From 86db61ca30e0507c85c755342428a215f171eece Mon Sep 17 00:00:00 2001
From: Achraf Reyani <achraf@reyani.dev>
Date: Fri, 9 Oct 2026 20:49:16 +0900
Subject: [PATCH] feat(libc): implement rand_r

Share rand's generator step with rand_r, which advances the caller's
state instead of the global one, as FreeBSD's rand and rand_r share
do_rand.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 core/libs/prx/libc/src/Math.cpp | 20 ++++++++++++++++----
 core/libs/tests/GuestMath.cpp   | 15 +++++++++++++++
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/core/libs/prx/libc/src/Math.cpp b/core/libs/prx/libc/src/Math.cpp
index 8beda4557..992f358e3 100644
--- a/core/libs/prx/libc/src/Math.cpp
+++ b/core/libs/prx/libc/src/Math.cpp
@@ -127,15 +127,27 @@ int APS5_VABI __signbitf_nid_postfix(float x) { return std::signbit(x) ? 1 : 0;
 static std::mutex g_randLock;
 static std::uint32_t g_randState = 1;
 
-int APS5_VABI rand_nid_postfix() {
-    std::lock_guard lock(g_randLock);
-    const std::int64_t x = static_cast<std::int64_t>(g_randState % 0x7ffffffeu) + 1;
+static int NextRand_nid_no_patch(std::uint32_t& state) {
+    const std::int64_t x = static_cast<std::int64_t>(state % 0x7ffffffeu) + 1;
     std::int64_t next = 16807 * (x % 127773) - 2836 * (x / 127773);
     if (next < 0) next += 0x7fffffff;
-    g_randState = static_cast<std::uint32_t>(next - 1);
+    state = static_cast<std::uint32_t>(next - 1);
     return static_cast<int>(next - 1);
 }
 
+int APS5_VABI rand_nid_postfix() {
+    std::lock_guard lock(g_randLock);
+    return NextRand_nid_no_patch(g_randState);
+}
+
+int APS5_VABI rand_r_nid_postfix(unsigned int* state) {
+    if (state == nullptr) throw std::invalid_argument("rand_r: null state");
+    std::uint32_t value = *state;
+    const int result = NextRand_nid_no_patch(value);
+    *state = value;
+    return result;
+}
+
 void APS5_VABI srand_nid_postfix(unsigned int seed) {
     std::lock_guard lock(g_randLock);
     g_randState = seed;
diff --git a/core/libs/tests/GuestMath.cpp b/core/libs/tests/GuestMath.cpp
index b0a96e78b..ae2333740 100644
--- a/core/libs/tests/GuestMath.cpp
+++ b/core/libs/tests/GuestMath.cpp
@@ -35,6 +35,9 @@ float APS5_VABI atan2f_nid_postfix(float, float);
 float APS5_VABI hypotf_nid_postfix(float, float);
 double APS5_VABI hypot_nid_postfix(double, double);
 float APS5_VABI tanf_nid_postfix(float);
+int APS5_VABI rand_nid_postfix();
+void APS5_VABI srand_nid_postfix(unsigned int);
+int APS5_VABI rand_r_nid_postfix(unsigned int*);
 float APS5_VABI log10f_nid_postfix(float);
 float APS5_VABI logbf_nid_postfix(float);
 double APS5_VABI exp2_nid_postfix(double);
@@ -234,6 +237,18 @@ int main() {
     Require(std::abs(acosf_nid_postfix(0.5f) - 1.0471976f) < 0.000001f);
     Require(std::abs(atan2f_nid_postfix(1.f, -1.f) - 2.3561945f) < 0.000001f);
     Require(tanf_nid_postfix(0.f) == 0.f);
+    unsigned int randomState = 1;
+    Require(rand_r_nid_postfix(&randomState) == 33613 && randomState == 33613u);
+    randomState = 0;
+    Require(rand_r_nid_postfix(&randomState) == 16806 && randomState == 16806u);
+    for (const unsigned int seed : {0u, 1u, 12345u, 0x7ffffffeu, 0xffffffffu}) {
+        srand_nid_postfix(seed);
+        randomState = seed;
+        for (int step = 0; step < 8; ++step) Require(rand_r_nid_postfix(&randomState) == rand_nid_postfix());
+    }
+    bool threw = false;
+    try { rand_r_nid_postfix(nullptr); } catch (...) { threw = true; }
+    Require(threw);
     Require(hypot_nid_postfix(3.0, 4.0) == 5.0 && hypot_nid_postfix(-3.0, -4.0) == 5.0 && hypotf_nid_postfix(3.f, -4.f) == 5.f);
     Require(std::abs(hypot_nid_postfix(1e308, 1e308) / 1.4142135623730951e308 - 1.0) < 1e-15);
     Require(std::abs(hypotf_nid_postfix(2e38f, 2e38f) / 2.8284271e38f - 1.f) < 1e-6f);