From 1ea2d884d2231ce6b6b4abd5d4e25e0ae55d9a3d Mon Sep 17 00:00:00 2001
From: Yuval Tassa <tassa@google.com>
Date: Sun, 5 Jul 2026 10:47:30 -0700
Subject: [PATCH] Change mju_round to use standard round() function.

PiperOrigin-RevId: 942873144
Change-Id: I3fd0d630643c104ff464d619387ff36582c2a5d8
---
 doc/changelog.rst                    |  2 ++
 src/engine/engine_util_misc.c        | 14 +++++---------
 test/engine/engine_util_misc_test.cc | 24 ++++++++++++++++++++++++
 3 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/doc/changelog.rst b/doc/changelog.rst
index 0b450ead..e7bbc3a5 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -27,6 +27,8 @@ General
      ``(nv x nC)``.
    - Removed the legacy sparse ancestor-walk inertia matrix ``mjData.qM``. The joint-space inertia matrix is now stored
      exclusively in the compressed sparse row (CSR) format ``mjData.M``.
+   - :ref:`mju_round` now breaks ties away from zero rather than towards :math:`+\infty`. This only affects
+     negative half-integers, e.g. ``mju_round(-2.5)`` now returns -3 rather than -2.
 
 Version 3.10.0 (June 22, 2026)
 ------------------------------
diff --git a/src/engine/engine_util_misc.c b/src/engine/engine_util_misc.c
index 3c438ba7..5242d040 100644
--- a/src/engine/engine_util_misc.c
+++ b/src/engine/engine_util_misc.c
@@ -15,13 +15,13 @@
 #include "engine/engine_util_misc.h"
 
 #include <ctype.h>
+#include <limits.h>
 #include <math.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
-#include <mujoco/mjdata.h>
 #include <mujoco/mjmacro.h>
 #include <mujoco/mjmodel.h>
 #include "engine/engine_array_safety.h"
@@ -1762,14 +1762,10 @@ mjtNum mju_sign(mjtNum x) {
 
 // round to nearest integer
 int mju_round(mjtNum x) {
-  mjtNum lower = floor(x);
-  mjtNum upper = ceil(x);
-
-  if (x-lower < upper-x) {
-    return (int)lower;
-  } else {
-    return (int)upper;
-  }
+  double d = (double)x;
+  if (d > INT_MAX) return INT_MAX;
+  if (d < INT_MIN) return INT_MIN;
+  return (int)round(d);
 }
 
 
diff --git a/test/engine/engine_util_misc_test.cc b/test/engine/engine_util_misc_test.cc
index ec1b457f..88b6b778 100644
--- a/test/engine/engine_util_misc_test.cc
+++ b/test/engine/engine_util_misc_test.cc
@@ -17,6 +17,7 @@
 #include "src/engine/engine_util_misc.h"
 
 #include <array>
+#include <climits>
 #include <cmath>
 #include <cstddef>
 #include <cstdint>
@@ -1720,5 +1721,28 @@ TEST_F(ShellTFITest, NoInteriorSmallGrid) {
   }
 }
 
+TEST_F(UtilMiscTest, Round) {
+  // basic rounding
+  EXPECT_EQ(mju_round(2.3), 2);
+  EXPECT_EQ(mju_round(2.7), 3);
+  EXPECT_EQ(mju_round(-2.3), -2);
+  EXPECT_EQ(mju_round(-2.7), -3);
+
+  // exact integers
+  EXPECT_EQ(mju_round(0.0), 0);
+  EXPECT_EQ(mju_round(3.0), 3);
+  EXPECT_EQ(mju_round(-3.0), -3);
+
+  // ties: round() rounds away from zero
+  EXPECT_EQ(mju_round(0.5), 1);
+  EXPECT_EQ(mju_round(1.5), 2);
+  EXPECT_EQ(mju_round(-0.5), -1);
+  EXPECT_EQ(mju_round(-1.5), -2);
+
+  // overflow clamps to INT_MAX/INT_MIN
+  EXPECT_EQ(mju_round(1e18), INT_MAX);
+  EXPECT_EQ(mju_round(-1e18), INT_MIN);
+}
+
 }  // namespace
 }  // namespace mujoco