INTERFACE MTextUnit;

IMPORT MText, ISOChar;

TYPE
  T = MText.T;

  (* This module imposes structure on character sequences in an
     "MText.T".  There are three different structures supported:
     {\it runs}, {\it lines}, and {\it paragraphs}.  Each
     structure defines a {\it unit}, in such a way that the
     entire mtext can be viewed as a sequence of non-overlapping
     units, possibly separated by characters that belong to no
     unit.

     A {\it run} is defined by a set of characters, typically
     "ISOChar.AlphaNumerics".  A unit then corresponds to a ``word'',
     and the mtext is seen as a sequence of ``words'' separated by
     ``non-word'' characters.

     A {\it line} is defined as a set of characters delimited by
     newlines.  A unit corresponds to a single line.  The mtext
     is viewed as a sequence of lines with 0-length separators.

     A {\it paragraph} is a maximal sequence of non-blank lines.

     Given an index "n" into the text, we define the {\it extent}
     "e" for runs, lines, and paragraphs as a record containing
     the left and right boundaries of the unit surrounding the
     index, plus a boolean value indicating whether the character
     at position "n" is inside a unit or between units.  Normally,
     "e.left <= index < e.right".  The only exception is when
     "index" is greater than or equal to the length of the text,
     in which case "e.left = e.right = length" and "e.inside
     = FALSE".

     There are also utilities for handling blank lines and for
     finding the leading and trailing blanks on a line.

     If you pass an index that's less than 0, it is treated as 0;
     if you pass an index that's greater than the length of the
     text, it is treated as the length of the text.
*)

  (* {\it Note for future work:  There is nothing in the nature
     of runs, lines, and paragraphs that is specific to
     "MText.T".  It would be at least as useful to provide this
     same functionality for "TEXT", "ARRAY" "OF CHAR", and
     seekable readers.} *)

CONST
  WordRun = ISOChar.AlphaNumerics;
  BlankRun = ISOChar.Spaces; (* {' ', '\n', '\t', '\r', '\f'} *)
  NonBlankRun = ISOChar.All - BlankRun;

TYPE
  Extent = RECORD
             left, right: INTEGER;
             inside     : BOOLEAN
           END;

PROCEDURE RunExtent (         t            : T;
                              index        : INTEGER;
                     READONLY includedChars := WordRun):
  Extent;

PROCEDURE StartOfRun (t    : T;
                      index: INTEGER;
                      READONLY includedChars := WordRun):
  INTEGER;

Return the largest i <= index for which IsStartOfRun(t, i, includedChars) is TRUE. Return -1 if there is no such i.

PROCEDURE IsStartOfRun (t    : T;
                        index: INTEGER;
                        READONLY includedChars := WordRun):
  BOOLEAN;

Equivalent to

      WITH e = RunExtent(t, index, includedChars) DO
       RETURN e.inside AND e.left = index END
   

PROCEDURE EndOfRun (t    : T;
                    index: INTEGER;
                    READONLY includedChars := WordRun):
  INTEGER;

Return the smallest i >= index for which IsEndOfRun(t, i, includedChars) is TRUE. Return -1 if there is no such i.

PROCEDURE IsEndOfRun (t    : T;
                      index: INTEGER;
                      READONLY includedChars := WordRun):
  BOOLEAN;

Equivalent to

      WITH e = RunExtent(t, index, includedChars) DO
        RETURN NOT e.inside AND e.left = index END
    

or

      IsStartOfRun(t, index, ISOChar.All - includedChars)
   

TYPE LineOption =
  {ExcludeBlanks, IncludeBlanks, IncludeNewline};

If the right option is ExcludeBlanks, then the subsequence will not include any trailing blanks. If the right option is IncludeBlanks, then it will. If the right option is IncludeNewline, then it will include both the trailing blanks and the following newline.

The default left option is IncludeBlanks; the default right option is IncludeNewline. This is consistent with the view of the text as a sequence of lines, with no intermediate ``gaps.''

PROCEDURE LineExtent (t: T; index: INTEGER): Extent;

Compute the extent of the line surrounding index. If a newline immediately precedes index, it uses the line that begins at index, not the one that ends there. LineExtent(t, index).inside is always TRUE.

TYPE
  LineRec = RECORD
              left       : INTEGER;  (* start of line *)
              leftMargin : INTEGER;  (* first non-blank *)
              rightMargin: INTEGER;  (* 1 + last non-blank *)
              rightEnd   : INTEGER;  (* final newline *)
              right      : INTEGER;  (* rightEnd + 1 *)
            END;

A LineRec contains the positions within a line. Normally,

      left <= leftMargin <= rightMargin <= rightEnd

and right = rightEnd + 1, i.e., the left of the next line, but there are exceptions. On a line consisting entirely of blanks (at least one),

      left < leftMargin = rightMargin = rightEnd

On the last line of the file (following the last newline), right = rightEnd.

If index = length(t) and there is a newline immediately preceding index, then all 5 values will be equal to index.

PROCEDURE LineInfo (t: T; index: INTEGER): LineRec;

PROCEDURE LineFacts (              t          : T;
                                   index      : INTEGER;
                     VAR (* out *) left       : INTEGER;
                     VAR (* out *) leftMargin : INTEGER;
                     VAR (* out *) rightMargin: INTEGER;
                     VAR (* out *) rightEnd   : INTEGER;
                     VAR (* out *) right      : INTEGER  );

PROCEDURE StartOfLine (t    : T;
                       index: INTEGER;
                       leftOption := LineOption.IncludeBlanks):
  INTEGER;

IF leftOption = ExcludeBlanks, then return LineInfo(t, index).leftMargin. Otherwise return LineInfo(t, index).left.

PROCEDURE IsStartOfLine (t    : T;
                         index: INTEGER;
                         leftOption := LineOption.IncludeBlanks):
  BOOLEAN;

Return index = StartOfLine(t, index, leftOption)

PROCEDURE EndOfLine (t    : T;
                     index: INTEGER;
                     rightOption := LineOption.IncludeNewline):
  INTEGER;

Return the rightMargin, rightEnd, or right field of LineInfo(t, index), depending on whether rightOption is ExcludeBlanks, IncludeBlanks, or IncludeNewline, respectively.

PROCEDURE IsEndOfLine (t    : T;
                       index: INTEGER;
                       rightOption := LineOption.IncludeNewline):
  BOOLEAN;

Return index = EndOfLine (t, index, rightOption)

PROCEDURE IsBlankLine (t: T; index: INTEGER): BOOLEAN;

Return TRUE if index is located on a line that consists entirely of blanks, tabs, and form-feeds, including empty lines.

PROCEDURE BlankLinesExtent (t: T; ndex: INTEGER): Extent;

Computes the extent of maximal sequence of blank lines surrounding index.

A {\it paragraph} is a maximal sequence of non-blank lines, in the sense of LineExtent where the left option is IncludeBlanks and the right option is IncludeNewline.

PROCEDURE ParagraphExtent (t: T; index: INTEGER): Extent;

PROCEDURE StartOfParagraph (t: T; index: INTEGER): INTEGER;

Return the largest i <= index for which IsStartOfParagraph(t, i) is TRUE. Return -1 if there is no such i.

PROCEDURE IsStartOfParagraph (t: T; index: INTEGER): BOOLEAN;

Equivalent to

      WITH e = ParagraphExtent(t, index) DO
       RETURN e.inside AND e.left = index END
   

PROCEDURE EndOfParagraph (t: T; index: INTEGER): INTEGER;

Return the smallest i >= index for which IsEndOfParagraph(t, i) is TRUE. Return -1 if there is no such i.

PROCEDURE IsEndOfParagraph (t: T; index: INTEGER): BOOLEAN;

Equivalent to

      WITH e = ParagraphExtent(t, index) DO
       RETURN NOT e.inside AND e.left = index END
   

END MTextUnit.