ffi helper demo: cairo text extents

[Matt Wette]

I have a small FFI Helper demo using cairo.
I used my ffi-helper, all written in Guile Scheme, to generate a (ffi cairo) 
module, in Scheme, from cairo.h.

Now compare the program below to the demo at 
https://www.cairographics.org/samples/text_extents/

Some notes:
1) The FFI Helper uses the bytestructures package to deal with C structs.
   The bs-ref macro defined below is used to save typing.
2) The FFI Helper uses Guile structs to wrap types:
    scheme@(guile-user)> extents
    $1 = #<cairo_text_extents_t 0x1079aaa70>
    scheme@(guile-user)> te
    $2 = #<bytestructure 0x108709930>
    scheme@(guile-user)> 
3) The procecure fh-object-val gets the object value (often a bytestructure) 
from the FFI-helper type.
4) You can use quoted symbols where enums are used in C code.
5) You can use strings in Scheme and the unwrapper within (ffi cairo) will 
apply string->pointer for you.
6) For reference, cairo.h contains
     typedef struct {
       double ascent;
       double descent;
       double height;
       double max_x_advance;
       double max_y_advance;
     } cairo_font_extents_t;
7) Below is a full script.  If saved as demo.scm, `guile demo.scm' will 
generate the expected .png file.


(use-modules (ffi cairo))
(use-modules (system ffi-help-rt))
(use-modules (bytestructures guile))

(define-syntax-rule (bs-ref bs ix ...)
  (bytestructure-ref bs ix ...))

(define pngfile "te.png")
(define srf (cairo_image_surface_create 'CAIRO_FORMAT_ARGB32 600 400))
(define cr (cairo_create srf))


;; -------------------------------------------------------
;; Go look at
;;   https://www.cairographics.org/samples/text_extents/
;; and compare to the following.

(define extents (make-cairo_text_extents_t))

(define utf8 "cairo")
(define x)
(define y)

(cairo_select_font_face cr "Sans"
                        'CAIRO_FONT_SLANT_NORMAL
                        'CAIRO_FONT_WEIGHT_NORMAL)

(cairo_set_font_size cr 100.0)
(cairo_text_extents cr utf8 (pointer-to extents))
(define te (fh-object-val extents))

(set! x 25.0)
(set! y 150.0)

(cairo_move_to cr x y)
(cairo_show_text cr utf8)

;; draw helping lines 
(cairo_set_source_rgba cr 1 0.2 0.2 0.6)
(cairo_set_line_width cr 6.0)
(cairo_arc cr x y 10.0 0 (* 2 3.15))
(cairo_fill cr)
(cairo_move_to cr x y)
(cairo_rel_line_to cr 0 (- (bs-ref te 'height)))
(cairo_rel_line_to cr (bs-ref te 'width) 0)
(cairo_rel_line_to cr (bs-ref te 'x_bearing) (- (bs-ref te 'y_bearing)))
(cairo_stroke cr)

;; -------------------------------------------------------

(cairo_surface_write_to_png srf pngfile)
(cairo_destroy cr)
(cairo_surface_destroy srf)