From 8bdbe3c6531b01219137576916ce50ec4018f2df Mon Sep 17 00:00:00 2001
From: fallenoak <git@fallenoak.me>
Date: Sat, 4 Feb 2023 14:46:03 -0600
Subject: [PATCH] feat(big): add MulMod

---
 storm/big/Ops.cpp | 10 ++++++++
 storm/big/Ops.hpp |  2 ++
 test/Big.cpp      | 62 +++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 74 insertions(+)

diff --git a/storm/big/Ops.cpp b/storm/big/Ops.cpp
index a2ff20a..3bf46cc 100644
--- a/storm/big/Ops.cpp
+++ b/storm/big/Ops.cpp
@@ -231,6 +231,16 @@ void Mul(BigBuffer& a, const BigBuffer& b, const BigBuffer& c, BigStack& stack)
     stack.UnmakeDistinct(a, aa);
 }
 
+void MulMod(BigBuffer& a, const BigBuffer& b, const BigBuffer& c, const BigBuffer& d, BigStack& stack) {
+    uint32_t allocCount = 0;
+    auto& scratch = stack.Alloc(&allocCount);
+
+    Mul(scratch, b, c, stack);
+    Div(scratch, a, scratch, d, stack);
+
+    stack.Free(allocCount);
+}
+
 void SetOne(BigBuffer& buffer) {
     buffer.SetCount(1);
     buffer[0] = 1;
diff --git a/storm/big/Ops.hpp b/storm/big/Ops.hpp
index d4f22e3..3b01e87 100644
--- a/storm/big/Ops.hpp
+++ b/storm/big/Ops.hpp
@@ -35,6 +35,8 @@ void Mul(BigBuffer& a, const BigBuffer& b, uint64_t c);
 
 void Mul(BigBuffer& a, const BigBuffer& b, const BigBuffer& c, BigStack& stack);
 
+void MulMod(BigBuffer& a, const BigBuffer& b, const BigBuffer& c, const BigBuffer& d, BigStack& stack);
+
 void SetOne(BigBuffer& buffer);
 
 void SetZero(BigBuffer& buffer);
diff --git a/test/Big.cpp b/test/Big.cpp
index e1f5807..66d509f 100644
--- a/test/Big.cpp
+++ b/test/Big.cpp
@@ -641,6 +641,68 @@ TEST_CASE("Mul", "[big]") {
     }
 }
 
+TEST_CASE("MulMod", "[big]") {
+    SECTION("multiplies 0xFFFFFFFF by 0x100 and mods the result by 0xABC") {
+        BigData* a;
+        SBigNew(&a);
+
+        BigData* b;
+        SBigNew(&b);
+        SBigFromUnsigned(b, 0xFFFFFFFF);
+
+        BigData* c;
+        SBigNew(&c);
+        SBigFromUnsigned(c, 0x100);
+
+        BigData* d;
+        SBigNew(&d);
+        SBigFromUnsigned(d, 0xABC);
+
+        MulMod(a->Primary(), b->Primary(), c->Primary(), d->Primary(), a->Stack());
+
+        a->Primary().Trim();
+
+        CHECK(a->Primary().Count() == 1);
+        CHECK(a->Primary()[0] == 0x624);
+
+        SBigDel(a);
+        SBigDel(b);
+        SBigDel(c);
+        SBigDel(d);
+    }
+
+    SECTION("multiplies 0x123456789ABCDEF0 by 0xFEDCBA9876543210 and mods the result by 0x10000000") {
+        BigData* a;
+        SBigNew(&a);
+
+        BigData* b;
+        SBigNew(&b);
+        uint64_t b_ = 0x123456789ABCDEF0;
+        SBigFromBinary(b, reinterpret_cast<uint8_t*>(&b_), sizeof(b_));
+
+        BigData* c;
+        SBigNew(&c);
+        uint64_t c_ = 0xFEDCBA9876543210;
+        SBigFromBinary(c, reinterpret_cast<uint8_t*>(&c_), sizeof(c_));
+
+        BigData* d;
+        SBigNew(&d);
+        SBigFromUnsigned(d, 0x10000000);
+
+        MulMod(a->Primary(), b->Primary(), c->Primary(), d->Primary(), a->Stack());
+
+        a->Primary().Trim();
+
+        CHECK(a->Primary().Count() == 1);
+        CHECK(a->Primary()[0] == 0x618CF00);
+
+        SBigDel(a);
+        SBigDel(b);
+        SBigDel(c);
+        SBigDel(d);
+    }
+}
+
 TEST_CASE("MakeLarge", "[big]") {
     SECTION("creates uint64_t out of 0xAABBCCDD and 0x11223344") {
         uint64_t value = MakeLarge(0xAABBCCDD, 0x11223344);