From 6759cb4d7cceecce0e4b717a359808f0bf8ecf5c Mon Sep 17 00:00:00 2001
From: Adam Heinermann <aheinerm@gmail.com>
Date: Fri, 15 Nov 2024 08:27:03 -0800
Subject: [PATCH] feat(big) add SBigIsEven

---
 storm/Big.cpp     |  4 ++++
 storm/Big.hpp     |  2 ++
 storm/big/Ops.cpp |  4 ++++
 storm/big/Ops.hpp |  2 ++
 test/Big.cpp      | 22 ++++++++++++++++++++++
 5 files changed, 34 insertions(+)

diff --git a/storm/Big.cpp b/storm/Big.cpp
index c04e91a..7e59fd3 100644
--- a/storm/Big.cpp
+++ b/storm/Big.cpp
@@ -69,6 +69,10 @@ void SBigInc(BigData* a, BigData* b) {
     Add(a->Primary(), b->Primary(), 1);
 }
 
+int32_t SBigIsEven(BigData* a) {
+    return IsEven(a->Primary());
+}
+
 void SBigMod(BigData* a, BigData* b, BigData* c) {
     uint32_t allocCount = 0;
     auto& scratch = a->Stack().Alloc(&allocCount);
diff --git a/storm/Big.hpp b/storm/Big.hpp
index bd7dac7..1f77ad7 100644
--- a/storm/Big.hpp
+++ b/storm/Big.hpp
@@ -28,6 +28,8 @@ void SBigFromUnsigned(BigData* num, uint32_t val);
 
 void SBigInc(BigData* a, BigData* b);
 
+int32_t SBigIsEven(BigData* a);
+
 void SBigMod(BigData* a, BigData* b, BigData* c);
 
 void SBigMul(BigData* a, BigData* b, BigData* c);
diff --git a/storm/big/Ops.cpp b/storm/big/Ops.cpp
index 4b2cdea..6d25914 100644
--- a/storm/big/Ops.cpp
+++ b/storm/big/Ops.cpp
@@ -211,6 +211,10 @@ void InsertLowPart(uint64_t& value, uint32_t low) {
     value = (value << 32) | low;
 }
 
+int32_t IsEven(const BigBuffer &num) {
+    return num.Count() == 0 || (num[0] & 1) == 0;
+}
+
 uint64_t MakeLarge(uint32_t low, uint32_t high) {
     return low + (static_cast<uint64_t>(high) << 32);
 }
diff --git a/storm/big/Ops.hpp b/storm/big/Ops.hpp
index 7a84657..2154dfc 100644
--- a/storm/big/Ops.hpp
+++ b/storm/big/Ops.hpp
@@ -33,6 +33,8 @@ uint32_t HighBitPos(const BigBuffer& buffer);
 
 void InsertLowPart(uint64_t& value, uint32_t low);
 
+int32_t IsEven(const BigBuffer &num);
+
 uint64_t MakeLarge(uint32_t low, uint32_t high);
 
 void Mul(BigBuffer& a, const BigBuffer& b, uint64_t c);
diff --git a/test/Big.cpp b/test/Big.cpp
index d6f93bf..6eae76c 100644
--- a/test/Big.cpp
+++ b/test/Big.cpp
@@ -426,6 +426,28 @@ TEST_CASE("SBigInc", "[big]") {
     }
 }
 
+TEST_CASE("SBigIsEven", "[big]") {
+    BigDataTest a;
+
+    SECTION("unset zero is even") {
+        CHECK(SBigIsEven(a));
+    }
+
+    SECTION("numbers are even") {
+        auto v = GENERATE(0ULL, 2ULL, 10ULL, 10000ULL, 0xFFFFFFFEULL, 0x9999888877776666ULL);
+
+        SBigFromStr(a, std::to_string(v).c_str());
+        CHECK(SBigIsEven(a));
+    }
+
+    SECTION("numbers are not even") {
+        auto v = GENERATE(1ULL, 3ULL, 37ULL, 999999999ULL, 0xFFFFFFFFFULL, 0x9999888877776667ULL);
+
+        SBigFromStr(a, std::to_string(v).c_str());
+        CHECK_FALSE(SBigIsEven(a));
+    }
+}
+
 TEST_CASE("SBigMod", "[big]") {
     SECTION("mods 7 by 4") {
         BigData* a;