Static nth index fix

Don't get caught in an infinite loop trying to adjust the bounds of a static index.

pyspelling/util/css_selectors.py

@@ -794,41 +794,43 @@ def match_nth(self, el, nth):
             count_incr = 1
             factor = -1 if last else 1
             index = len(parent.contents) - 1 if last else 0
-
-            # Abort if our nth index is out of bounds and only getting further out of bounds as we increment.
-            # Otherwise, increment to try to get in bounds.
-            idx = last_idx = a * count + b if var else a
-            while idx < 1 or idx > last_index:
-                diff_low = 0 - idx
-                if idx < 0:
-                    count += count_incr
-                    idx = last_idx = a * count + b if var else a
-                    diff = 0 - idx
-                    if diff > diff_low:
-                        break
-                    diff_low = diff
-                diff_high = idx - last_index
-                if idx > last_index:
-                    count += count_incr
-                    idx = last_idx = a * count + b if var else a
-                    diff = idx - last_index
-                    if diff > diff_high:
-                        break
-                    diff_high = diff
-
-            # If a < 0, our count is working backwards, so floor the index by increasing the count.
-            # Find the count that yields the lowest, in bound value and use that.
-            # Lastly reverse count increment so that we'll increase our index.
-            lowest = count
-            if a < 0:
-                while idx >= 1:
-                    lowest = count
-                    count += count_incr
-                    idx = last_idx = a * count + b if var else a
-                count_incr = -1
-            count = lowest
             idx = last_idx = a * count + b if var else a
 
+            # We can only adjust bounds within a variable index
+            if var:
+                # Abort if our nth index is out of bounds and only getting further out of bounds as we increment.
+                # Otherwise, increment to try to get in bounds.
+                while idx < 1 or idx > last_index:
+                    diff_low = 0 - idx
+                    if idx < 0:
+                        count += count_incr
+                        idx = last_idx = a * count + b if var else a
+                        diff = 0 - idx
+                        if diff >= diff_low:
+                            break
+                        diff_low = diff
+                    diff_high = idx - last_index
+                    if idx > last_index:
+                        count += count_incr
+                        idx = last_idx = a * count + b if var else a
+                        diff = idx - last_index
+                        if diff >= diff_high:
+                            break
+                        diff_high = diff
+
+                # If a < 0, our count is working backwards, so floor the index by increasing the count.
+                # Find the count that yields the lowest, in bound value and use that.
+                # Lastly reverse count increment so that we'll increase our index.
+                lowest = count
+                if a < 0:
+                    while idx >= 1:
+                        lowest = count
+                        count += count_incr
+                        idx = last_idx = a * count + b if var else a
+                    count_incr = -1
+                count = lowest
+                idx = last_idx = a * count + b if var else a
+
             # Evaluate elements while our calculated nth index is still in range
             while 1 <= idx <= last_index:
                 child = None

tests/filters/test_html.py

@@ -668,6 +668,33 @@ def test_css_nth_even(self):
             ]
         )
 
+    def test_css_nth_child_bad(self):
+        """Test HTML."""
+
+        config = self.dedent(
+            """
+            matrix:
+            - name: html_css
+              sources:
+              - '{}/**/*.txt'
+              aspell:
+                lang: en
+              hunspell:
+                d: en_US
+              pipeline:
+              - pyspelling.filters.html:
+                  mode: html5
+                  ignores:
+                  - 'p:nth-child(-2)'
+            """
+        ).format(self.tempdir)
+        self.mktemp('.html5.yml', config, 'utf-8')
+        self.assert_spellcheck(
+            '.html5.yml', [
+                'aaaa', 'bbbb', 'cccc', 'dddd', 'eeee', 'ffff', 'gggg', 'hhhh', 'iiii', 'jjjj', 'kkkk', 'llll'
+            ]
+        )
+
     def test_css_nth_child1(self):
         """Test HTML."""