From 219c7b895cf0077ec595aa27a19cf31a741a708d Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Fri, 28 Aug 2026 09:50:44 +0200
Subject: [PATCH] test(core): add arithmetic and logic opcode tests

---
 tests/Chip8.Tests/ArithmeticTests.cs | 140 +++++++++++++++++++++++++++
 1 file changed, 140 insertions(+)
 create mode 100644 tests/Chip8.Tests/ArithmeticTests.cs

diff --git a/tests/Chip8.Tests/ArithmeticTests.cs b/tests/Chip8.Tests/ArithmeticTests.cs
new file mode 100644
index 0000000..bfb0529
--- /dev/null
+++ b/tests/Chip8.Tests/ArithmeticTests.cs
@@ -0,0 +1,140 @@
+using Chip8.Core;
+using Xunit;
+
+namespace Chip8.Tests;
+
+public class ArithmeticTests
+{
+    [Fact]
+    public void addImmediateWrapsWithoutTouchingFlag()
+    {
+        var machine = TestHarness.withOpcode(0x70FF);
+        machine.v[0] = 0x05;
+        machine.v[0xF] = 0xAB;
+        machine.step();
+        Assert.Equal(0x04, machine.v[0]);
+        Assert.Equal(0xAB, machine.v[0xF]);
+    }
+
+    [Fact]
+    public void moveCopiesVyIntoVx()
+    {
+        var machine = TestHarness.withOpcode(0x8010);
+        machine.v[1] = 0x42;
+        machine.step();
+        Assert.Equal(0x42, machine.v[0]);
+    }
+
+    [Fact]
+    public void orAndXorProduceExpectedBits()
+    {
+        var or = TestHarness.withOpcode(0x8011);
+        or.v[0] = 0b1100; or.v[1] = 0b1010; or.step();
+        Assert.Equal(0b1110, or.v[0]);
+
+        var and = TestHarness.withOpcode(0x8012);
+        and.v[0] = 0b1100; and.v[1] = 0b1010; and.step();
+        Assert.Equal(0b1000, and.v[0]);
+
+        var xor = TestHarness.withOpcode(0x8013);
+        xor.v[0] = 0b1100; xor.v[1] = 0b1010; xor.step();
+        Assert.Equal(0b0110, xor.v[0]);
+    }
+
+    [Fact]
+    public void logicLeavesFlagUntouchedOnModernButClearsOnCosmac()
+    {
+        var modern = TestHarness.withOpcode(0x8011, Quirks.modern());
+        modern.v[0] = 1; modern.v[1] = 2; modern.v[0xF] = 0x7;
+        modern.step();
+        Assert.Equal(0x7, modern.v[0xF]);
+
+        var cosmac = TestHarness.withOpcode(0x8011, Quirks.cosmacVip());
+        cosmac.v[0] = 1; cosmac.v[1] = 2; cosmac.v[0xF] = 0x7;
+        cosmac.step();
+        Assert.Equal(0, cosmac.v[0xF]);
+    }
+
+    [Fact]
+    public void addSetsCarryFlag()
+    {
+        var overflow = TestHarness.withOpcode(0x8014);
+        overflow.v[0] = 200; overflow.v[1] = 100;
+        overflow.step();
+        Assert.Equal(44, overflow.v[0]);
+        Assert.Equal(1, overflow.v[0xF]);
+
+        var noOverflow = TestHarness.withOpcode(0x8014);
+        noOverflow.v[0] = 20; noOverflow.v[1] = 30;
+        noOverflow.step();
+        Assert.Equal(50, noOverflow.v[0]);
+        Assert.Equal(0, noOverflow.v[0xF]);
+    }
+
+    [Fact]
+    public void addIntoVfKeepsCarryNotSum()
+    {
+        var machine = TestHarness.withOpcode(0x8F14);
+        machine.v[0xF] = 200; machine.v[1] = 100;
+        machine.step();
+        Assert.Equal(1, machine.v[0xF]);
+    }
+
+    [Fact]
+    public void subtractSetsNotBorrowFlag()
+    {
+        var positive = TestHarness.withOpcode(0x8015);
+        positive.v[0] = 50; positive.v[1] = 20;
+        positive.step();
+        Assert.Equal(30, positive.v[0]);
+        Assert.Equal(1, positive.v[0xF]);
+
+        var borrow = TestHarness.withOpcode(0x8015);
+        borrow.v[0] = 20; borrow.v[1] = 50;
+        borrow.step();
+        Assert.Equal(226, borrow.v[0]);
+        Assert.Equal(0, borrow.v[0xF]);
+    }
+
+    [Fact]
+    public void subtractReverseUsesVyMinusVx()
+    {
+        var machine = TestHarness.withOpcode(0x8017);
+        machine.v[0] = 20; machine.v[1] = 50;
+        machine.step();
+        Assert.Equal(30, machine.v[0]);
+        Assert.Equal(1, machine.v[0xF]);
+    }
+
+    [Fact]
+    public void shiftRightUsesConfiguredSource()
+    {
+        var modern = TestHarness.withOpcode(0x8016, Quirks.modern());
+        modern.v[0] = 0b0000_0101; modern.v[1] = 0xFF;
+        modern.step();
+        Assert.Equal(0b0000_0010, modern.v[0]);
+        Assert.Equal(1, modern.v[0xF]);
+
+        var cosmac = TestHarness.withOpcode(0x8016, Quirks.cosmacVip());
+        cosmac.v[0] = 0x00; cosmac.v[1] = 0b0000_0101;
+        cosmac.step();
+        Assert.Equal(0b0000_0010, cosmac.v[0]);
+        Assert.Equal(1, cosmac.v[0xF]);
+    }
+
+    [Fact]
+    public void shiftLeftUsesConfiguredSource()
+    {
+        var modern = TestHarness.withOpcode(0x801E, Quirks.modern());
+        modern.v[0] = 0b1000_0001;
+        modern.step();
+        Assert.Equal(0b0000_0010, modern.v[0]);
+        Assert.Equal(1, modern.v[0xF]);
+
+        var cosmac = TestHarness.withOpcode(0x801E, Quirks.cosmacVip());
+        cosmac.v[0] = 0x00; cosmac.v[1] = 0b1000_0001;
+        cosmac.step();
+        Assert.Equal(0b0000_0010, cosmac.v[0]);
+        Assert.Equal(1, cosmac.v[0xF]);
+    }
+}