From f01c906aa12f7b902a2322216b7302f2c4ca6415 Mon Sep 17 00:00:00 2001
From: fallenoak <git@fallenoak.me>
Date: Fri, 3 Feb 2023 00:43:20 -0600
Subject: [PATCH] feat(big): add ExtractLowPartLargeSum

---
 storm/big/Ops.cpp |  8 ++++++++
 storm/big/Ops.hpp |  2 ++
 test/Big.cpp      | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 57 insertions(+)

diff --git a/storm/big/Ops.cpp b/storm/big/Ops.cpp
index 3e094bc..2affe61 100644
--- a/storm/big/Ops.cpp
+++ b/storm/big/Ops.cpp
@@ -58,6 +58,14 @@ uint32_t ExtractLowPart(uint64_t& value) {
     return low;
 }
 
+uint32_t ExtractLowPartLargeSum(uint64_t& value, uint64_t add) {
+    auto carry = (value += add) < add;
+    auto low = ExtractLowPart(value);
+    value += static_cast<uint64_t>(carry) << 32;
+
+    return low;
+}
+
 uint32_t ExtractLowPartSx(uint64_t& value) {
     auto low = static_cast<uint32_t>(value);
     value >>= 32;
diff --git a/storm/big/Ops.hpp b/storm/big/Ops.hpp
index 99fc457..94b502b 100644
--- a/storm/big/Ops.hpp
+++ b/storm/big/Ops.hpp
@@ -15,6 +15,8 @@ void Div(BigBuffer& a, uint32_t* b, const BigBuffer& c, uint64_t d);
 
 uint32_t ExtractLowPart(uint64_t& value);
 
+uint32_t ExtractLowPartLargeSum(uint64_t& value, uint64_t add);
+
 uint32_t ExtractLowPartSx(uint64_t& value);
 
 void FromBinary(BigBuffer& buffer, const void* value, uint32_t bytes);
diff --git a/test/Big.cpp b/test/Big.cpp
index c1ba4dd..e22c214 100644
--- a/test/Big.cpp
+++ b/test/Big.cpp
@@ -288,6 +288,53 @@ TEST_CASE("ExtractLowPart", "[big]") {
     }
 }
 
+TEST_CASE("ExtractLowPartLargeSum", "[big]") {
+    SECTION("extracts low part after adding 2 and 4") {
+        uint64_t value = 2;
+        uint64_t add = 4;
+        auto low = ExtractLowPartLargeSum(value, add);
+
+        CHECK(low == 6);
+        CHECK(value == 0);
+    }
+
+    SECTION("extracts low part after adding 0x1111111122222222 and 0x3333333344444444") {
+        uint64_t value = 0x1111111122222222;
+        uint64_t add = 0x3333333344444444;
+        auto low = ExtractLowPartLargeSum(value, add);
+
+        CHECK(low == 0x66666666);
+        CHECK(value == 0x44444444);
+    }
+
+    SECTION("extracts low part after adding 0xCCCCCCCCCCCCCCCC and 0x5555555555555555") {
+        uint64_t value = 0xCCCCCCCCCCCCCCCC;
+        uint64_t add = 0x5555555555555555;
+        auto low = ExtractLowPartLargeSum(value, add);
+
+        CHECK(low == 0x22222221);
+        CHECK(value == 0x122222222);
+    }
+
+    SECTION("extracts low part after adding 0xFFFFFFFFFFFFFFFF and 0") {
+        uint64_t value = 0xFFFFFFFFFFFFFFFF;
+        uint64_t add = 0;
+        auto low = ExtractLowPartLargeSum(value, add);
+
+        CHECK(low == 0xFFFFFFFF);
+        CHECK(value == 0xFFFFFFFF);
+    }
+
+    SECTION("extracts low part after adding 0xFFFFFFFFFFFFFFFF and 0xFFFFFFFFFFFFFFFF") {
+        uint64_t value = 0xFFFFFFFFFFFFFFFF;
+        uint64_t add = 0xFFFFFFFFFFFFFFFF;
+        auto low = ExtractLowPartLargeSum(value, add);
+
+        CHECK(low == 0xFFFFFFFE);
+        CHECK(value == 0x1FFFFFFFF);
+    }
+}
+
 TEST_CASE("ExtractLowPartSx", "[big]") {
     SECTION("extracts low part of 0") {
         uint64_t value = 0;