Copyright (C) 1992, Digital Equipment Corporation 
 All rights reserved. 
 See the file COPYRIGHT for a full description. 
 Last modified on Mon Nov 23 12:29:27 PST 1992 by meehan 
      modified on Tue Jun 30 19:31:32 1992 by mhb 
 modified on Tue Jun 16 13:16:28 PDT 1992 by muller 
 modified on Fri Oct 7 14:05:01 1988 by chan 
 modified on Tue Jan 27 11:34:42 1987 by mbrown 

An MText is a mutable sequence of characters whose internal representation is a balanced binary tree. An MText is an appropriate abstraction for use in implementing a text editor, and perhaps other applications that perform lots of text modifications. More documentation is at the end of this interface.

MText is related to text and readers. You should consult Text.i3 and Rd.i3 for information on these abstractions.

This interface is monitored. Readers are also synchronized with updates to the MText so that a reader fails when it tries to refill its buffer and discovers that the MText has been changed.

If a procedure raises an exception, the mtext is left in an undefined state. However, the client should be able to call Close on it.

MText procedures use the ``begin/end/length'' semantics defined by the Text interface: if begin or begin + length or end exceed Length(m), then Length(m) will be used. (A previous implementation raised checked runtime errors in such cases.)


  T <: REFANY;
  Index = CARDINAL;             (* obsolete, but keep for
                                   compatibility *)
\subsection {Creating MTexts}

PROCEDURE New (t := ""; bufMax: CARDINAL := 256): T;
Returns a new MText containing the characters of t. bufMax specifies the size of the mutable buffer used by Replace when it cannot perform editing within the buffer. The buffer is useful both during straight typing by a user and during insertions by a program (as in a typescript).

PROCEDURE ChangeBufMax (m: T; bufMax: CARDINAL);

PROCEDURE Close (m: T);

      = MText.Replace(m, 0, MText.Length(m), "")
 \subsection {Examining MTexts and Extracting Characters} 

Note that readers on MText are available (see the MTextRd interface). It is often more appropriate to create a reader than to create a text.

Returns the number of characters in m.

PROCEDURE GetChar (m: T; index: CARDINAL): CHAR;
Returns the character at index in the text. A checked error if index >= Length(m). Warning: this is an expensive operation. If it is going to be done several times, it would be better to use an MTextRd.T.

PROCEDURE GetText (m    : T;
                   begin: CARDINAL := 0;
                   end  : CARDINAL := LAST (CARDINAL)): TEXT;
Returns a sub-sequence of m. The result will be empty if begin >= Length(m); otherwise the range of indexes of the subsequence is [begin .. MIN (end, Length (m)) -1].
 \subsection {Modifying MTexts} 

See ``Efficiency considerations'' below for a discussion of the performance implications of the various ways of modifying mtexts. To delete characters from an mtext, use Replace with an empty string as the value of newtext. To insert characters, use any of the procedures below with begin = end.

PROCEDURE Replace (m: T; begin, end: CARDINAL; newtext: TEXT);
Delete characters in the range [begin .. end-1], and insert the characters of newtext at begin.

PROCEDURE ReplaceChars (m: T; begin, end: CARDINAL; READONLY str: ARRAY OF CHAR);
= Replace (m, begin, end, Text.FromChars (str))

PROCEDURE ReplaceFile (m         : T;
                       begin, end: CARDINAL;
                       rd        : Rd.T;
                       start     : CARDINAL   := 0;
                       numChars  : CARDINAL   := LAST (CARDINAL));
Delete characters in the range [begin .. end-1], then insert the characters [start .. start+numChars-1] from rd at begin.

While this procedure is intended to be used primarily for file-readers, any seekable, non-intermittent reader is acceptable.

Characters from rd are read only as needed, not necessarily at the time ReplaceFile is called. The client should therefore not call Rd.Close(rd) or make any changes to the underlying text.

END MText.
Introduction to Mutable Text (MText)

A Mutable Text or MText is a REF representing a mutable sequence of characters. The characters of an MText are indexed starting from 0, like those of a Text. MText is an appropriate abstraction for use in implementing a text editor, and perhaps other applications that perform lots of text modifications.

MText is well integrated with the more basic Text abstraction. It is easy to create an MText from a Text, and to extract Texts from an MText. It is also easy to create an MText from a file. The performance of these operations is generally good.

Efficiency considerations

The implementation of MText is a 'piece table'. A piece table is (logically) a linear list of pieces, where each piece is some representation of a character sequence. The piece table represents the concatenation of the character sequences.

MText supports three types of pieces: subsequence of characters in an immutable Text, subsequence of characters in a reader (e.g., a readonly file), and a mutable buffer. The mutable buffer is supported to avoid creating a huge number of pieces when performing individual character-insertions during typein.

The number of pieces in an MText may increase linearly with the number of editing operations applied to it. So MText uses a balanced binary tree to represent the list of pieces. This means that reading and editing the MText can still be quite fast when the number of pieces is large. The time to perform GetText(mtext,begin,end) is proportional to (log n + r) where n is the number of pieces in the MText and r is the number of pieces contributing characters to the result. The fact that this is linear in r is not wonderful. <<Eventually we should give the constant factors.>>

When a reader is used as a text source, the MText implementation reads characters into virtual memory only when those characters are referenced. This means that creating an MText from a reader takes constant time; it also means that if the reader's target is changed while the MText still includes the reader, the contents of the MText become undefined.