test(core): add save state round-trip tests

tests/GameBoy.Core.Tests/SaveStateTests.cs

@@ -0,0 +1,72 @@
+using Xunit;
+using Xunit.Abstractions;
+
+namespace GameBoy.Tests
+{
+    public sealed class SaveStateTests
+    {
+        private readonly ITestOutputHelper output;
+
+        public SaveStateTests(ITestOutputHelper output)
+        {
+            this.output = output;
+        }
+
+        [Fact]
+        public void RestoreContinuesIdenticallyToUninterruptedRun()
+        {
+            byte[] rom = TestRoms.Load("cpu_instrs.gb");
+
+            var reference = new Emulator(rom);
+            for (int frame = 0; frame < 300; frame++)
+                reference.RunFrame();
+
+            byte[] snapshot = reference.SaveState();
+
+            for (int frame = 0; frame < 150; frame++)
+                reference.RunFrame();
+
+            var restored = new Emulator(rom);
+            restored.LoadState(snapshot);
+            for (int frame = 0; frame < 150; frame++)
+                restored.RunFrame();
+
+            output.WriteLine($"snapshot size: {snapshot.Length} bytes");
+            output.WriteLine($"reference cycles: {reference.TotalCycles}, restored cycles: {restored.TotalCycles}");
+
+            Assert.Equal(reference.TotalCycles, restored.TotalCycles);
+            Assert.Equal(reference.SerialOutput, restored.SerialOutput);
+            Assert.Equal(reference.Framebuffer, restored.Framebuffer);
+
+            Assert.Equal(reference.Cpu.AF, restored.Cpu.AF);
+            Assert.Equal(reference.Cpu.BC, restored.Cpu.BC);
+            Assert.Equal(reference.Cpu.DE, restored.Cpu.DE);
+            Assert.Equal(reference.Cpu.HL, restored.Cpu.HL);
+            Assert.Equal(reference.Cpu.SP, restored.Cpu.SP);
+            Assert.Equal(reference.Cpu.PC, restored.Cpu.PC);
+        }
+
+        [Fact]
+        public void SnapshotTakenMidTransferSurvivesRoundTrip()
+        {
+            byte[] rom = TestRoms.Load("dmg-acid2.gb");
+
+            var reference = new Emulator(rom);
+            for (int frame = 0; frame < 120; frame++)
+                reference.RunFrame();
+
+            byte[] snapshot = reference.SaveState();
+            var restored = new Emulator(rom);
+            restored.LoadState(snapshot);
+
+            for (int frame = 0; frame < 90; frame++)
+            {
+                reference.RunFrame();
+                restored.RunFrame();
+            }
+
+            Assert.Equal(reference.Framebuffer, restored.Framebuffer);
+            Assert.Equal(reference.Cpu.PC, restored.Cpu.PC);
+        }
+    }
+}