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It is possible to use the cursor optimization functions of this screen package
without the overhead and additional size of the screen updating functions.
The screen updating functions are designed for uses where parts of the screen
are changed, but the overall image remains the same.  This includes such
programs as and

actually uses these functions, does not.  Certain other programs will find it
difficult to use these functions in this manner without considerable
unnecessary program overhead.  For such applications, such as some ".(f

Graphics programs designed to run on character-oriented terminals.  I could
name many, but they come and go, so the list would be quickly out of date.
Recently, there have been programs such as and and optimizing programs, all
that is needed is the motion optimizations.  This, therefore, is a description
of what some of what goes on at the lower levels of this screen package.  The
descriptions assume a certain amount of familiarity with programming problems
and some finer points of C.  None of it is terribly difficult, but you should
be forewarned.  In order to use a terminal's features to the best of a
program's abilities, it must first know what they are.

If this comes as any surprise to you, there's this tower in Paris they're
thinking of junking that I can let you have for a song.  The   describes
these, but a certain amount of decoding is necessary, and there are, of
course, both efficient and inefficient ways of reading them in.  The algorithm
that the uses is taken from and is hideously efficient.  It reads them in a
tight loop into a set of variables whose names are two uppercase letters with
some mnemonic value.  For example, is a string which moves the cursor to the
"home" position.

These names are identical to those variables used in the
 to describe each capability.  See Appendix A for a complete list of those
read, and the manual page for a full description.  As there are two types of
variables involving ttys, there are two routines.  The first, sets some
variables based upon the tty modes accessed by and The second, a larger task
by reading in the descriptions from the  .  This is the way these routines are
used by if (isatty(0)) {
       gettmode();
       if ((sp=getenv("TERM")) != NULL)
               setterm(sp);   else             setterm(Defterm); } else
       setterm(Defterm); puts(TI); puts(VS); checks to see if file descriptor
0 is a terminal.

is defined in the default C library function routines.  It does a on the
descriptor and checks the return value.  If it is, sets the terminal
description modes from a is then called to get the name of the terminal, and
that value (if there is one) is passed to which reads in the variables from
associated with that terminal.  returns a pointer to a string containing the
name of the terminal, which we save in the character pointer If returns false,
the default terminal is used.  The and sequences initialize the terminal is a
macro which uses (see and to put out a string).  undoes these things.  Now
that we have all this useful information, it would be nice to do something
with it.

Actually, it be emotionally fulfilling just to get the information.  This is
usually only true, however, if you have the social life of a kumquat.  The
most difficult thing to do properly is motion optimization.  When you consider
how many different features various terminals have (tabs, backtabs, non-
destructive space, home sequences, absolute tabs, .....) you can see that
deciding how to get from here to there can be a decidedly non-trivial task.
The editor uses many of these features, and the routines it uses to do this
take up many pages of code.  Fortunately, I was able to liberate them with the
author's permission, and use them here.  After using and to get the terminal
descriptions, the function deals with this task.  It usage is simple: you
simply tell it where you are now and where you want to go.  For example
mvcur(00LINES/2COLS/2) (would move the cursor from the home position (00) to
the middle of the screen.  If you wish to force absolute addressing, you can
use the function from the routines, or you can tell that you are impossibly
far away, like Cleveland.  For example, to absolutely address the lower left
hand corner of the screen from anywhere just claim that you are in the upper
right hand corner: mvcur(0COLS-1LINES-10)

                              September 26, 2016                            ()

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