wait(nonempty) END;
FOR i := 0 TO Nc-1 DO x[i] := buf[out]; INC(out) END;
IF out = N THEN out := 0 END;
ne := ne + Nc;
IF ne >= 0 THEN Signals.Send(nonfull) END
END fetch;
BEGIN
ne := N; nf := 0; in := 0; out := 0;
Signals.Init(nonfull); Signals.Init(nonempty)
END Buffer.
1.7.4 Textual Input and Output
By standard input and output we understand the transfer of data to (from) a computer system from (to)
genuinely external agents, in particular its human operator. Input may typically originate at a keyboard and
output may sink into a display screen. In any case, its characteristic is that it is readable, and it typically
consists of a sequence of characters. It is a text. This readability condition is responsible for yet another
complication incurred in most genuine input and output operations. Apart from the actual data transfer, they
also involve a transformation of representation. For example, numbers, usually considered as atomic units
and represented in binary form, need be transformed into readable, decimal notation. Structures need to be
represented in a suitable layout, whose generation is called formatting.
Whatever the transformation may be, the concept of the sequence is once again instrumental for a
considerable simplification of the task. The key is the observation that, if the data set can be considered as
a sequence of characters, the transformation of the sequence can be implemented as a sequence of
(identical) transformations of elements.
T() =
We shall briefly investigate the necessary operations for transforming representations of natural numbers
for input and output. The basis is that a number x represented by the sequence of decimal digits d = has the value
x = Si: i = 0 .. n-1: d i * 10 i