% From the book
% PROLOG PROGRAMMING IN DEPTH
% by Michael A. Covington, Donald Nute, and Andre Vellino
% (Prentice Hall, 1997).
% Copyright 1997 Prentice-Hall, Inc.
% For educational use only

% File READI16.PL
% Routines to read 16-bit binary integers from a file

% Assumes less significant byte comes first
% (as on PC's, not Sparcstations).
% If this is not the case, swap Lo and Hi in read_u16/1.


:- ensure_loaded('readbyte.pl').  % or use reconsult if necessary


% read_u16(-Integer)
%  Reads 2 bytes as a 16-bit unsigned integer, LSB first.

read_u16(I) :-              % 16-bit unsigned integer
   read_bytes(2,[Lo,Hi]),
   I is Hi*256 + Lo.



% read_i16(-Integer)
%  Reads 2 bytes as a 16-bit signed integer, LSB first.

read_i16(I) :-              % 16-bit signed integer
  read_u16(U),
  u16_to_i16(U,I).



% u16_to_i16(+U,-I)
%  Converts 16-bit unsigned to signed integer.

u16_to_i16(U,I) :- U > 32767, !, I is U - 65536.

u16_to_i16(U,U).