diff --git a/src/Chip8.Replay/ReplayEngine.cs b/src/Chip8.Replay/ReplayEngine.cs
new file mode 100644
index 0000000..7b23022
--- /dev/null
+++ b/src/Chip8.Replay/ReplayEngine.cs
@@ -0,0 +1,119 @@
+using System;
+using Chip8.Core;
+
+namespace Chip8.Replay;
+
+public enum ReplayMode
+{
+    Record,
+    Playback,
+}
+
+public sealed class ReplayEngine
+{
+    public Machine machine { get; }
+    public InputMovie movie { get; }
+    public ReplayMode mode { get; }
+    public int instructionsPerFrame { get; }
+    public int frameIndex { get; private set; }
+
+    ReplayEngine(Machine machine, InputMovie movie, ReplayMode mode)
+    {
+        this.machine = machine;
+        this.movie = movie;
+        this.mode = mode;
+        instructionsPerFrame = movie.instructionsPerFrame;
+    }
+
+    public static ReplayEngine record(Machine machine, InputMovie movie) =>
+        new ReplayEngine(machine, movie, ReplayMode.Record);
+
+    public static ReplayEngine playback(InputMovie movie, byte[] rom) =>
+        new ReplayEngine(MachineBuilder.fromMovie(movie, rom), movie, ReplayMode.Playback);
+
+    public static ReplayEngine playback(InputMovie movie, Machine machine) =>
+        new ReplayEngine(machine, movie, ReplayMode.Playback);
+
+    public bool finished => frameIndex >= movie.length || machine.exited;
+
+    public void pressKey(int key)
+    {
+        requireRecord();
+        machine.pressKey(key);
+    }
+
+    public void releaseKey(int key)
+    {
+        requireRecord();
+        machine.releaseKey(key);
+    }
+
+    public void setKeys(KeyMask keys)
+    {
+        requireRecord();
+        keys.applyTo(machine.keypad);
+    }
+
+    public void advanceFrame()
+    {
+        if (mode == ReplayMode.Record)
+        {
+            KeyMask observed = KeyMask.fromKeypad(machine.keypad);
+            movie.recordFrame(frameIndex, observed);
+            machine.runFrame(instructionsPerFrame);
+        }
+        else
+        {
+            KeyMask injected = movie.keysAt(frameIndex);
+            injected.applyTo(machine.keypad);
+            machine.runFrame(instructionsPerFrame);
+        }
+
+        frameIndex++;
+
+        if (mode == ReplayMode.Record)
+        {
+            movie.extendLength(frameIndex);
+        }
+    }
+
+    public void advanceFrames(int count)
+    {
+        for (int frame = 0; frame < count; frame++)
+        {
+            advanceFrame();
+        }
+    }
+
+    public void playToEnd()
+    {
+        while (!finished)
+        {
+            advanceFrame();
+        }
+    }
+
+    public ReplayOutcome outcome() => ReplayOutcome.capture(machine, frameIndex);
+
+    public static ReplayOutcome replayToEnd(InputMovie movie, byte[] rom)
+    {
+        var engine = playback(movie, rom);
+        engine.playToEnd();
+        return engine.outcome();
+    }
+
+    public static bool isDeterministic(InputMovie movie, byte[] rom)
+    {
+        ReplayOutcome first = replayToEnd(movie, rom);
+        ReplayOutcome second = replayToEnd(movie, rom);
+        return first.sameAs(second);
+    }
+
+    void requireRecord()
+    {
+        if (mode != ReplayMode.Record)
+        {
+            throw new InvalidOperationException("input can only be supplied while recording");
+        }
+    }
+}