From dfdbda44064e65914496d10f51f18aecd3c721ba Mon Sep 17 00:00:00 2001
From: Emi Simpson <emi@alchemi.dev>
Date: Mon, 6 Mar 2023 14:16:03 -0500
Subject: [PATCH] Fixed a bug which caused unnecessary infinite recursion

---
 resolution.py | 58 ++++++++++++++++++++++++++++++++-------------------
 1 file changed, 37 insertions(+), 21 deletions(-)

diff --git a/resolution.py b/resolution.py
index b9af348..48ffc2f 100644
--- a/resolution.py
+++ b/resolution.py
@@ -134,8 +134,8 @@ i.e. the empty clause
 
 
 def next_generation(
-    previous_generations: KnowledgeBase_,
-    current_generation: KnowledgeBase_,
+    previous_generations: KnowledgeBase,
+    current_generation: KnowledgeBase,
 ) -> KnowledgeBase:
 	"""
 	Advance resolution by one step
@@ -154,13 +154,13 @@ def next_generation(
 	empty list for previous generations indicates that this is the zeroth (original)
 	knowledge base.
 
-	>>> next_generation([
-	... ], [
-	...     [IRNeg(IRProp('animal', [IRProp('Kim')]))],
-	...     [IRNeg(IRProp('dog', [IRVar('x0')])), IRProp('animal', [IRVar('x0')])],
-	...     [IRNeg(IRProp('cat', [IRVar('x1')])), IRProp('animal', [IRVar('x1')])],
-	...     [IRProp('dog', [IRProp('Kim')])],
-	... ]) #doctest: +NORMALIZE_WHITESPACE
+	>>> next_generation(fset(
+	... ), fset(
+	...     fset( IRNeg(IRProp('animal', (IRProp('Kim'),)))                              ),
+	...     fset( IRNeg(IRProp('dog', (IRVar('x0'),))), IRProp('animal', (IRVar('x0'),)) ),
+	...     fset( IRNeg(IRProp('cat', (IRVar('x1'),))), IRProp('animal', (IRVar('x1'),)) ),
+	...     fset( IRProp('dog', (IRProp('Kim'),))                                        ),
+	... )) #doctest: +NORMALIZE_WHITESPACE
 	{
 		{ animal(Kim()) },
 		{ ¬cat(Kim())   },
@@ -171,24 +171,40 @@ def next_generation(
 	the first generation have been moved to the previous generations knowledge base, and
 	the current generation contains only the results from the first call.
 
-	>>> next_generation([
-	...     [IRNeg(IRProp('animal', [IRProp('Kim')]))],
-	...     [IRNeg(IRProp('dog', [IRVar('x0')])), IRProp('animal', [IRVar('x0')])],
-	...     [IRNeg(IRProp('cat', [IRVar('x1')])), IRProp('animal', [IRVar('x1')])],
-	...     [IRProp('dog', [IRProp('Kim')])],
-	... ], [
-	...     [IRNeg(IRProp('dog', [IRProp('Kim')]))],
-	...     [IRNeg(IRProp('cat', [IRProp('Kim')]))],
-	...     [IRProp('animal', [IRProp('Kim')])],
-	... ])
+	>>> next_generation(fset(
+	...     fset( IRNeg(IRProp('animal', (IRProp('Kim'),)))                              ),
+	...     fset( IRNeg(IRProp('dog', (IRVar('x0'),))), IRProp('animal', (IRVar('x0'),)) ),
+	...     fset( IRNeg(IRProp('cat', (IRVar('x1'),))), IRProp('animal', (IRVar('x1'),)) ),
+	...     fset( IRProp('dog', (IRProp('Kim'),))                                        ),
+	... ), fset(
+	...     fset( IRNeg(IRProp('dog', (IRProp('Kim'),)))                                 ),
+	...     fset( IRNeg(IRProp('cat', (IRProp('Kim'),)))                                 ),
+	...     fset( IRProp('animal', (IRProp('Kim'),))                                     ),
+	... ))
 	{ { } }
 
 	The return of a knowledge base containing false (ie `[]`) indicates that resolution
 	has found a contradiction.  In this case, there are two:  One from ¬dog(Kim) and
 	dog(Kim), and one from animal(Kim) and ¬animal(Kim).
-	"""
-	return fset(*derive(current_generation), *derive2(current_generation, previous_generations))
 
+	Excluded from the next generation is any term which exists in a previous generation.
+	For example:
+
+	>>> next_generation(fset(
+	... ), fset(
+	...     fset( IRNeg(IRProp('day')), IRProp('day') ),
+	...     fset( IRProp('day')                       ),
+	... ))
+	{ }
+
+	This configuration could theoretically derive day() again, however, it does not,
+	because that is already known, and is present in the current generation.
+	"""
+	return FSet(
+		(derive(current_generation) | derive2(current_generation, previous_generations))
+		- current_generation
+		- previous_generations
+	)
 def derives_false(
 	knowledge_base: KnowledgeBase,
 	previous_generations: KnowledgeBase = FSet(),