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FUNNELWEB MANUAL WEB PAGE
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Copyright (c) Ross N. Williams 1992,1999. All rights reserved.
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Permission is granted to redistribute and use this manual in
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any medium, with or without modification, provided that all
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notices (including, without limitation, the copyright
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modifications since the manual's original release by Ross N.
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Williams (www.ross.net) consist of translations or other
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content. THIS MANUAL IS PROVIDED "AS IS" AND WITHOUT ANY
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<TITLE>3.5 Implementing Text Indentation</TITLE>
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<A HREF="index.html"><FONT COLOR="#FFFFFF"><B>Developer</B></FONT></A><BR>
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<A HREF="compile.html"><FONT COLOR="#FFFFFF">1 Compile</FONT></A><BR>
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<A HREF="design.html"><FONT COLOR="#FFFFFF">2 Design</FONT></A><BR>
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<A HREF="implement.html"><FONT COLOR="#FFFFFF">3 Implement</FONT></A><BR>
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<A HREF="modify.html"><FONT COLOR="#FFFFFF">4 Modify</FONT></A><BR>
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<P><FONT SIZE="5">3.5 Implementing Text Indentation</FONT><BR>
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<P>At one point during the development of FunnelWeb, text
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indentation was fully implemented. However, it was
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subsequently removed because it was considered a dangerous
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feature. This section records the way in which text
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indentation was implemented so that if the feature ever has
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to be put back, this technique can be used again.
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<P>1. Create a new field in the <SAMP>sc_t</SAMP> record call
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<SAMP>sc_postn</SAMP>.
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char *sc_postn; /* Pointer in the range [sc_first,sc_last+1]. */
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/* It is the minimum possible value of sc_postn for */
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/* which EOL does not appear in *sc_postn..*sc_last. */
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/* i.e. Points to the byte following the first EOL in */
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/* the scrap or sc_first if EOL does not appear. */
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<P>2. Modify the scanner so that it generates this field.
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Sendtext should be modified so that
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it accepts an argument for the <SAMP>p_postn</SAMP> value.
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LOCAL void sendtext P_((p_ps_t,char *,char *,char *,bool));
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LOCAL void sendtext(p_tkps,p_first,p_last,p_postn,is_white)
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/* Appends a text token to the end of the token list. */
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/* IN: p_ps is a pointer to a position structure giving the position of the */
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/* first character of the token. */
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/* IN: p_first and p_last point to the first and last byte of the text scrap. */
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/* IN: p_postn has the same definition as sc_postn (see fwdata.h). */
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/* IN: is_white should be set to TRUE iff scrap is entirely whitespace. */
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/* Empty text scraps should never be generated. */
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assert(p_first<=p_last,"sendtext: Text scrap bounds are bad.");
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/* If ch=EOL then we should be scanning more text, not shipping it! */
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assert(ch!=EOL,"senttext: Shipping text while still more to scan.");
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/* Check that p_postn is in range. See definition in fwdata.h. */
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assert(p_first<=p_postn && p_postn<=p_last+1,
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"sendtext: p_postn is out of range.");
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/* Debug: Check the p_postn field using a brute force check. */
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for (i=p_first;i<=p_last;i++)
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assert(j==p_postn,"sendtext: sc_postn field is incorrect.");
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/* Load the text token. */
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token.tk_kind = TK_TEXT;
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ASSIGN(token.tk_ps,*p_tkps);
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token.tk_sc.sc_first = p_first;
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token.tk_sc.sc_last = p_last;
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token.tk_sc.sc_postn = p_postn;
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token.tk_white = is_white;
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ls_add(token_list,PV &token);
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<P>Then all the calls to sendtext have to be changed:
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/* @ instructs FunnelWeb to replace the special construct with the */
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/* special character. Luckily one appears just before the @ !! */
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/* Note: FALSE is OK because space is not a legal specialch. */
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/* Note: Setting parameter p_postn to p_ch-1 is OK as EOL is not a */
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/* legal specialch. */
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sendtext(ps_spec,p_ch-1,p_ch-1,p_ch-1,FALSE);
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/* + instructs FunnelWeb to insert an EOL. We can't look to the end of */
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/* the previous line to find an EOL as this might be the first line. */
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/* Running ahead to the end of the line is expensive, and having the */
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/* liner mini-package maintain a variable for it would be extra */
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/* housekeeping. Instead of all this, we just point to a static. */
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{CONST static char stateol = EOL;
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sendtext(&ps_spec,&stateol,&stateol,(&stateol)+1,TRUE);}
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/* If we hit something that ends a text token */
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/* then we can transmit a white text token. */
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if (ch==specialch || ch==EOFCH)
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{sendtext(&ps_start,p_first,p_ch-1,MAX(p_sol,p_first),TRUE); return;}
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/* Otherwise we have some more (non-white) text to scan. */
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/* We can then send a non-white text token. */
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while (ch!=specialch && ch!=EOFCH)
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sendtext(&ps_start,p_first,p_ch-1,MAX(p_sol,p_first),FALSE);
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The dump code needs to be changed too!
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assert(token->tk_sc.sc_first !=NULL,"dm_tkls: NULL ptr1.");
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assert(token->tk_sc.sc_last !=NULL,"dm_tkls: NULL ptr2.");
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for (i=token->tk_sc.sc_first; i<=token->tk_sc.sc_last; i++)
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if (i==token->tk_sc.sc_postn)
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wf_str(p_wf,"<postn>");
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dm_byte(p_wf,*((ubyte_ *) i));
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if (i==token->tk_sc.sc_postn)
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wf_str(p_wf,"<postn>");
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<P>3. Over in the Tangle module, create a massive array of
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pointers to scraps to be used as a stack. Maintain pointers
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into the stack called <SAMP>current</SAMP> and <I>base</I>
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(similar to the blank indentation variables). Implement the
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<LI> To write out a scrap, scan it byte by byte. Output
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each byte. When you hit an EOL, pop the stack back to
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<SAMP>base</SAMP>. Then write out an EOL followed by the stack
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contents but writing each scrap only from <SAMP>postn</SAMP> to
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end end of each scrap. When you have finished the new scrap,
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push it on the stack.
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<LI> When you hit a new macro to expand, save
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<SAMP>base</SAMP>. Restore it later.
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<P>The <SAMP>postn</SAMP> field solves the big problem of how
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to cope with something like this:
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falls mainly @<on the plain@>
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<P>The trouble is that we want to text indent the lines in
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<SAMP>@<on the plain@></SAMP> with just "<TT>falls
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mainly </TT>". However, this string is only part of a scrap. The
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solution is to get the scanner to record, in the
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<SAMP>postn</SAMP> field of each scrap, the position of the
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first byte with a EOL-free run to the end of the scrap.
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<P>This scheme is very efficient because all we are doing
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is pushing and popping pointers to scraps on a stack array.
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The main disadvantage is that the array must necessarily be
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finite and would impose a limit on the depth of indentation
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