reorder_glyphs: check the length and set of glyphs of new_glyph_order…

… matches the font's

src/nanoemoji/reorder_glyphs.py

@@ -218,6 +218,20 @@ def _access_path(path: SubTablePath):
 
 
 def reorder_glyphs(font: ttLib.TTFont, new_glyph_order: List[str]):
+    old_glyph_order = font.getGlyphOrder()
+    if len(new_glyph_order) != len(old_glyph_order):
+        raise ValueError(
+            f"New glyph order contains {len(new_glyph_order)} glyphs, "
+            f"but font has {len(old_glyph_order)} glyphs"
+        )
+
+    if set(old_glyph_order) != set(new_glyph_order):
+        raise ValueError(
+            "New glyph order does not contain the same set of glyphs as the font:\n"
+            f"* only in new: {set(new_glyph_order) - set(old_glyph_order)}\n"
+            f"* only in old: {set(old_glyph_order) - set(new_glyph_order)}"
+        )
+
     # Changing the order of glyphs in a TTFont requires that all tables that use
     # glyph indexes have been fully.
     # Cf. https://github.com/fonttools/fonttools/issues/2060

tests/reorder_glyphs_test.py

@@ -151,3 +151,19 @@ def _pair_pos(font):
 
         # Confirm swap applied to Coverage and pair pos parallel array
         assert _pair_pos(font) == (("b", [("c", -16), ("b", -14)]), ("a", [("b", -12)]))
+
+
+def test_invalid_new_glyph_order():
+    font = ttLib.TTFont()
+    font.setGlyphOrder([".notdef", "a", "b", "c"])
+
+    with pytest.raises(
+        ValueError, match="New glyph order contains 3 glyphs, but font has 4 glyphs"
+    ):
+        reorder_glyphs(font, new_glyph_order=[".notdef", "a", "b"])
+
+    with pytest.raises(
+        ValueError,
+        match="New glyph order does not contain the same set of glyphs as the font:",
+    ):
+        reorder_glyphs(font, new_glyph_order=[".notdef", "a", "b", "d"])