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       cachechars, agefont, loadchar, Subfont, Fontchar, Font  - font utilities

       #include <u.h>
       #include <libc.h>
       #include <libg.h>

       int  cachechars(Font *f, char **s, ushort *c, int n, int *widp)

       int  loadchar(Font *f, Rune r, Cacheinfo *c, int h, int noclr)

       void agefont(Font *f)

       A  Font	may  contain  too many characters to hold in memory simultaneously.  The graphics
       library and bitblt device (see bit(3)) cooperate to solve this problem  by  maintaining	a
       cache  of  recently  used  character  images.  The details of this cooperation need not be
       known by most programs: binit and its associated  font  variable,  rdfontfile,  charwidth,
       string, and ffree are sufficient for most purposes.  The routines described below are used
       internally by the graphics library to maintain the font cache.

       A Subfont is a set of images for a contiguous range of characters, stored as a single bit-
       map  with the characters placed side-by-side on a common baseline.  It is described by the
       following data structures.

	      struct Fontchar {
		    ushort   x;        /* left edge of bits */
		    uchar    top;      /* first non-zero scan-line */
		    uchar    bottom;   /* last non-zero scan-line */
		    char     left;     /* offset of baseline */
		    uchar    width;    /* width of baseline */
	      } Fontchar;

	      struct Subfont {
		    short    n;        /* number of chars in subfont */
		    uchar    height;   /* height of bitmap */
		    char     ascent;   /* top of bitmap to baseline */
		    Fontchar *info;    /* n+1 Fontchars */
		    int      id;       /* id as known in /dev/bitblt */
	      } Subfont;

       The bitmap fills the rectangle (0, 0, w, height), where w is the  sum  of  the  horizontal
       extents (of non-zero pixels) for all characters.  The pixels to be displayed for character
       c are in the rectangle (i->x, i->top, (i+1)->x, i->bottom) where i  is  &subfont->info[c].
       When a character is displayed at Point p in a bitmap, the character rectangle is placed at
       (p.x+i->left, p.y) and the next character of the string	is  displayed  at  (p.x+i->width,
       p.y).   The  baseline  of  the characters is rows down from the top of the subfont bitmap.
       The array has n+1 elements, one each for characters 0 to n-1 plus an additional	entry  so
       the  size of the last character can be calculated.  Thus the width, w, of the Bitmap asso-
       ciated with a Subfont s is s->info[s->n].x.

       A Font consists of an overall height and ascent and a collection of subfonts together with
       the  ranges  of	runes  (see utf(6)) they represent.  Fonts are described by the following

	      struct Cachefont {
		    Rune      min;	/* value of 0th char in subfont */
		    Rune      max;	/* value+1 of last char in subfont */
		    int       offset;	/* posn in subfont of char at min */
		    int       abs;	/* name has been made absolute */
		    char      *name;
	      } Cachefont;

	      struct Cacheinfo {
		    Rune      value;	/* of char at this slot in cache */
		    ushort    age;
		    ulong     xright;	/* right edge of bits */
	      } Cacheinfo;

	      struct Cachesubf {
		    ulong     age;	/* for replacement */
		    Cachefont *cf;	/* font info that owns us */
		    Subfont   *f;	/* attached subfont */
	      } Cachesubf;

	      struct Font {
		    char      *name;
		    uchar     height;	/* max ht of bitmap;interline space*/
		    char      ascent;	/* top of bitmap to baseline */
		    char      width;	/* widest so far; used in caching */
		    char      ldepth;	/* of images */
		    short     id;	/* of font */
		    short     nsub;	/* number of subfonts */
		    ulong     age;	/* increasing counter; for LRU */
		    int       ncache;	/* size of cache */
		    int       nsubf;	/* size of subfont list */
		    Cacheinfo *cache;
		    Cachesubf *subf;
		    Cachefont **sub;	/* as read from file */
	      } Font;

       The and fields of Font are described in graphics(2).  is that of the  cache,  set  by  the
       argument  to  rdfontfile.   contains  pointers  to Cachefonts.  A Cachefont connects runes
       through inclusive, to the subfont with file name it corresponds to  a  line  of	the  file
       describing the font.

       The  characters	are taken from the subfont starting at character number (usually zero) in
       the subfont, permitting selection of parts of subfonts.	Thus the  image  for  rune  r  is
       found in position r-min+offset of the subfont.

       For  each  font,  the library, with support from the graphics server, maintains a cache of
       subfonts and a cache of recently used character images.	The subf  and  cache  fields  are
       used  by  the library to maintain these caches.	The of a font is the maximum of the hori-
       zontal extents of the characters in the cache.  String draws a string by loading the cache
       and  emitting  a  sequence of cache indices to draw.  Cachechars guarantees the images for
       the characters pointed to by *s are in the cache of f.  It calls loadchar to  put  missing
       characters into the cache.  Cachechars translates the character string into a set of cache
       indices which it loads into the array c, up to a maximum of n indices or the length of the
       string.	 Cachechars returns in c the number of cache indices emitted, updates *s to point
       to the next character to be processed, and sets *widp to the total width of the characters
       processed.   Cachechars may return before the end of the string if it cannot proceed with-
       out destroying active data in the caches.  It can return zero if  it  is  unable  to  make
       progress because it is unable to resize the caches.

       Loadchar  loads a character image into the character cache.  First, if necessary, it loads
       the subfont containing the character.  Then it tells the graphics server to copy the char-
       acter  into  position  h  in the character cache.  If the current font is smaller than the
       horizontal extent of the character being loaded, loadfont clears the cache and  resets  it
       to  accept  characters  with  the bigger width, unless noclr is set, in which case it just
       returns -1.  If the character does not exist in the font at all, loadfont returns 0; if it
       is  unable to load the character without destroying cached information, it returns -1.  It
       returns 1 to indicate success.

       The fields record when subfonts and characters have been  used.	 The  font  is	increased
       every  time  the  font  is used (agefont does this).  A character or subfont is set to the
       font age.  Thus, characters or subfonts with  small  ages  are  the  best  candidates  for
       replacement when the cache is full.


       graphics(2), balloc(2), bitblt(2), subfalloc(2), bitmap(6), font(6)

       All of the functions use the graphics error function (see graphics(2)).

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