left recursion memo size may be limited

3 files changed, 82 insertions, 3 deletions

diff --git a/pyparsing/core.py b/pyparsing/core.py
index 4b4a437..c346aeb 100644
--- a/pyparsing/core.py
+++ b/pyparsing/core.py
@@ -1,6 +1,7 @@
 #
 # core.py
 #
+from typing import Optional
 from abc import ABC, abstractmethod
 from enum import Enum
 import string
@@ -23,6 +24,8 @@ from .util import (
     _escapeRegexRangeChars,
     _bslash,
     _flatten,
+    LRUMemo as _LRUMemo,
+    UnboundedMemo as _UnboundedMemo,
 )
 from .exceptions import *
 from .actions import *
@@ -790,7 +793,7 @@ class ParserElement(ABC):
         ParserElement._parse = ParserElement._parseNoCache
 
     @staticmethod
-    def enable_left_recursion(*, force=False):
+    def enable_left_recursion(cache_size_limit: Optional[int] = None, *, force=False):
         """
         Enables "bounded recursion" parsing, which allows for both direct and indirect
         left-recursion. During parsing, left-recursive :class:`Forward` elements are
@@ -821,6 +824,12 @@ class ParserElement(ABC):
             ParserElement.disable_memoization()
         elif ParserElement._packratEnabled:
             raise RuntimeError("Packrat and Bounded Recursion are not compatible")
+        if cache_size_limit is None:
+            ParserElement.recursion_memos = _UnboundedMemo()
+        elif cache_size_limit > 0:
+            ParserElement.recursion_memos = _LRUMemo(capacity=cache_size_limit)
+        else:
+            raise NotImplementedError("Memo size of %s" % cache_size_limit)
         ParserElement._left_recursion_enabled = True
 
     @staticmethod
@@ -4505,7 +4514,9 @@ class Forward(ParseElementEnhance):
                             # replace the match for doActions=False as well,
                             # in case the action did backtrack
                             prev_loc, prev_result = memo[peek_key] = memo[act_key]
+                            del memo[peek_key], memo[act_key]
                             return prev_loc, prev_result.copy()
+                        del memo[peek_key]
                         return prev_loc, prev_peek.copy()
                     # the match did get better: see if we can improve further
                     else:
diff --git a/pyparsing/util.py b/pyparsing/util.py
index 3cb69d2..875799d 100644
--- a/pyparsing/util.py
+++ b/pyparsing/util.py
@@ -119,6 +119,52 @@ class _FifoCache:
         self.clear = types.MethodType(clear, self)
 
 
+class LRUMemo:
+    """
+    A memoizing mapping that retains `capacity` deleted items
+
+    The memo tracks retained items by their access order; once `capacity` items
+    are retained, the least recently used item is discarded.
+    """
+    def __init__(self, capacity):
+        self._capacity = capacity
+        self._active = {}
+        self._memory = collections.OrderedDict()
+
+    def __getitem__(self, key):
+        try:
+            return self._active[key]
+        except KeyError:
+            self._memory.move_to_end(key)
+            return self._memory[key]
+
+    def __setitem__(self, key, value):
+        self._memory.pop(key, None)
+        self._active[key] = value
+
+    def __delitem__(self, key):
+        try:
+            value = self._active.pop(key)
+        except KeyError:
+            pass
+        else:
+            while len(self._memory) >= self._capacity:
+                self._memory.popitem(last=False)
+            self._memory[key] = value
+
+    def clear(self):
+        self._active.clear()
+        self._memory.clear()
+
+
+class UnboundedMemo(dict):
+    """
+    A memoizing mapping that retains all deleted items
+    """
+    def __delitem__(self, key):
+        pass
+
+
 def _escapeRegexRangeChars(s):
     # escape these chars: ^-[]
     for c in r"\^-[]":
diff --git a/tests/test_unit.py b/tests/test_unit.py
index e431b87..aed2db3 100644
--- a/tests/test_unit.py
+++ b/tests/test_unit.py
@@ -8076,11 +8076,28 @@ class Test8_WithUnboundedPackrat(Test2_WithoutPackrat):
 
 class Test9_WithLeftRecursionParsing(Test2_WithoutPackrat):
     """
-    rerun Test2 tests, now with unbounded packrat cache
+    rerun Test2 tests, now with unbounded left recursion cache
     """
 
     def setUp(self):
-        recursion_suite_context.restore()
+        ParserElement.enable_left_recursion(force=True)
+
+    def tearDown(self):
+        default_suite_context.restore()
+
+    def test000_assert_packrat_status(self):
+        print("Left-Recursion enabled:", ParserElement._left_recursion_enabled)
+        self.assertTrue(ParserElement._left_recursion_enabled, "left recursion not enabled")
+        self.assertIsInstance(ParserElement.recursion_memos, pp.util.UnboundedMemo)
+
+
+class Test10_WithLeftRecursionParsingBoundedMemo(Test2_WithoutPackrat):
+    """
+    rerun Test2 tests, now with bounded left recursion cache
+    """
+
+    def setUp(self):
+        ParserElement.enable_left_recursion(cache_size_limit=4, force=True)
 
     def tearDown(self):
         default_suite_context.restore()
@@ -8088,6 +8105,11 @@ class Test9_WithLeftRecursionParsing(Test2_WithoutPackrat):
     def test000_assert_packrat_status(self):
         print("Left-Recursion enabled:", ParserElement._left_recursion_enabled)
         self.assertTrue(ParserElement._left_recursion_enabled, "left recursion not enabled")
+        self.assertIsInstance(ParserElement.recursion_memos, pp.util.LRUMemo)
+        # check that the cache matches roughly what we expect
+        # – it may be larger due to action handling
+        self.assertLessEqual(ParserElement.recursion_memos._capacity, 4)
+        self.assertGreater(ParserElement.recursion_memos._capacity * 3, 4)
 
 
 class TestLR1_Recursion(ppt.TestParseResultsAsserts, TestCase):