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

              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 font.

       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 graphics(3g)).