From c4a57999e5d9181790e9b314434f32aebcccdd5d Mon Sep 17 00:00:00 2001
From: Aayush Sabharwal <aayush.sabharwal@gmail.com>
Date: Thu, 24 Oct 2024 16:29:20 +0530
Subject: [PATCH] feat: add iteration limit for `fixpoint_sub`

---
 src/variable.jl | 12 ++++++++----
 test/utils.jl   |  8 ++++++++
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/src/variable.jl b/src/variable.jl
index b5230f07b..d7374b507 100644
--- a/src/variable.jl
+++ b/src/variable.jl
@@ -503,21 +503,25 @@ function SymbolicIndexingInterface.symbolic_evaluate(ex::Union{Num, Arr, Symboli
 end
 
 """
-    fixpoint_sub(expr, dict; operator = Nothing)
+    fixpoint_sub(expr, dict; operator = Nothing, maxiters = 10000)
 
 Given a symbolic expression, equation or inequality `expr` perform the substitutions in
 `dict` recursively until the expression does not change. Substitutions that depend on one
 another will thus be recursively expanded. For example,
 `fixpoint_sub(x, Dict(x => y, y => 3))` will return `3`. The `operator` keyword can be
-specified to prevent substitution of expressions inside operators of the given type.
+specified to prevent substitution of expressions inside operators of the given type. The
+`maxiters` keyword is used to limit the number of times the substitution can occur to avoid
+infinite loops in cases where the substitutions in `dict` are circular
+(e.g. `[x => y, y => x]`).
 
 See also: [`fast_substitute`](@ref).
 """
-function fixpoint_sub(x, dict; operator = Nothing)
+function fixpoint_sub(x, dict; operator = Nothing, maxiters = 10000)
     y = fast_substitute(x, dict; operator)
-    while !isequal(x, y)
+    while !isequal(x, y) && maxiters > 0
         y = x
         x = fast_substitute(y, dict; operator)
+        maxiters -= 1
     end
 
     return x
diff --git a/test/utils.jl b/test/utils.jl
index 616dee642..6c876faba 100644
--- a/test/utils.jl
+++ b/test/utils.jl
@@ -38,3 +38,11 @@ end
     @test var_from_nested_derivative(p) == (p, 0)
     @test var_from_nested_derivative(D(p(x))) == (p(x), 1)
 end
+
+@testset "fixpoint_sub maxiters" begin
+    @variables x y
+    expr = fixpoint_sub(x, Dict(x => y, y => x))
+    @test isequal(expr, x)
+    expr = fixpoint_sub(x, Dict(x => y, y => x); maxiters = 9)
+    @test isequal(expr, y)
+end