move-pos: calculates the effect of printing a displayed string on

_move-pos_: calculates the effect of printing a displayed string on
line/column positions.

Danny Yoo (dyoo@cs.wpi.edu / dyoo@hkn.eecs.berkeley.edu)


Introduction
------------

_move-pos.ss_ is a simple library that figures out what line and
column we're on after a given string is displayed.  It takes as input
the location that we're at and the displayed string, and returns a
location that represents the cursor movement.


Example
-------

    > (require (planet "move-pos.ss" ("dyoo" "move-pos.plt" 1)))
    >
    > (let ([line 0]
            [column 0]
            [pos 0])
        (apply-move (get-move (open-input-string "hello"))
                    (make-loc line column pos)))
    #(struct:loc 0 5 5)
    >
    > (let ([line 0]
            [column 13]
            [pos 13])
        (apply-move (get-move (open-input-string "hello"))
                    (make-loc line column pos)))
    #(struct:loc 0 18 18)
    >
    > (let ([line 0]
            [column 13]
            [pos 13])
        (apply-move (get-move (open-input-string "hello\nworld"))
                    (make-loc line column pos)))
    #(struct:loc 1 5 24)


Data defintions
---------------

There are two data definitions: a _location_ to represent where we
are, and a _move_ to represent the line/column shifting after a
display.


A _location_ is a:

   (define-struct loc (line col pos) #f)

where line, column, and pos are natural numbers.


A _move_ is an opaque structure that can be applied on locations to
make them shift around.



API
---

> loc?: any -> boolean

Returns #t if the datum is a location structure.


> move?: any -> boolean

Returns #t if the datum is a move structure.


> get-move: input-port -> move

Constructs a move that simulates the line/column shifting effect of 
displaying the content on the given input-port.


> apply-move: move loc -> loc

Given a move, shifts the given location.


> move-compose: move move -> move

Creates a new move whose effect is the first move followed by the second.  So

    (apply-move (move-compose move-1 move-2) pos)

should be equivalent to:

    (apply-move move-2 (apply-move move-1 pos))


Parameters
----------

> current-line-break-mode: (parameterof symbol)

Defines the line-breaking lexer what interpretation to choose for the
line breaking character sequence, similar to what _read-line_ uses.
Supported values are 'linefeed, 'return, 'return-linefeed, 'any, and
'anyone.  The default is 'any.

> current-tab-break-length: (parameterof natural-number)

Defines the tab break length we use when we see a #\tab.  The default
value is 8.