render: font: Allow rendering of substrings.

* chickadee/render/font.scm (draw-text*, draw-text): Add start and end
arguments.
* doc/api.texi (Fonts): Document new arguments.

2 files changed, 12 insertions, 4 deletions

diff --git a/chickadee/render/font.scm b/chickadee/render/font.scm
index 3654a13..d5995a2 100644
--- a/chickadee/render/font.scm
+++ b/chickadee/render/font.scm
@@ -311,7 +311,8 @@ extension must be either .xml or .fnt."
 (define draw-text*
   (let ((cursor (vec2 0.0 0.0))
         (rect (make-rect 0.0 0.0 0.0 0.0)))
-    (lambda* (font text matrix #:key (blend-mode 'alpha))
+    (lambda* (font text matrix #:key (blend-mode 'alpha)
+                   (start 0) (end (string-length text)))
       ;; TODO: Respect kerning.
       (define (render-char c)
         ;; TODO: What if "?" isn't in the font?
@@ -331,7 +332,7 @@ extension must be either .xml or .fnt."
                            (font-char-advance char))))))
       (set-vec2-x! cursor 0.0)
       (set-vec2-y! cursor 0.0)
-      (string-for-each render-char text))))
+      (string-for-each render-char text start end))))
 
 (define %default-scale (vec2 1.0 1.0))
 (define %null-vec2 (vec2 0.0 0.0))
@@ -345,7 +346,9 @@ extension must be either .xml or .fnt."
               (origin %null-vec2)
               (rotation 0)
               (scale %default-scale)
-              (blend-mode 'alpha))
+              (blend-mode 'alpha)
+              (start 0)
+              (end (string-length text)))
       "Draw the string TEXT with the first character starting at
 POSITION using FONT."
       (matrix4-2d-transform! matrix
@@ -353,4 +356,5 @@ POSITION using FONT."
                              #:position position
                              #:rotation rotation
                              #:scale scale)
-      (draw-text* font text matrix #:blend-mode blend-mode))))
+      (draw-text* font text matrix #:blend-mode blend-mode
+                  #:start start #:end end))))
diff --git a/doc/api.texi b/doc/api.texi
index 82cde8f..8295b38 100644
--- a/doc/api.texi
+++ b/doc/api.texi
@@ -892,6 +892,7 @@ Return @code{#t} if @var{font} is an italicized font.
 
 @deffn {Procedure} draw-text @var{font} @var{text} @var{position}
        [#:origin] [#:scale] [#:rotation] [#:blend-mode]
+       [#:start 0] [#:end @code{(string-length text)}]
 
 Draw the string @var{text} with the first character starting at
 @var{position} using @var{font}.
@@ -900,6 +901,9 @@ Draw the string @var{text} with the first character starting at
 (draw-text font "Hello, world!" (vec2 128.0 128.0))
 @end example
 
+To render a substring of @var{text}, use the @var{start} and @var{end}
+arguments.
+
 Refer to @code{draw-sprite} (@pxref{Sprites}) for information about
 the other arguments.
 @end deffn