feat(buchstabensuppe): font fallback

By default we now try to fallback to rendering 0xFFFD if a glyph is
missing from a grapheme in all fonts.

Also introduces BS_RENDER_NO_FALLBACK flag to disable this behavior.

Implementation reuses bs_render_grapheme_append (recursively) which is
probably overkill, but is concise and prevents code duplication.

2 files changed, 19 insertions, 1 deletions

diff --git a/buchstabensuppe.c b/buchstabensuppe.c
index ec9b211..5919073 100644
--- a/buchstabensuppe.c
+++ b/buchstabensuppe.c
@@ -12,6 +12,8 @@
 
 #include <buchstabensuppe.h>
 
+#define FALLBACK_CODEPOINT 0xFFFD
+
 #define FONT_SCALE_MULTIPLIER 1
 
 #define LOG(...) \
@@ -374,6 +376,21 @@ bool bs_render_grapheme_append(bs_context_t *ctx, bs_bitmap_t *target, bs_cursor
     font_index++;
   }
 
+  if(!have_glyphs && !(ctx->bs_rendering_flags & BS_RENDER_NO_FALLBACK)) {
+    bs_utf32_buffer_t fallback_grapheme = bs_utf32_buffer_new(1);
+    bs_utf32_buffer_append_single(FALLBACK_CODEPOINT, &fallback_grapheme);
+
+    if(fallback_grapheme.bs_utf32_buffer_len > 0) {
+      // avoid infinite recursion
+      ctx->bs_rendering_flags |= BS_RENDER_NO_FALLBACK;
+      have_glyphs = bs_render_grapheme_append(ctx,
+        target, cursor, fallback_grapheme, 0, 1);
+      ctx->bs_rendering_flags ^= BS_RENDER_NO_FALLBACK;
+    }
+
+    bs_utf32_buffer_free(&fallback_grapheme);
+  }
+
   return have_glyphs;
 }
 
diff --git a/include/buchstabensuppe.h b/include/buchstabensuppe.h
index fc24698..7573aff 100644
--- a/include/buchstabensuppe.h
+++ b/include/buchstabensuppe.h
@@ -50,7 +50,8 @@ typedef struct bs_font {
 } bs_font_t;
 
 enum bs_rendering_flag {
-  BS_RENDER_BINARY    = 0x01,
+  BS_RENDER_BINARY      = 0b001,
+  BS_RENDER_NO_FALLBACK = 0b100,
 };
 
 typedef struct bs_context {