From 1bebd2ccba5ebfff44a94d41c570094871f79065 Mon Sep 17 00:00:00 2001
From: Juan Mauricio Matera <matera@fisica.unlp.edu.ar>
Date: Sat, 31 Aug 2024 16:06:14 -0300
Subject: [PATCH] Blanks as singleton (#1081)

Looking at the initialization process, `Blank*` PatternObjets are
created several times. This PR makes that these elements be created just
once when a parameter is not provided. This would also help to optimize
the pattern-matching algorithm when we face that problem.
---
 CHANGES.rst                 |  6 ++++++
 mathics/builtin/patterns.py | 16 ++++++++++++++++
 mathics/core/builtin.py     | 25 ++++++++++++-------------
 3 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/CHANGES.rst b/CHANGES.rst
index 5f041f999..e8a99c4bc 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -19,6 +19,12 @@ Internals
 Operator precedence has been gone over and is picked up in tables from the Mathics Scanner project.
 
 
+Performance
+-----------
+
+* `Blank*` patterns without arguments are now singletons.
+
+  
 7.0.0
 -----
 
diff --git a/mathics/builtin/patterns.py b/mathics/builtin/patterns.py
index 0d60dc60b..7117bf7a8 100644
--- a/mathics/builtin/patterns.py
+++ b/mathics/builtin/patterns.py
@@ -1169,6 +1169,22 @@ def get_default_value(
 class _Blank(PatternObject):
     arg_counts = [0, 1]
 
+    _instance = None
+
+    def __new__(cls, *args, **kwargs):
+        if kwargs.get("expression", None) is False:
+            return super().__new__(cls, *args, **kwargs)
+
+        num_elem = len(args[0].elements)
+        assert num_elem < 2, f"{cls} should have at most an element."
+
+        if num_elem != 0:
+            return super().__new__(cls, *args, **kwargs)
+        # no arguments. Use the singleton
+        if cls._instance is None:
+            cls._instance = super().__new__(cls, *args, **kwargs)
+        return cls._instance
+
     def init(
         self, expr: Expression, evaluation: OptionalType[Evaluation] = None
     ) -> None:
diff --git a/mathics/core/builtin.py b/mathics/core/builtin.py
index b97f0cbd6..78504c0e0 100644
--- a/mathics/core/builtin.py
+++ b/mathics/core/builtin.py
@@ -46,7 +46,8 @@
 from mathics.core.list import ListExpression
 from mathics.core.number import PrecisionValueError, dps, get_precision, min_prec
 from mathics.core.parser.util import PyMathicsDefinitions, SystemDefinitions
-from mathics.core.rules import BuiltinRule, Pattern, Rule
+from mathics.core.pattern import Pattern
+from mathics.core.rules import BuiltinRule, Rule
 from mathics.core.symbols import (
     BaseElement,
     BooleanType,
@@ -191,16 +192,15 @@ def __new__(cls, *args, **kwargs):
         # well documented.
         # Notice that this behavior was used extensively in
         # mathics.builtin.inout
-
         if kwargs.get("expression", None) is not False:
             return to_expression(cls.get_name(), *args)
-        else:
-            instance = super().__new__(cls)
-            if not instance.formats:
-                # Reset formats so that not every instance shares the same
-                # empty dict {}
-                instance.formats = {}
-            return instance
+
+        instance = super().__new__(cls)
+        if not instance.formats:
+            # Reset formats so that not every instance shares the same
+            # empty dict {}
+            instance.formats = {}
+        return instance
 
     def __init__(self, *args, **kwargs):
         super().__init__()
@@ -472,12 +472,11 @@ def is_literal(self) -> bool:
 class BuiltinElement(Builtin, BaseElement):
     def __new__(cls, *args, **kwargs):
         new_kwargs = kwargs.copy()
+        # In a Builtin element, we never return an Expression object,
+        # so we create it with the option `expression=False`.
         new_kwargs["expression"] = False
         instance = super().__new__(cls, *args, **new_kwargs)
-        if not instance.formats:
-            # Reset formats so that not every instance shares the same empty
-            # dict {}
-            instance.formats = {}
+        # If `expression` is not `False`, we need to initialize the object:
         if kwargs.get("expression", None) is not False:
             try:
                 instance.init(*args, **kwargs)