From 9df424631a8c2efa39aeda2c1cca668bbd41ce26 Mon Sep 17 00:00:00 2001
From: Ben Kelly <bk@ancilla.ca>
Date: Thu, 18 Jan 2018 10:30:49 -0500
Subject: [PATCH] Generate `ignore sub` tables to constrain ligature
 conversion.

This matches the behaviour of Fira Code, where long runs of identical
characters don't get ligaturized; for example, with this change,
`######` will get ligaturized as six `#` characters, rather than as
three `##` characters.
---
 ligaturize.py | 39 ++++++++++++++++++++++++++++++++-------
 1 file changed, 32 insertions(+), 7 deletions(-)

diff --git a/ligaturize.py b/ligaturize.py
index f7d0037..580611e 100644
--- a/ligaturize.py
+++ b/ligaturize.py
@@ -85,7 +85,6 @@ class LigatureCreator(object):
         except ValueError:
             return False
 
-
     def add_ligature(self, input_chars, firacode_ligature_name):
         if firacode_ligature_name is None:
             # No ligature name -- we're just copying a bunch of individual characters.
@@ -137,16 +136,42 @@ class LigatureCreator(object):
 
 
         calt_lookup_name = 'calt.{}'.format(self._lig_counter)
-        self.font.addLookup(calt_lookup_name, 'gsub_contextchain', (), (('calt', (('DFLT', ('dflt',)), ('arab', ('dflt',)), ('armn', ('dflt',)), ('cyrl', ('SRB ', 'dflt')), ('geor', ('dflt',)), ('grek', ('dflt',)), ('lao ', ('dflt',)), ('latn', ('CAT ', 'ESP ', 'GAL ', 'ISM ', 'KSM ', 'LSM ', 'MOL ', 'NSM ', 'ROM ', 'SKS ', 'SSM ', 'dflt')), ('math', ('dflt',)), ('thai', ('dflt',)))),))
+        self.font.addLookup(calt_lookup_name, 'gsub_contextchain', (),
+            (('calt', (('DFLT', ('dflt',)),
+                       ('arab', ('dflt',)),
+                       ('armn', ('dflt',)),
+                       ('cyrl', ('SRB ', 'dflt')),
+                       ('geor', ('dflt',)),
+                       ('grek', ('dflt',)),
+                       ('lao ', ('dflt',)),
+                       ('latn', ('CAT ', 'ESP ', 'GAL ', 'ISM ', 'KSM ', 'LSM ', 'MOL ', 'NSM ', 'ROM ', 'SKS ', 'SSM ', 'dflt')),
+                       ('math', ('dflt',)),
+                       ('thai', ('dflt',)))),))
+        #print('CALT %s (%s)' % (calt_lookup_name, firacode_ligature_name))
         for i, char in enumerate(input_chars):
-            ctx_subtable_name = 'calt.{}.{}'.format(self._lig_counter, i)
-            ctx_spec = '{prev} | {cur} @<{lookup}> | {next}'.format(
+            self.add_calt(calt_lookup_name, 'calt.{}.{}'.format(self._lig_counter, i),
+                '{prev} | {cur} @<{lookup}> | {next}',
                 prev = ' '.join(cr_name(j) for j in range(i)),
                 cur = char,
                 lookup = lookup_name(i),
-                next = ' '.join(input_chars[i+1:]),
-            )
-            self.font.addContextualSubtable(calt_lookup_name, ctx_subtable_name, 'glyph', ctx_spec)
+                next = ' '.join(input_chars[i+1:]))
+
+        # Add ignore rules
+        self.add_calt(calt_lookup_name, 'calt.{}.{}'.format(self._lig_counter, i+1),
+            '| {first} | {rest} {last}',
+            first = input_chars[0],
+            rest = ' '.join(input_chars[1:]),
+            last = input_chars[-1])
+        self.add_calt(calt_lookup_name, 'calt.{}.{}'.format(self._lig_counter, i+2),
+            '{first} | {first} | {rest}',
+            first = input_chars[0],
+            rest = ' '.join(input_chars[1:]))
+
+    def add_calt(self, calt_name, subtable_name, spec, **kwargs):
+        spec = spec.format(**kwargs)
+        #print('    %s: %s ' % (subtable_name, spec))
+        self.font.addContextualSubtable(calt_name, subtable_name, 'glyph', spec)
+
 
 
 def change_font_names(font, fontname, fullname, familyname, copyright_add, unique_id):