Use box drawing for sextants

See the Symbols for Legacy Computing section in the Unicode standard
https://www.unicode.org/charts/PDF/U1FB00.pdf

docs/changelog.rst

@@ -37,6 +37,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 - Fix unfocused windows in which a bell occurs not changing their border color
   to red until a relayout
 
+- Implement special rendering for various characters from the set of "Symbols
+  for Legacy Computing" from the Unicode 13 standard
+
 
 0.18.3 [2020-08-11]
 -------------------

kitty/fonts.c

@@ -600,6 +600,7 @@ START_ALLOW_CASE_RANGE
         case 0xe0ba: //   
         case 0xe0bc: // 
         case 0xe0be: //   
+        case 0x1fb00 ... 0x1fb3b:  // sextants
             return BOX_FONT;
         default:
             ans = in_symbol_maps(fg, cpu_cell->ch);
@@ -637,8 +638,10 @@ START_ALLOW_CASE_RANGE
             return ch - 0x2500; // IDs from 0x00 to 0x9f
         case 0xe0b0 ... 0xe0d4:
             return 0xa0 + ch - 0xe0b0; // IDs from 0xa0 to 0xc4
+        case 0x1fb00 ... 0x1fb3b:
+            return 0xc5 + ch - 0x1fb00;
         default:
-            return 0xff;
+            return 0xffff;
     }
 END_ALLOW_CASE_RANGE
 }

kitty/fonts/box_drawing.py

@@ -582,6 +582,35 @@ def quad(buf: BufType, width: int, height: int, x: int = 0, y: int = 0) -> None:
             buf[off + c] = 255
 
 
+def sextant(buf: BufType, width: int, height: int, level: int = 1, which: int = 0) -> None:
+
+    def draw_sextant(row: int = 0, col: int = 0) -> None:
+        if row == 0:
+            y_start, y_end = 0, height // 3
+        elif row == 1:
+            y_start, y_end = height // 3, 2 * height // 3
+        else:
+            y_start, y_end = 2 * height // 3, height
+        if col == 0:
+            x_start, x_end = 0, width // 2
+        else:
+            x_start, x_end = width // 2, width
+        for r in range(y_start, y_end):
+            off = r * width
+            for c in range(x_start, x_end):
+                buf[c + off] = 255
+
+    def add_row(q: int, r: int) -> None:
+        if q & 1:
+            draw_sextant(r)
+        if q & 2:
+            draw_sextant(r, col=1)
+
+    add_row(which % 4, 0)
+    add_row(which // 4, 1)
+    add_row(which // 16, 2)
+
+
 box_chars: Dict[str, List[Callable]] = {
     '─': [hline],
     '━': [p(hline, level=3)],
@@ -710,6 +739,14 @@ for chars, func_ in (('╒╕╘╛', dvcorner), ('╓╖╙╜', dhcorner), ('
         box_chars[ch] = [p(cast(Callable, func_), which=ch)]
 
 
+c = 0x1fb00
+for i in range(1, 63):
+    if i in (20, 40):
+        continue
+    box_chars[chr(c)] = [p(sextant, which=i)]
+    c += 1
+
+
 def render_box_char(ch: str, buf: BufType, width: int, height: int, dpi: float = 96.0) -> BufType:
     global _dpi
     _dpi = dpi