diff --git a/src/Chip8.Replay/Chip8.Replay.csproj b/src/Chip8.Replay/Chip8.Replay.csproj
new file mode 100644
index 0000000..c64a386
--- /dev/null
+++ b/src/Chip8.Replay/Chip8.Replay.csproj
@@ -0,0 +1,16 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net6.0</TargetFramework>
+    <ImplicitUsings>disable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <RootNamespace>Chip8.Replay</RootNamespace>
+    <AssemblyName>Chip8.Replay</AssemblyName>
+    <LangVersion>10</LangVersion>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Chip8.Core\Chip8.Core.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/src/Chip8.Replay/MachineBuilder.cs b/src/Chip8.Replay/MachineBuilder.cs
new file mode 100644
index 0000000..611abd8
--- /dev/null
+++ b/src/Chip8.Replay/MachineBuilder.cs
@@ -0,0 +1,31 @@
+using Chip8.Core;
+
+namespace Chip8.Replay;
+
+public static class MachineBuilder
+{
+    public static Machine fromMovie(InputMovie movie)
+    {
+        return new Machine(QuirksCodec.clone(movie.quirks), movie.randomSeed, movie.platform);
+    }
+
+    public static Machine fromMovie(InputMovie movie, byte[] rom)
+    {
+        var machine = fromMovie(movie);
+        machine.loadRom(rom);
+        return machine;
+    }
+
+    public static InputMovie headerFor(Machine machine, byte[] rom, string romName, int instructionsPerFrame, int? randomSeed)
+    {
+        var movie = new InputMovie
+        {
+            platform = machine.platform,
+            quirks = QuirksCodec.clone(machine.quirks),
+            randomSeed = randomSeed,
+            instructionsPerFrame = instructionsPerFrame,
+        };
+        movie.setRom(rom, romName);
+        return movie;
+    }
+}
diff --git a/src/Chip8.Replay/MovieFrame.cs b/src/Chip8.Replay/MovieFrame.cs
new file mode 100644
index 0000000..1d3afe9
--- /dev/null
+++ b/src/Chip8.Replay/MovieFrame.cs
@@ -0,0 +1,32 @@
+using System;
+
+namespace Chip8.Replay;
+
+public readonly struct MovieFrame : IEquatable<MovieFrame>
+{
+    public int index { get; }
+    public KeyMask keys { get; }
+
+    public MovieFrame(int index, KeyMask keys)
+    {
+        if (index < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(index), "movie frame index cannot be negative");
+        }
+        this.index = index;
+        this.keys = keys;
+    }
+
+    public MovieFrame(int index, ushort keyBits)
+        : this(index, new KeyMask(keyBits))
+    {
+    }
+
+    public bool Equals(MovieFrame other) => index == other.index && keys == other.keys;
+
+    public override bool Equals(object? obj) => obj is MovieFrame other && Equals(other);
+
+    public override int GetHashCode() => (index * 397) ^ keys.bits;
+
+    public override string ToString() => "frame " + index + " " + keys.toKeyList();
+}
diff --git a/src/Chip8.Replay/RomHash.cs b/src/Chip8.Replay/RomHash.cs
new file mode 100644
index 0000000..56e8d6d
--- /dev/null
+++ b/src/Chip8.Replay/RomHash.cs
@@ -0,0 +1,28 @@
+using System;
+using System.Globalization;
+
+namespace Chip8.Replay;
+
+public static class RomHash
+{
+    const ulong fnvOffsetBasis = 14695981039346656037UL;
+    const ulong fnvPrime = 1099511628211UL;
+
+    public static ulong compute(byte[] rom)
+    {
+        ulong hash = fnvOffsetBasis;
+        foreach (byte value in rom)
+        {
+            hash ^= value;
+            hash *= fnvPrime;
+        }
+        return hash;
+    }
+
+    public static string toHex(byte[] rom) => format(compute(rom));
+
+    public static string format(ulong hash) => hash.ToString("X16", CultureInfo.InvariantCulture);
+
+    public static ulong parse(string hex) =>
+        ulong.Parse(hex, NumberStyles.HexNumber, CultureInfo.InvariantCulture);
+}
diff --git a/src/Chip8.Replay/TimelineSnapshot.cs b/src/Chip8.Replay/TimelineSnapshot.cs
new file mode 100644
index 0000000..2857f17
--- /dev/null
+++ b/src/Chip8.Replay/TimelineSnapshot.cs
@@ -0,0 +1,17 @@
+namespace Chip8.Replay;
+
+public sealed class TimelineSnapshot
+{
+    public int frameIndex { get; }
+    public byte[] state { get; }
+
+    public TimelineSnapshot(int frameIndex, byte[] state)
+    {
+        this.frameIndex = frameIndex;
+        this.state = state;
+    }
+
+    public int stateSize => state.Length;
+
+    public override string ToString() => "snapshot frame=" + frameIndex + " bytes=" + state.Length;
+}