Chapter 7: Redisplay

One, two! One, two! And through and through
  The vorpal blade went snicker-snack!

The previous chapter described a way of dividing the implementation into parts and covered one of those parts, the internal sub-editor. This chapter describes the redisplay part.

This chapter will start by discussing the general constraints that affect redisplay. It will then describe the external interface and some of the internal interfaces used by redisplay (the "procedure interface" definitions). It goes on to discuss many of the considerations that affect implementations of the algorithms. Finally, it describes the redisplay algorithms.


Redisplay, or incremental redisplay, to give it its full name, is that part of the implementation that is responsible for ensuring that all changes to the buffer are promptly reflected on the user's display. As is evident from the definition, there are two parts to redisplay's job.

The first part is to ensure that all changes are indeed tracked. In the absence of the second part, this part would be quite easy.

The second part of the job is to ensure that the changes are made "promptly." In this context, "promptly" means that the amount of clock time required to make the updates visible is minimized. Clock time is the combination of transmission time, CPU time, and disk access time that is perceived by the user as the delay from when a command has been entered to when the display has been updated.

In general, the buffer's contents will change only a small amount during any one command. The screen will thus only have to be changed by a small amount in order to reflect the changed buffer contents. Hence, the algorithms concentrate on incrementally redisplaying the buffer; the entire process is thus referred to as incremental redisplay. Fortunately, it turns out that in cases where the buffer is changed drastically, the increment-oriented approach to redisplay works quite well and so there is no need for multiple algorithms.

This discussion of incremental redisplay assumes a model of the system where the editing is done on a main processor which communicates with a display. If the main processor is the same as the display, the bandwidth of the CPU to display communications channel can be very high. However, the considerations remain unchanged: only the relative weights change. Incremental redisplay is an optimization between CPU time, display-processing time, and communications-channel time, with a few memory considerations thrown in.

The first major constraint is the speed of the communications channel. Typical speeds that are available are 300, 1200, 2400, and 9600 bps. Memory-mapped and other built-in displays run at bus speeds: communications speed is essentially infinite.

A typical video display has a 24 x 80 character screen. At 300 bps, it takes three seconds to reprint a line and over a minute to refresh the whole screen. At 1200 bps, less than one second is required to reprint a line and about sixteen seconds to refresh the screen. At 9600 bps, it will take one to two seconds to refresh the screen. The speed of the communication thus greatly affects the amount of optimization that is desired. At 300 bps, the user may notice even one extra transmitted character, while at 9600 bps, reprinting entire lines does not take an appreciable amount of time. One dimension of the optimization is thus clear: the importance of optimizing the number of characters sent increases in proportion to the slowness of the communication line.

The second major constraint is the speed of the display device. It takes time for the display to handle each command, and this time can affect the choice of commands sent to the display. For example, if a line ending in:


was changed to:


the redisplay code could elect to position the cursor just before the "w", insert three characters, then send "nar". Alternatively, it could position to the same place and just send "narwhale". The latter would be more efficient unless the display could accept and perform the "insert three characters" command in less than five character times (possible). As has been mentioned before, some memory-mapped displays actually process commands very slowly. As the effective display speed is so low, it is important to use good redisplay algorithms on those displays.

User interface considerations also affect which command sequences should be sent. For example, while it might be acceptable from a pure clock-time point of view to reprint an entire line, users do not like to see text which has not changed in the buffer "change" by being reprinted. The flickering that is generated by the reprinting process attracts the user's attention to that text, which is undesirable as the text has not, after all, changed. Thus, avoiding extraneous flickering and movement of text is good. The amount of perceived flicker will vary from display to display, being highly dependent upon such factors as display command set, display speed, internal display data-structures, timing, and phosphor.

The third major constraint is the CPU speed. On some computers, computing an optimal redisplay sequence takes longer than is saved by the optimizations ("optimal" considering only the communications channel and display). On those machines, the correct optimization is to send the "less-than-optimal" sequence.

CPU time must be spent in order to perform any optimizations. If the CPU time spent exceeds some small amount of clock time, the user will perceive response to be sluggish. It is therefore desirable to minimize the CPU time spent on optimizing the redisplay. However, the communications channel speed also makes a difference. If the line is slow, extra CPU time can and should be spent (at 300 bps, it is worthwhile to spend up to 30 msec. of CPU time to eliminate one character from being transmitted (which takes about 30 msec.)). However, at higher speeds it is generally not practical to heavily optimize the number of characters sent, as it can easily take longer to compute the optimizations than to transmit the extra data. This relaxation of the optimization is subject to the user interface constraint outlined above.

The fourth constraint is the memory size. For example, one technique stores a copy of the entire screen, character by character. This technique works quite well in general. However, where memory is tight this technique may not be feasible.

Procedure Interface Definitions

This section describes two interfaces. The first one is the (external) interface that redisplay presents to the rest of the editor implementation (i.e., the sub-editor and the user-oriented commands). The second interface is the internal interface used by the redisplay to isolate the display-specific portions of its internal code.

In this section, the term display refers to the hardware operated by the user. A display has a keyboard, screen, and perhaps a graphical input device. The screen is the part of the display that shows output. A window is a logical screen. One window can occupy the entire screen, or more than one window can share the screen, perhaps even overlapping. A window can never be larger than the screen.

Editor Procedures

	status Window_Init(char *display);
	status Window_Fini(void);
	status Window_Save(FILE *fptr);
	status Window_Load(FILE *fptr);

Window_Init is the basic set-up-housekeeping call. It is called once, upon editor invocation. It should perform all required one-time initialization, including keyboard and screen initialization. No other editor interface call except for Window_Fini can be legally called unless Window_Init returns a successful status. The parameter indicates the display type. Presumably, if the parameter is null, the routine will determine a default display.

Window_Fini terminates all state information. Once called, Window_Init must be called again before other editor interface calls can be legally made.

Window_Save saves the current redisplay state information in the file opened on the specified file descriptor. Presumably, World_Save opened the file, then called this routine.

Window_Load loads all redisplay state information from the file opened on the specified file descriptor.

If you are creating a "stripped down" editor, then these routines would not do anything. They can be put in as stubs if there is a reasonable possibility that the editor will be embellished later.

	void Redisplay(void);
	void Recenter(void);
	void Refresh_Screen(void);

Redisplay performs one incremental redisplay. If it runs to completion, the screen will accurately reflect the buffer. However, this routine also checks for type ahead. If the user does type (or use the graphical input device) before the redisplay has finished, this routine will notice that event and abort the redisplay in a safe manner. Presumably, the user will quit typing ahead at some point and redisplay can then complete.

Recenter operates as does Redisplay, except that it moves the point to the center of the window (technically, the preferred percentage). This procedure is needed because there is typically a user-level command to perform this operation.

Refresh_Screen operates as does Recenter, except that it assumes that the screen has been corrupted. Thus, this routine ensures that the screen is correct no matter what else may have happened.

	void Set_Pref_Pct(int percent);
	int Get_Point_Row(void);
	int Get_Point_Col(void);

Set_Pref_Pct sets the preferred percentage. After a Recenter or other similar operation, the point will be on a line approximately this percentage of the way through the window. A good default value is about forty percent. With this default on a 22-line window, the point will be on line 9.

Get_Point_Row returns the number of the row within the window that the point is at.

Get_Point_Col returns the number of the column within the window that the point is at. This may not be the same as the column returned by Get_Column, as that routine does not take into account line wrap.

	window_data Window_Create(window_data wind);
	status Window_Destroy(window_data wind);
	status Window_Grow(window_data wind, int amt);
	int Get_Window_Top_Line(window_data wind);
	int Get_Window_Bot_Line(window_data wind);
	location Get_Window_Top(window_data wind);
	location Get_Window_Bot(window_data wind);

These routines are used to manipulate multiple windows, if you choose to offer that feature. The definitions provided here only allow for horizontal windows (i.e., all windows occupy the full width of the display). Vertical windows and overlapping windows are not covered, although they are often implemented.

Window_Create creates a new window. It operates by splitting the supplied window in two. Both windows initially show the same data. It returns a window descriptor for the second window.

Window_Destroy destroys the supplied window. The window above this one expands to occupy the vacant screen space. Note that Window_Destroy(Window_Create(wind)) results in no change.

Window_Grow grows the specified window by the specified number of lines. The window is grown by moving the top line up.

Get_Window_Top_Line returns the screen line that contains the top line of the specified window.

Get_Window_Bot_Line returns the screen line one after the bottom of the specified window. This is the same value as Get_Window_Top_Line of the next lower window.

Get_Window_Top returns the location in the buffer just before the character that is at the top left part in the window. Several of the user commands use this information.

Get_Window_Bot returns the location in the buffer just after the character that is at the bottom right part of the window (or as close as you can get to it). This is the same location that would be returned by Get_Window_Top if the window were exactly one window below its current position. Several of the user commands use this information.

Display Independent Procedures

This section describes the display functions used by redisplay. The routines that implement these functions are not part of redisplay itself. A full discussion of this topic is beyond the scope of this book (but is covered in Linhart 1980). In essence, the problem is that every display manufacturer has decided on a different set of features and offers different ways of accessing those features. To solve the problem, a set of routines is needed which can isolate these differences, as well as a way of selecting among different sets of such routines as the display changes.

Although not recommended, it is possible to cover a lot of displays by assuming that the display accepts the ANSI escape sequences (i.e., the display is a DEC VT100). Most modern displays accept these sequences. However, many older displays do not. In addition, not all displays take the same amount of time to process a given command. Thus, there is still per-display information to consider.

There is one piece of existing technology to mention, and that is exemplified by the curses package available on many UNIX systems (similar packages are available under other names for other systems). It not only provides display independent functions, albeit with a somewhat different interface, it also performs the redisplay for you! However, as the purpose of this chapter is to explain how redisplay works, that package will receive no further mention.

The following set of procedures will allow display-independent operations for most displays. The procedure interfaces isolate the operations that are used by redisplay.

	status Key_Init(char *display);
	status Key_Fini(void);
	char Key_Get(void);
	FLAG Key_IsInput(void);
	private Key_FunctionKeys(void);

Key_Init and Key_Fini operate in the by now familiar fashion. They will be called by the Window_Init and Window_Fini routines. In particular, though, these routines make sure that all processing of input characters is turned off (i.e., set it for "raw" input) and all configuration information is loaded.

Key_Get waits for a key to be pressed and returns it. Keys that send multiple characters (e.g., function keys) are returned one character at a time.

Key_IsInput returns True if input is available or False if it is not. It is used, for example, by Redisplay to determine whether to abort.

Key_FunctionKeys returns information about the function keys available on this keyboard. This information includes key placement, key labelling, and the codes returned by the keys. The information is returned in an implementation-defined manner (i.e., you get to invent your own representation).

	status Screen_Init(char *screen);
	status Screen_Fini(void);
	int Screen_Rows(void);
	int Screen_Columns(void);
	private Screen_Atrributes(void);

Screen_Init and Screen_Fini operate in the by now familiar fashion. They, too will be called by the Window_Init and Window_Fini routines. In particular, though, these routines make sure that all processing of output characters is turned off (i.e., set it for "raw" output) and all configuration information is loaded.

Screen_Rows returns the number of rows in the screen. In this case, a row is the granularity of screen output. On a graphics screen, a row would be one pixel.

Screen_Columns returns the number of columns in the screen. In this case, a column is the granularity of screen output. On a graphics screen, a column would be one pixel.

Screen_Attributes returns information about the attributes (e.g., boldface, reverse video, blinking, etc.) that the screen supports. The information is returned in an implementation-defined manner (i.e., you get to invent your own representation again).

	void Set_Cursor(int row, int column);
	void Set_Row(int row);
	void Set_Column(int column);
	void Set_Attr(private attributes);
	int Get_Row(void);
	int Get_Column(void);
	private Get_Attr(void);
	void Put_Char(char c);
	void Put_String(char *str);
	void Beep(void);

Set_Cursor sets the cursor to the specified row and column. It is assumed that the optimal (i.e., least cost) command sequence will be selected.

Set_Row sets the cursor to the specified row, without affecting the column. Instead of a separate routine, this could be multiplexed onto Set_Cursor, say by one of the following:

	Set_Cursor(row, -1);
	Set_Cursor(row, Get_Column());

Set_Column sets the cursor to the specified column without affecting the row. The functionality provided by this routine could also be multiplexed onto Set_Cursor.

Set_Attr sets the current attributes to those specified.

Get_Row returns the row that the cursor is on.

Get_Column returns the column that the cursor is on.

Get_Attr returns the current attributes.

Put_Char outputs the supplied character to the screen, updating the cursor position. The character is always displayed: it is never part or all of a command sequence.

Put_String outputs the supplied string and leaves the cursor after the string. It otherwise works as per Put_Char. These strings are always displayed, even if they appear to contain screen commands. Commands may be sent to the screen only by means of the supplied procedures.

Beep rings the screen's bell or flashes the screen.


CLEOL sends the command sequence that optimally clears from the current cursor position to the end of the current line. The cursor does not move.

Clear_Line sends the command sequence that optimally clears the entire current line, and leaves the cursor in column 0 (you are assuming a zero-origin on all of these numbers, aren't you?).

CLEOS sends the command sequence that optimally clears from the current cursor position to the end of the screen. Lines after the current one are completely cleared. The cursor does not move.

Clear_Screen sends the command sequence that optimally clears the entire screen, and leaves the cursor at the upper-left corner (row 0, column 0).

	void Insert_String(char *str);
	void Delete_Chars(int count);
	void Insert_Lines(int count);
	void Delete_Lines(int count);
	void Scroll_Lines(int from, int to, int count);

These commands are available on advanced displays only (by the definition of an advanced display from Chapter 2), and each is assumed to send the optimal command sequences to effect its purpose.

Insert_String takes a string, determines the optimal command sequence required to insert it, and inserts it starting at the current cursor location. Line wrap is not performed: excess characters are dropped off the right edge of the screen. This routine could have been defined to accept a count instead of a string and to insert blanks. However, it is easier to optimize command sequences by having the string to be inserted available.

Delete_Chars accepts a count and deletes that many columns. Line wrap is not performed. Blank columns are inserted from the right edge of the screen.

Insert_Lines accepts a count and inserts that number of blank lines, starting with the line that the cursor is on (thus, you can insert lines at the very top of the screen).

Delete_Lines accepts a count and deletes that number of lines, starting with the line that the cursor is on (thus, the line at the very top of the screen can be deleted). Blank lines are scrolled in from the bottom.

Scroll_Lines accepts a from line, a to line, and a count. The lines starting with the from line, and up to but not including the to line, are scrolled by count lines (positive scrolls the lines up and negative scrolls the lines down).

	private Screen_Timings(private goal);

Screen_Timings accepts a description of a goal, and returns timing information on the various choices of screen routines that could be used to achieve that result. The description takes into account the current screen status. The information is to help the redisplay code select the best screen routine, not to help the screen routines optimize their own performance (such optimizations are assumed to be done anyway). As with the other private data types, the information is returned in an implementation-defined manner (i.e., you get to invent your own representation). Note that the two private data types in this procedure's definition (that for goal and the procedure itself) refer to different data types with different representations.


This section describes various considerations that go into the redisplay algorithm. In other words, these are the ways in which the algorithm gets complicated. While none of these ways are particularly difficult to implement in themselves, collectively they would clutter the redisplay algorithms presented later. Hence, you should keep these topics in mind when reviewing the algorithms.

The topics in this section are only vaguely related to each other and are in no particular order.

Status Line

In general, each buffer will have some lines of status information. In addition, there may be general editor status information. Finally, there may be lines of separators between windows. (One hopes that on "small" screens (i.e., those with less than, say, fifty lines), the numbers for these are "one," "none," and "none: use the buffer status line as a window separator" in order to devote as many lines as possible to showing the text being edited.)

In any event, this "framework" information must be retained and displayed. The user-oriented command routines and redisplay must work together to provide this infrastructure.

Here are some sample types of per-buffer status information:

Of course, any one editor implementation will only show some of this information at a time. This list is not definitive.

Here are some sample types of editor status information:

Again, any one implementation may only show some of this information, and this list is not definitive.

End of the Buffer

There are two cases that must be handled.

First, if the entire buffer fits in the window, you will run out of buffer before you run out of window. The caveat here is to ensure that this case is properly detected and that the entire buffer is shown, with the start of the buffer at the upper-left corner.

Second, if the entire buffer does not fit in the window but the end of the buffer appears, the end should be close to but not at the bottom of the window.

Those portions of the window that follow the end of the buffer can be left blank or marked in some fashion. As a rule, Emacs-type editors leave that part of the window blank.

Horizontal Scrolling

A window has a finite width. Some lines will not fit within that width. There are two popular ways of handling such a situation: horizontal scrolling and line wrap. Ideally, your editor should offer the user a choice between them. This section will describe the first.

When performing horizontal scrolling, a line longer than the window width will spill off the edge: the part of the line that does not fit thus will not be visible to the user. As the user types, the text being displayed will adjust so that the text around the point is always visible. In addition, the user should be provided with commands to move the window left or right (with a few characters of overlap). In addition, the status line should contain indicators that show whether text is currently lost off of either the left or the right sides (use separate indicators).

Line Wrap

When performing line wrap, the window never moves left or right at all. Instead, the text that would have been clipped off of the right edge of the window is wrapped to the next line. If the line is sufficiently long, it may wrap two or more times.

In this type of display, no window motion commands are required. In addition, the status indicators are also not required, although you may wish to mark the wrapped lines.

Line wrap introduces a new problem that must be handled properly: that of single lines that, when wrapped, occupy the entire window. Although rare, such lines do show up from time to time in non-text files.

When horizontal scrolling and line wrap are compared, neither comes out a clear winner and both offer valuable features, hence the assertion that your implementation should support both line wrap and horizontal scrolling.

The advantages to horizontal scrolling are that it is easy to implement, and can be processed quickly.

One of the disadvantages is that it requires a fast display. Consider the case when the user has a 160-column line displayed in an 80-column window. On the average, the window will have to be shifted twice per line of typing. Another disadvantage is that clipped text appears to have been deleted. It can be rather disconcerting to the user to have this text vanish and reappear.

One of the advantages of line wrap is that all of the text is always visible. In addition, when editing a very long line, the entire window shows the immediate context. In contrast, when editing the end of a long line when using horizontal scrolling, most or all of the remainder of the window will be blank, having been scrolled off the left edge.

The main disadvantages to line wrap are the additional complexity in the redisplay required to handle the line wrap, the very poor presentation when lines are only slightly wider than the window, and the disconcerting multi-line "shifting" that occurs when a user is inserting or deleting near the start of a wrapped line.

Word Wrap

Once you have line wrap, the next logical step is to break the line on a word boundary instead of a character boundary. You then offer "word wrap," a feature found in almost every word processor available today. Typically, a word processor will store each paragraph of text as a single line and simply perform word wrap upon it. Ruler lines are used to adjust the margins and change the type of justification.

This is a very nice feature to offer. It does have some pitfalls for the unwary implementor, however:

If you do implement word wrap, you may as well go the whole way and support flushing right, centering, and justification of text during display.


Tab characters can be handled in two ways. The first way is to not handle them at all. Instead, convert them to spaces upon entry. In this case, the redisplay code never sees those characters and hence doesn't need to deal with them.

The second - and by far the most common method - is to treat the tab as a "cursor control command" that in effect says "think of me as n blanks, where n is the number of units to the next tab stop." Thus, when the redisplay code encounters a tab, it computes n, then pretends that it is displaying n consecutive blanks (or a single blank of width n). N can be computed in one of three ways.

First, tab stops can be set every c columns (or characters). In a zero-origin numbering system, tabs set every c columns are set at columns 0, C, 2*C, 3*C, ... For example, when c is 8, tabs are in columns 0, 8, 16, 24, ... The C language expression to compute n is:

	n = c - x % c;

where x is the current column.

The second way to set tab stops is to allow them to be set at arbitrary column positions. This way is often used in ruler lines in simple word processors. In this case, you must decide on a representation such as a bit array or an array of the columns where tabs are set.

The third way to set tab stops is to allow them to be set at arbitrary positions, where the positions are measured in units such as inches, millimeters, etc. This way is most useful on graphics screens.

So far, only "traditional" tabs have been described. These might be termed "left" tabs because the left edge of the text is placed at the tab stop. Other types of tabs have become popular (again) with the advent of word processors:

Again, other variations are possible. Note that only the (traditional) left tabs can be implemented without some sort of look ahead.

Control Characters

Control characters are those that are not printing characters, a space, a newline, or a tab. ("Printing characters" means just that: if your system supported "extended" or "enhanced" character sets, then those characters may not count as control characters.) In addition, a word processor may store some information "in band." That information would be either interpreted or skipped on redisplay.

However, even in a word processor or on a system with an extended character set, there should be a way to view (and edit) a "pure" binary file. In order to view such a file, there must be a standard representation for non-printing characters.

One representation is to show such characters in octal ("\###") or hexadecimal ("\x##").

However, the most common representation -- and possibly the most useful one -- is to show such characters in caret notation (for a complete list of the caret notation, see Appendix E). The easiest way to define this notation is with a code excerpt:

	void Caret(char c)
		if (c == NL) {
			...handle newlines...
		if (c == TAB) {
			...handle tabs...
		if (c & 0x80) {
			c &= 0x7f;
		if (c < SP || c > '~') {
			c ^= '@'

When handling these multiple-character characters, your implementation must be consistent. For example, be sure that your cursor-positioning code takes the extra characters into account. Your implementation must also properly handle the case where such a character spans a line boundary. It doesn't matter which choice is made here (i.e., the choice is between splitting the character at the boundary and moving the whole character to the next line), only that your implementation handle it consistently and correctly.

Proportionally Spaced Text

Once you have tabs and control characters down, displaying text in a proportionally spaced font is not too difficult. The main variation is that you no longer assume that all printing characters are the same width. Instead, you look up the width of each one as you display it. Actually, you can even support kerning by looking up each consecutive pair of characters to decide how to handle them.

The main "gotcha" in supporting proportionally spaced text is that one character no longer always exactly overwrites another on the screen. Thus, if you change an "m" to an "i", you have to figure out what to do with the extra width. Fortunately most displays that handle proportionally spaced text (mainly graphics displays) offer a high-performance primitive to scroll a region of the screen.

Attributes, Fonts, and Scripts

The next level of generality is the support of attributes, fonts, and scripts. Attributes include such modifiers as boldface, italics, underscoring, and superscripting. Fonts include the different typefaces such as Times Roman and Helvetica. Scripts include language families such as European and Japanese.

With these, your support can be as complex as you wish. Especially when it comes to scripts, your time and energy are going to give out long before you can provide support for all languages.

However, each one is fairly simple to handle. The first step is to store the attribute, font, and script information somewhere (see Chapter 5). The second step is to interpret that information.

Breaking Out Between Lines

As was mentioned in the procedure interface definitions, the redisplay process does not have to run to completion before editing resumes. Instead, it can get to a convenient spot and check for any user input. If input has arrived, the redisplay can be aborted ("broken out of") and the input processed. It is important to keep in mind that the purpose of redisplay is to provide feedback to the user. If the user has already typed something, there is no immediate need for the feedback. Hence, redisplay can be broken out of and then restarted after the user's input has been processed.

In order to keep the amount of state information to a minimum, it may make sense to not abort instantly, but instead to finish a current chunk of redisplay (say, a line) before checking for input. At the minimum, you must keep track of how far along you had proceeded, so that you don't wind up redisplaying your redisplayed text.

The presence of between-line breakout can affect how your redisplay is done. For example, if resources are tight, it may make sense to start by redisplaying the line that the point is on, then to go on to the other lines as you have time. In that way, the information that is most important to the user is the first to get updated.

Lest there be any doubt: between-line breakout is a very important feature and should only be left out of the very simplest implementations or those implementations that can complete even the most complex redisplay in under 100 msec.

Multiple Windows

Supporting multiple windows implies that the screen is divided into sections, with each section showing a possibly different buffer or part of the buffer. There are several ways that multiple windows can be supported:

The main thing to keep in mind when implementing multiple windows is that, when two or more windows contain the same text, changes made to one should be immediately reflected in the other.

If you do support multiple windows, you can implement status and prompt lines as buffers in themselves and simply fit them in as additional windows to be displayed. In this way, you no longer have to consider them as special cases.

Redisplay Itself

The basic role of redisplay is to ensure that all changes to the sub-editor are promptly reflected on the screen. Two major approaches are used by implementors to performing redisplay.

				   | user commands |
				     /		 \
				    /		  \
				   v		   v
			--------------		-------------
			| sub-editor |		| redisplay |
			--------------		-------------

				     First Approach

The first approach is for the routines which are invoked by the user to tell the redisplay code exactly what they did (e.g., "I deleted 5 characters from here"). This approach is not a very clean one and it is prone to error, as the same information must be given twice (once to the sub-editor and once to redisplay), and hence an implementation must handle the situation where the two sets of instructions are not consistent (e.g. the application tells the sub-editor to delete a line but tells redisplay to insert a line). This is an especially important consideration because we would like to encourage novice users to write their own commands. The extra effort of getting the redisplay correct might discourage such efforts.

				   | user commands |
			--------------		-------------
			| sub-editor |<---->| redisplay |
			--------------		-------------

				     Second Approach

The second -- and preferred -- approach is to have the redisplay code communicate with the sub-editor to track the changes. This approach also has two methods of operation.

The first method (which might be called "sub-editor-driven") is to have the sub-editor calls communicate directly with the redisplay. For example, Insert_Char would make a call to display saying, "I inserted this character at this place." The second method (which might be called "redisplay-driven") is to have the redisplay operate on its own and ask the sub-editor for information.

The sub-editor-driven method appears to be simple to implement, but upon closer examination turns out to be quite complex. This complexity arises for several reasons.

First, the desirable operations for a sub-editor to offer (as shown in the sub-editor procedure interface definitions) do not match well to the available operations on displays. Hence, the redisplay code will have to perform this conversion. An example would be deleting a line. The code to perform the delete might be:

	void Delete_Line(void)
		mark_name beg;

		Find_First_In_Backward(NEWLINE);/* to start of line */
		if (Mark_Create(&beg) != OK) return;

		Find_First_In_Forward(NEWLINE);	/* to end of line */
		Point_Move(1);			/* skip over newline */

		Copy_Region(kills, beg);	/* save in kill buffer */
		Delete_Region(beg);		/* gone */


The sub-editor operation is "delete a region" and the region just happens to contain a line. Somebody has to examine the region to determine that it contains a line and that a "delete line" call to the display might be the correct one to use.

Second, the redisplay code will have to filter the sub-editor operations (and subsequent directives) that happen outside the window.

Third, every change made in the buffer does not necessarily imply a change in the display. For example, if the buffer contains the text:

	Here is a line.
	Here is a line.
	Here is a line.
	Here is a line.

and the first line is deleted, the following lines do not in fact change. That particular case may be rare, but the following happens fairly often:

	Here is line 1.
	Here is line 2.
	Here is line 3.
	Here is line 4.

In this case, the "Here is line " strings should not be redisplayed.

Fourth, the change might be no change at all. For example, the "lower case region" command applied to the text:

	Most people believe the Unicorn to be a mythical animal.

might have in its inner loop:


This would have the effect of telling redisplay 56 times that a character had changed, when in fact only two of those characters were changed. One might argue that the Replace_Char routine could check to see whether the new character was in fact different before informing redisplay, however:

			c = Get_Char();

The most telling reason for not using the sub-editor-driven method, however, is more fundamental. The sub-editor's responsibility is to handle the buffer, not redisplay. It is the redisplay's responsibility to handle the redisplay function.

The algorithms presented in the remainder of this chapter illustrate the basic algorithms. They do not handle all possible error cases, nor do they handle many of the options listed above, such as variable width characters, line wrap, between-line breakout, and others.

The Framer

The framer is that part of the redisplay code that decides what part of the buffer will appear in the window. The redisplay code maintains two marks, one at the top of the window and the other at the bottom. The algorithm is fairly simple. Here it is:

	int num_lines_window;	/* the number of lines in the window */
	int point_pct;		/* the preferred percentage */

	void Framer(void)
		mark_name saved;
		location new_start_loc;
		int cnt;

			/* remember where we started */
		if (Mark_Create(&saved) != OK) {
			Fatal("can't create mark for redisplay");

			/* count at most one window's worth of lines */
		for (cnt = 0; cnt < num_lines_window; cnt++) {

				/* stop at the start of the window */
			if (Is_Point_At_Mark(top_of_window)) break;

				/* stop at the start of the buffer */
			if (Compare_Locations(Buffer_Start, Point_Get) >= 0)

				/* record where a fresh screen would start,
				just in case we need it */
			if (cnt == point_pct * num_lines_window)
				new_start_loc = Point_Get();


			/* has the window moved? */
		if (cnt >= num_lines_window)
			Mark_Set(top_of_screen, new_start_loc);


In essence, the algorithm followed by this routine is: "so long as the point would still wind up in the window, leave the start of window unchanged. If the point would not wind up in the window, place it at the preferred percentage."

This version of the algorithm assumes that a buffer line will always occupy exactly one window line and that all buffer lines are the same height.

The Basic Algorithm

The basic redisplay algorithm is as follows:

	int num_lines_window;	/* the number of lines in the window */
	int num_chars_window;	/* the number of characters in the
				window (its width) */
	char window[MAX_ROWS][MAX_COLS];	/* window contents */	

	void Redisplay(void)
		mark_name saved;
		int row;
		int col;
		int i;
		int point_row;
		int point_col;
		char c;

			/* remember where we started */
		if (Mark_Create(&saved) != OK) {
			Fatal("can't create mark for redisplay");


			/* loop over the whole window */
		for (row = 0; row < num_lines_window; row++) {
			for (col = 1; col < num_chars_window; col++) {

/* save the coordinates of the point so that we can put the cursor
there later */

				if (Is_Point_At_Mark(saved)) {
					point_row = row;
					point_col = col;

				c = Get_Char();
				if (c == NL) {	/* at a newline? */

/* check whether the rest of the window line is blank.  if it is not, clear it */
					for (i = col; i < num_chars_window; i++) {
						if (window[row][i] != SP) {
							Set_Cursor(row, i);
							memset(&window[row][i], SP,
								num_chars_window - i);

/* no newline, so has there been a change in the sub-editor? */

			else if (window[row][col] != c) {
				Set_Cursor(row, col);
				window[row][col] = c;

/* clean up */

		Set_Cursor(pointrow, pointcol);


The preceeding code shows your basic, garden variety redisplay algorithm. It will work on any screen that supports cursor positioning (the CLEOL call can be simulated by sending Space characters). It will work quite well on communications channels running at 4800 bps or over. Its only memory requirements are an array large enough to hold the window (typically 1920 characters). There are no special redisplay "hooks" in the sub-editor management code.

This algorithm is sufficient (and nearly optimal) in those cases where CPU and memory are plentiful and the screen does not perform insert/delete line or character operations. If memory is tight, the algorithm can be modified to only retain a complete copy of the current line. If you must be prepared to emulate the CLEOL operation, it may be worthwhile to record the last non-blank column in each screen line. Doing so minimizes the number of Space characters that must be sent.

Sub-Editor Interaction

The basic algorithm can be sped up tremendously if some redisplay-specific hooks are placed into the sub-editor. There are a number of different ways that the hooks can be introduced. All of these methods track the changes made to the buffer in one way or another.

The first way is to keep a separate modification flag that tells whether any changes were made to the buffer since the last redisplay. If no changes were made, then redisplay will consist of either a simple cursor motion or a complete screen regeneration.

The seond way, and much more useful, is to keep the modification flag on a per-window-line basis. A general interface to accomplish this that works with all sub-editor implementation schemes is to define a third type of mark, called a window mark. This mark has a flag associated with it. There is one window mark for each line in the window. Just after a redisplay has been completed, all the flags for all window marks are clear. Each time the sub-editor changes any of the buffers' contents, it sets the flag on the window mark that is located before and closest to the change. The redisplay code can examine the flags. Only window lines that have their corresponding window marks set need to be examined closely during redisplay.

Note that window marks need not be located at the start of a buffer line. If lines are being wrapped, one will be at each wrap point. If horizontal scrolling is being performed, one may be at the start of the buffer line and another at the right edge of the window. In this way, changes made to the right of the window won't cause the redisplay code to examine unchanged text.

This interaction is easy to define and implement in the sub-editor. It is inexpensive to implement as the marks have to be examined for updating anyway. It is also highly effective at reducing CPU overhead, as most commands change only a single line. And, although redisplay has to examine the flags for every line, most of the time only one or two will show changes..

A third way is to associate a unique identifier with each window mark instead of a flag. This identifier would be changed by the sub-editor whenever the associated text changes (i.e., instead of setting a flag it changes the unique identifier). Typically, the identifier will be a 32-bit integer. Whenever an identifier is required, the current value of the integer is used and the integer is incremented.

The only problem that can arise with using unique identifiers is if a unique identifier is not in fact unique. This problem can arise if all 2^32 unique identifiers are consumed before all lines in the window have changed.

Some sub-editors that use the linked-line scheme use the addresses of line structures as the unique identifiers. While doing so is space efficient, the sub-editor must ensure that if a line is freed, the address is not re-used until all windows have been completely redisplayed.

Finaly, there is one more flag that can help redisplay a great deal. This flag is only useful if the point is located at the end of a buffer line. The flag would say whether any buffer modification other than "insert one or more characters" has been performed. If the flag says not, all that redisplay needs to do is to output those characters. As this situation is very common, it can save a significant amount of computation.

The Advanced Algorithm

The advanced-redisplay algorithm has two improvements over the basic algorithm. First, it provides a way of efficiently taking advantage of the insert/delete line and character functions which are supplied with many screens. Second, it provides a low CPU overhead way of performing a redisplay on basic displays.

The basic idea used by this algorithm is to assign a unique identifier to each window line. (See the preceding section.) When the redisplay encounters a modified line (the unique identifiers don't match), it performs a pattern match on the unique identifiers for the remainder of the window. It then uses the information derived from that match to determine the best sequence of insert/delete line commands to issue to the screen.

In more detail, this algorithm loops over the window lines, checking each saved unique identifier against the current identifier returned by the sub-editor. If they match, no work needs to be done and the algorithm proceeds to the next line. If they don't match, it can be for one of three reasons.

The first reason could be that an additional line or lines were inserted between the two window lines. This condition is detected by comparing the window-line unique identifier against the rest of the unique identifiers returned by the sub-editor and finding a match. (Remember that the window-line unique identifiers are the unique identifiers returned by the sub-editor one redisplay iteration ago.) The insertion case is where we once had lines:


and now have:


We determine how many lines are in "C" (because we know how far down we had to go to find a match) and tell the screen to insert that many lines. (If there is information after this window on the screen, you will first have to delete that many lines from the end of the window.)

The second reason could be that a line (or lines) was deleted. This condition is detected by comparing the unique identifier returned by the sub-editor for the next line against the unique identifiers of the rest of the window lines and finding a match. The deletion case is where we once had lines:


and now have:


We determine how many lines are in "B" (because we know how far down we had to go to find a match) and tell the screen to delete that many lines. (If there is information after this window on the screen, you will eventually have to insert that many lines at the end of the window.)

The third reason could be that the line was changed. This condition is detected by comparing the unique identifiers of the following window lines against the unique identifiers returned by the sub-editor. This case is either where we once had lines:


and now have:




In other words, neither the insertion condition nor deletion condition was met. Knowing now that a line has been changed, the next step is to determine exactly how the line has changed.

The algorithm starts by comparing the buffer line against the window line and determining how many leading characters are in common. (If the whole line is common, no changes need to be made to the screen and the algorithm stops.) For example, if the window line is:


and the buffer line is:


the two have three characters in common from the start.

The next step is to repeat the comparison, but work backwards starting from the end. The example strings have two characters in common from the end.

The third step is to compare the line lengths. If the two lines are the same length, only the changed part in the middle needs to be updated on the screen. In the example strings, the lengths are the same (six6). This optimization can be done even on a basic display.

If the two lines are not the same length (for example, the buffer line is "abcxyzef"), the characters in the window line that are replaced by characters in the buffer line can be rewritten (in the example, the "x" replaces the "d"), then the requisite number of characters can either be inserted or deleted and the remainder of the changes written (insert two characters, "yz"). If there is no common text at the end of the line and the buffer line is shorter than the window line, a CLEOL call can be used instead of deleting characters.

Line wrap can pose a problem. The window and buffer lines may have no end text in common, and yet an insert or delete character operation might be the appropriate one. For example, consider the case where the window width is six characters, the window line is "abcdef", and the buffer line is "abcxdef". Here, the buffer line will ultimately become two window lines, "abcxde" and "f". This case is detected by having no common portion at the end and noticing that the line wraps. A more complicated matching process can detect the situation and appropriate action can be taken.

This entire section considered only the (admittedly very common) case where line and character insertions and deletions were only made in one place. It is very reasonable and appropriate to use more general pattern-matching techniques to properly optimize multiple insertions and deletions (Miller 1987).

Redisplay for Memory-Mapped Displays

Redisplay for memory-mapped displays boils down to one of three cases. Each case is relatively simple.

First is the case where both reading from and writing to the screen causes flicker. The solution is to use the basic redisplay algorithm.

Second is the case where reading does not cause flicker but writing does. The solution is to use the basic redisplay scheme, but change it to use the actual window memory for storing the window array.

In the third case, neither reading nor writing causes flicker. On each redisplay cycle, merely copy the buffer text into window memory, not forgetting to process new lines, etc., as needed.

Questions to Probe Your Understanding

Define a set of editor procedures to handle vertical windows. (Easy) Extend that set to handle overlapping windows. (Medium)

Implement the procedures that you just defined. (Hard)

Define a representations for the private data types mentioned here (function keys, attributes, command times). (one is Easy, all together are Medium)

Identify the places where left-to-right, top-to-bottom biases are built into the interface definitions. English and European languages have this bias. (Easy)

Rework the interface definitions to remove this bias and to be able to handle all eight (yes, eight) combinations of directions. (Medium)

Outlining is popular these days. "Outlining" is the ability to selectively skip over parts of the text during redisplay. For example, one level might only display the chapter and section titles. Another level might include all titles and the first sentence of each paragraph. Identify how adding outlining would affect redisplay. (Medium)

Identify how the editor's redisplay algorithm changes when it makes use of the UNIX curses library. (Hard)

How do the presence of ligatures and contextual forms used by non-Roman languages affect cursor motion? (Medium)

What modifications to the redisplay algorithm are required to handle ligatures or contextual forms used by non-Roman languages? (Hard)

Copyright 1999 by Craig A. Finseth.

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