add notcurses_canbraille() capability test

NEWS.md

@@ -1,6 +1,9 @@
 This document attempts to list user-visible changes and any major internal
 rearrangements of Notcurses.
 
+* 2.1.9 (not yet released)
+  * Add `notcurses_canbraille()` capability predicate.
+
 * 2.1.8 (2021-02-03):
   * The `notcurses-tetris` binary has been renamed `nctetris`.
   * The new function `channel_set_palindex()` has been added.

USAGE.md

@@ -300,6 +300,12 @@ bool notcurses_canutf8(const struct notcurses* nc);
 
 // Can we blit to Sixel? This requires being built against libsixel.
 bool notcurses_cansixel(const struct notcurses* nc);
+
+// Can we draw sextants? This requires Unicode 13.
+bool notcurses_cansextants(const struct notcurses* nc);
+
+// Can we draw Braille? The Linux console cannot.
+bool notcurses_canbraille(const struct notcurses* nc);
 ```
 
 ## Direct mode

doc/man/man3/notcurses_capabilities.3.md

@@ -28,6 +28,8 @@ notcurses_capabilities - runtime capability detection
 
 **bool notcurses_cansextant(const struct notcurses* ***nc***);**
 
+**bool notcurses_canbraille(const struct notcurses* ***nc***);**
+
 **bool notcurses_cansixel(const struct notcurses* ***nc***);**
 
 # DESCRIPTION
@@ -64,7 +66,8 @@ UTF-8 encoding, and the locale was successfully loaded.
 that the terminal can properly render Unicode 13 sextants.
 
 **notcurses_cansixel** returns **true** if the terminal advertises
-support for Sixel.
+support for Sixel. **notcurses_canbraille** returns **true** if Braille
+is expected to work on the terminal.
 
 # BUGS
 

include/notcurses/notcurses.h

@@ -1228,6 +1228,9 @@ API bool notcurses_canutf8(const struct notcurses* nc);
 // Can we reliably use Unicode 13 sextants?
 API bool notcurses_cansextant(const struct notcurses* nc);
 
+// Can we reliably use Unicode Braille?
+API bool notcurses_canbraille(const struct notcurses* nc);
+
 // Can we blit to Sixel?
 API bool notcurses_cansixel(const struct notcurses* nc);
 

src/lib/notcurses.c

@@ -2053,6 +2053,10 @@ bool notcurses_cansextant(const notcurses* nc){
   return nc->tcache.sextants && nc->tcache.utf8;
 }
 
+bool notcurses_canbraille(const notcurses* nc){
+  return nc->tcache.braille && nc->tcache.utf8;
+}
+
 bool notcurses_canfade(const notcurses* nc){
   return nc->tcache.CCCflag || nc->tcache.RGBflag;
 }

tests/plot.cpp

@@ -146,6 +146,9 @@ TEST_CASE("Plot") {
   }
 
   SUBCASE("BraillePlot") {
+    if(!notcurses_canbraille(nc_)){
+      return;
+    }
     ncplane_options nopts = {
       .y = 1, .x = 1, .rows = 6, .cols = 50,
       .userptr = nullptr, .name = "plot", .resizecb = nullptr, .flags = 0,
@@ -250,6 +253,9 @@ TEST_CASE("Plot") {
   }
 
   SUBCASE("BraillePlot1Row") {
+    if(!notcurses_canbraille(nc_)){
+      return;
+    }
     ncplane_options nopts = {
       .y = 1, .x = 1, .rows = 1, .cols = 25,
       .userptr = nullptr, .name = "plot", .resizecb = nullptr, .flags = 0,