Added support for longer ligatures

- Breaking out the ligature substitution rules like this also allows for
 easy logical extensions, to handle edge cases as FiraCode does.

Scripts/features.py

@@ -44,36 +44,41 @@ def rule(liga):
                  [LIG   f i] LIG
                  [ f    f i] LIG }
     """
-    if len(liga) == 2:
-        return dedent('''\
-          lookup {0}_{1} {{
-            ignore sub {0} {0}' {1};
-            ignore sub {0}' {1} {1};
-            sub {0}'  {1}  by LIG;
-            sub LIG {1}' by {0}_{1}.liga;
-          }} {0}_{1};
-        ''').format(*liga)
-    elif len(liga) == 3:
-        return dedent('''\
-          lookup {0}_{1}_{2} {{
-            ignore sub {0} {0}' {1} {2};
-            ignore sub {0}' {1} {2} {2};
-            sub {0}' {1}  {2} by LIG;
-            sub LIG  {1}' {2} by LIG;
-            sub LIG  LIG  {2}' by {0}_{1}_{2}.liga;
-          }} {0}_{1}_{2};
-        ''').format(*liga)
-    elif len(liga) == 4:
-        return dedent('''\
-            lookup {0}_{1}_{2}_{3} {{
-               ignore sub {0} {0}' {1} {2} {3};
-               ignore sub {0}' {1} {2} {3} {3};
-               sub {0}'   {1}   {2}  {3}  by LIG;
-               sub LIG  {1}'  {2}  {3}  by LIG;
-               sub LIG LIG  {2}' {3}  by LIG;
-               sub LIG LIG LIG {3}' by {0}_{1}_{2}_{3}.liga;
-            }} {0}_{1}_{2}_{3};
-        ''').format(*liga)
+    # ignores:
+    #   ignore sub {0} {0}' {1};
+    #   ignore sub {0}' {1} {1};
+    rules = [
+        ignore(prefix=liga[:1], head=liga[0], suffix=liga[1:]),
+        ignore(head=liga[0], suffix=(liga[1:] + [liga[-1]])),
+    ]
+
+    name = "_".join(liga)
+    # substitution logic
+    #   sub {0}'  {1}  by LIG;
+    #   sub LIG {1}' by {0}_{1}.liga;
+    for i in range(len(liga)):
+        init = _join(["LIG" for lig in liga[:i]])
+        tail = _join(liga[i + 1 :])
+        replace = "LIG" if (i + 1 < len(liga)) else (name + ".liga")
+        rules.append("sub{0} {1}'{2} by {3};".format(init, liga[i], tail, replace))
+
+    # put it all together
+    lines = (
+        ["lookup " + name + " {"] + ["  " + r for r in rules] + ["}} {0};".format(name)]
+    )
+    return "\n".join(lines)
+
+
+def _join(items):
+    return (" " + " ".join(items)) if items else ""
+
+
+def ignore(prefix=None, head=None, suffix=None):
+    """ don't substitute `head` if it's surrounded by `prefix` and `suffix` """
+    assert head
+    pref = _join(prefix)
+    rest = _join(suffix)
+    return "ignore sub{0} {1}'{2};".format(pref, head, rest)
 
 
 def indent(text, prefix):