IMPLEMENTATION MODULE Strings;
(* The descriptor's header holds the logical length; Assign/Concat
   update it (m2setlen), so Length reflects content, not capacity. *)

PROCEDURE m2setlen(VAR s : ARRAY OF CHAR; n : INTEGER);
  EXTERNAL;

PROCEDURE Length(VAR s : ARRAY OF CHAR) : INTEGER;
BEGIN
  RETURN LEN(s)
END Length;

PROCEDURE Assign(VAR src, dst : ARRAY OF CHAR);
VAR i, n, cap : INTEGER;
BEGIN
  n := LEN(src);
  cap := LEN(dst);
  IF n > cap THEN n := cap END;
  i := 0;
  WHILE i < n DO
    dst[i] := src[i];
    i := i + 1
  END;
  m2setlen(dst, n)
END Assign;

PROCEDURE Copy(VAR src, dst : ARRAY OF CHAR);
BEGIN
  Assign(src, dst)
END Copy;

PROCEDURE Concat(VAR s1, s2 : ARRAY OF CHAR; VAR dst : ARRAY OF CHAR);
VAR i, n1, n2, cap : INTEGER;
BEGIN
  n1 := LEN(s1);
  n2 := LEN(s2);
  cap := LEN(dst);
  IF n1 + n2 > cap THEN
    n1 := cap - n2;
    IF n1 < 0 THEN n1 := 0 END
  END;
  i := 0;
  WHILE i < n1 DO
    dst[i] := s1[i];
    i := i + 1
  END;
  i := 0;
  WHILE i < n2 DO
    dst[n1 + i] := s2[i];
    i := i + 1
  END;
  m2setlen(dst, n1 + n2)
END Concat;

PROCEDURE Compare(VAR s1, s2 : ARRAY OF CHAR) : INTEGER;
VAR i, n1, n2, n : INTEGER;
BEGIN
  n1 := LEN(s1);
  n2 := LEN(s2);
  n := n1;
  IF n2 < n THEN n := n2 END;
  i := 0;
  WHILE i < n DO
    IF s1[i] < s2[i] THEN RETURN -1
    ELSIF s1[i] > s2[i] THEN RETURN 1
    END;
    i := i + 1
  END;
  IF n1 < n2 THEN RETURN -1
  ELSIF n1 > n2 THEN RETURN 1
  END;
  RETURN 0
END Compare;

END Strings.