[ttLib.name] Add nameTable.findMultilingualName() method (#1921)

* add nameTable.findMultilingualName(), to find an existing multilingual name
* Make addMultilingualName() reuse nameIDs if possible when asking for a new nameID, by calling findMultilingualName()

Lib/fontTools/ttLib/tables/_n_a_m_e.py

@@ -184,6 +184,57 @@ class table__n_a_m_e(DefaultTable.DefaultTable):
 			raise ValueError("nameID must be less than 32768")
 		return nameID
 
+	def findMultilingualName(self, names, windows=True, mac=True):
+		"""Return the name ID of an existing multilingual name that
+		matches the 'names' dictionary, or None if not found.
+
+		'names' is a dictionary with the name in multiple languages,
+		such as {'en': 'Pale', 'de': 'Blaß', 'de-CH': 'Blass'}.
+		The keys can be arbitrary IETF BCP 47 language codes;
+		the values are Unicode strings.
+
+		If 'windows' is True, the returned name ID is guaranteed
+		exist for all requested languages for platformID=3 and
+		platEncID=1.
+		If 'mac' is True, the returned name ID is guaranteed to exist
+		for all requested languages for platformID=1 and platEncID=0.
+		"""
+		# Gather the set of requested
+		#   (string, platformID, platEncID, langID)
+		# tuples
+		reqNameSet = set()
+		for lang, name in sorted(names.items()):
+			if windows:
+				windowsName = _makeWindowsName(name, None, lang)
+				if windowsName is not None:
+					reqNameSet.add((windowsName.string,
+					                windowsName.platformID,
+					                windowsName.platEncID,
+					                windowsName.langID))
+			if mac:
+				macName = _makeMacName(name, None, lang)
+				if macName is not None:
+					reqNameSet.add((macName.string,
+				                    macName.platformID,
+				                    macName.platEncID,
+				                    macName.langID))
+
+		# Collect matching name IDs
+		matchingNames = dict()
+		for name in self.names:
+			key = (name.string, name.platformID,
+			       name.platEncID, name.langID)
+			if key in reqNameSet:
+				nameSet = matchingNames.setdefault(name.nameID, set())
+				nameSet.add(key)
+
+		# Return the first name ID that defines all requested strings
+		for nameID, nameSet in sorted(matchingNames.items()):
+			if nameSet == reqNameSet:
+				return nameID
+
+		return None  # not found
+
 	def addMultilingualName(self, names, ttFont=None, nameID=None,
 	                        windows=True, mac=True):
 		"""Add a multilingual name, returning its name ID
@@ -199,7 +250,8 @@ class table__n_a_m_e(DefaultTable.DefaultTable):
 		names that otherwise cannot get encoded at all.
 
 		'nameID' is the name ID to be used, or None to let the library
-		pick an unused name ID.
+		find an existing set of name records that match, or pick an
+		unused name ID.
 
 		If 'windows' is True, a platformID=3 name record will be added.
 		If 'mac' is True, a platformID=1 name record will be added.
@@ -207,6 +259,10 @@ class table__n_a_m_e(DefaultTable.DefaultTable):
 		if not hasattr(self, 'names'):
 			self.names = []
 		if nameID is None:
+			# Reuse nameID if possible
+			nameID = self.findMultilingualName(names, windows=windows, mac=mac)
+			if nameID is not None:
+				return nameID
 			nameID = self._findUnusedNameID()
 		# TODO: Should minimize BCP 47 language codes.
 		# https://github.com/fonttools/fonttools/issues/930

Tests/ttLib/tables/_n_a_m_e_test.py

@@ -144,6 +144,48 @@ class NameTableTest(unittest.TestCase):
 		rec2 = table.getName(2, 1, 0, 0)
 		self.assertEqual(str(rec2), "Regular")
 
+	@staticmethod
+	def _get_test_names():
+		names = {
+			"en": "Width",
+			"de-CH": "Breite",
+			"gsw-LI": "Bräiti",
+		}
+		namesSubSet = names.copy()
+		del namesSubSet["gsw-LI"]
+		namesSuperSet = names.copy()
+		namesSuperSet["nl"] = "Breedte"
+		return names, namesSubSet, namesSuperSet
+
+	def test_findMultilingualName(self):
+		table = table__n_a_m_e()
+		names, namesSubSet, namesSuperSet = self._get_test_names()
+		nameID = table.addMultilingualName(names)
+		assert nameID is not None
+		self.assertEqual(nameID, table.findMultilingualName(names))
+		self.assertEqual(nameID, table.findMultilingualName(namesSubSet))
+		self.assertEqual(None, table.findMultilingualName(namesSuperSet))
+
+	def test_addMultilingualNameReuse(self):
+		table = table__n_a_m_e()
+		names, namesSubSet, namesSuperSet = self._get_test_names()
+		nameID = table.addMultilingualName(names)
+		assert nameID is not None
+		self.assertEqual(nameID, table.addMultilingualName(names))
+		self.assertEqual(nameID, table.addMultilingualName(namesSubSet))
+		self.assertNotEqual(None, table.addMultilingualName(namesSuperSet))
+
+	def test_findMultilingualNameNoMac(self):
+		table = table__n_a_m_e()
+		names, namesSubSet, namesSuperSet = self._get_test_names()
+		nameID = table.addMultilingualName(names, mac=False)
+		assert nameID is not None
+		self.assertEqual(nameID, table.findMultilingualName(names, mac=False))
+		self.assertEqual(None, table.findMultilingualName(names))
+		self.assertEqual(nameID, table.findMultilingualName(namesSubSet, mac=False))
+		self.assertEqual(None, table.findMultilingualName(namesSubSet))
+		self.assertEqual(None, table.findMultilingualName(namesSuperSet))
+
 	def test_addMultilingualName(self):
 		# Microsoft Windows has language codes for “English” (en)
 		# and for “Standard German as used in Switzerland” (de-CH).