From 60215deb60462d362ceeb4ea0eac414070a71deb Mon Sep 17 00:00:00 2001
From: chenyu <chenyu@fastmail.com>
Date: Mon, 9 Mar 2026 03:40:39 -0400
Subject: [PATCH] tiebreak in fold_divmod_congruence (#15190)

need to try both direction
---
 test/null/test_uop_symbolic.py |  5 +++++
 tinygrad/uop/divandmod.py      | 12 +++++++-----
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/test/null/test_uop_symbolic.py b/test/null/test_uop_symbolic.py
index b3cdeaaf36..5219cb657e 100644
--- a/test/null/test_uop_symbolic.py
+++ b/test/null/test_uop_symbolic.py
@@ -286,6 +286,11 @@ class TestSymbolic(unittest.TestCase):
                               "(((z+(x*-1))+(y*-1))+7)")
     self.helper_test_variable((10+12*Variable("x",0,2)+Variable("y", 0, 4)%3)%13, 8, 12, "(((x*-1)+(y%3))+10)")
 
+  def test_mod_congruence_tied_remainder(self):
+    # when f%c == c/2, both r and r-c have equal abs — try both signs
+    self.helper_test_variable((3+2*Variable("x",0,1)+3*Variable("y",0,1))%4, 0, 3, "((x*-2)+(y*-1)+3)")
+    self.helper_test_variable((3+6*Variable("x",0,1)+7*Variable("y",0,1))%4, 0, 3, "((x*-2)+(y*-1)+3)")
+
   def test_div_congruence(self):
     self.helper_test_variable((3+3*Variable("a",0,3))//4, 0, 3, "a")
     self.helper_test_variable((18+17*Variable("a",0,2)+17)//18, 1, 3, "(a+1)")
diff --git a/tinygrad/uop/divandmod.py b/tinygrad/uop/divandmod.py
index c8ddd24c18..52dd8d9dd2 100644
--- a/tinygrad/uop/divandmod.py
+++ b/tinygrad/uop/divandmod.py
@@ -1,4 +1,4 @@
-import functools
+import functools, itertools
 from tinygrad.uop.ops import PatternMatcher, UPat, Ops, UOp
 from tinygrad.dtype import dtypes
 from tinygrad.helpers import cdiv, cmod, CORRECT_DIVMOD_FOLDING, unwrap
@@ -48,10 +48,12 @@ def fold_divmod_general(d: UOp, correct_divmod_folding: bool) -> UOp|None:
 
     # fold_divmod_congruence: fold if a is congruent to an expression whose range is between 0 and c
     if not (x.vmin<0 and correct_divmod_folding):
-      rems = [min((r:=f%c), r-c, key=abs) for f in factors]
-      if (rem:=sum(r*v for r,v in zip(rems,terms))+const%c).vmin//c==rem.vmax//c:
-        if d.op is Ops.MOD: return rem - rem.vmin//c*c
-        return sum((f-r)//c * v for f,r,v in zip(factors,rems,terms)) + (const-const%c+rem.vmin//c*c)//c
+      # when f%c == c//2, abs(r) == abs(r-c) is a tie, try both signs since either may fit in one period
+      rem_choices = [((r:=f%c), r-c) if (r:=f%c)*2 == c else (min(r, r-c, key=abs),) for f in factors]
+      for rems in itertools.product(*rem_choices):
+        if (rem:=sum(r*v for r,v in zip(rems,terms))+const%c).vmin//c==rem.vmax//c:
+          if d.op is Ops.MOD: return rem - rem.vmin//c*c
+          return sum((f-r)//c * v for f,r,v in zip(factors,rems,terms)) + (const-const%c+rem.vmin//c*c)//c
 
     # gcd_with_remainder: factor out common gcd from numerator
     # Note: this rule uses uops_no_const to exclude the additive constant from the GCD calculation