CACHECHARS(3G)                                                  CACHECHARS(3G)

       cachechars, Subfont, Cachesubf, Fontchar, Font  - font utilities

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

       int  cachechars(Font *f, char **s, XChar2b *c, int n, int *widp, ushort *fp)

       A Font may contain too many characters to hold in memory
       simultaneously.  The graphics library and X server 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:
       xtbinit and its associated *p9font X resource, 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 may be considered to be a set of images for a contiguous
       range of characters, stored as a single bitmap with the characters
       placed side-by-side on a common baseline.  It is described by the
       following data structures.

              struct Fontchar {
                    ushort   cwidth;   /* width of glyph */
                    uchar    top;      /* first non-zero scan-line */
                    uchar    bottom;   /* last non-zero scan-line */
                    char     left;     /* offset of baseline */
                    uchar    width;    /* advance to next char's origin */
              } Fontchar;

              struct Subfont {
                    short    minrow;   /* first character row in font (for X subfonts) */
                    short    mincol;   /* first character col in font (for X subfonts) */
                    short    minchar;  /* first char code in font (for X subfonts) */
                    short    maxchar;  /* last char code in font (for X subfonts) */
                    short    width;    /* number of chars in row (for X subfonts) */
                    short    n;        /* number of chars in subfont */
                    unsigned char height;/* height of bitmap */
                    char     ascent;   /* top of bitmap to baseline */
                    Fontchar *info;    /* n+1 character descriptors */
                    int      id;       /* of font */
              } Subfont;

       The actual bitmap for each character is stored on the server but the
       metrics associated with character c are cached locally in
       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+(i+1)->left,
       p.y).  The baseline of the characters is rows down from the top of the
       subfont bitmap.  Each Fontchar has two widths associated with it: width
       is the width of the rectangular bitmap to contain the character; cwidth
       is the true width of the character, that is, the number of pixels
       between the leftmost and rightmost pixels of the character glyph.

       A Font consists of an overall height and ascent and a collection of
       subfonts together with the ranges of runes (see utf(5g)) they
       represent.  Fonts are described by the following structures.

              struct Cachesubf {
                    Rune      min;      /* rune value of 0th char in subfont */
                    Rune      max;      /* rune value+1 of last char in subfont */
                    char      *name;
                    Subfont   *f;       /* attached subfont */
              } Cachefont;

              struct Font {
                    char      *name;
                    uchar     height;   /* max height of bitmap, interline spacing */
                    char      ascent;   /* top of bitmap to baseline */
                    char      width;    /* widest so far; used in caching only */
                    char      ldepth;   /* of images */
                    short     id;       /* of font */
                    short     nsubf;    /* number of subfonts */
                    Cachesubf *subf;    /* as read from file */
              } Font;

       The and fields of Font are described in graphics(3g).  contains
       pointers to Cachesubfs.  A Cachesubf connects runes through inclusive,
       to the subfont it corresponds to a line of the file describing the

       The image for rune r is found in position r−min+minchar of the subfont.

       For each font, the library, with support from the graphics server,
       maintains a list of subfonts.  The of a font is the maximum of the
       horizontal extents of the characters in the cache.  [ is unused in the
       X implementation of libg.]  String draws a string by calling cachechars
       and emitting a sequence of X font indices to draw.  Cachechars
       calculates the subfont/index pairs for the characters pointed to by *s.
       It calls getsubfont to fetch subfonts which are not yet in the subfont
       list of f.  Cachechars translates the character string into a set of
       subfont/index pairs.  It loads the character indices into the array c,
       up to a maximum of n indices or the length of the string.  It also
       loads the corresponding element of fp with the subfont index for each
       character.  Cachechars returns in c the number of subfont 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.  It can
       return zero if it is unable to make progress because a character code
       does not covered by any subfont range.

       graphics(3g), balloc(3g), bitblt(3g), font(5g).

       All of the functions use the graphics error function (see