diff --git a/StoneRed.LogicSimulator.Test/CircuitDefinition.cs b/StoneRed.LogicSimulator.Test/CircuitDefinition.cs
new file mode 100644
index 0000000..dcb2b36
--- /dev/null
+++ b/StoneRed.LogicSimulator.Test/CircuitDefinition.cs
@@ -0,0 +1,177 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace StoneRed.LogicSimulator.Test;
+
+public sealed class CircuitDefinition
+{
+    private readonly List<GateKind> gateKinds = new();
+    private readonly List<(int FromGate, int ToGate, byte ToInputBit)> connections = new();
+    private readonly List<int> inputPins = new();
+    private readonly List<int> outputPins = new();
+    private readonly List<MacroInstanceDef> macroInstances = new();
+
+    public IReadOnlyList<GateKind> GateKinds => gateKinds;
+    public IReadOnlyList<(int FromGate, int ToGate, byte ToInputBit)> Connections => connections;
+    public IReadOnlyList<int> InputPins => inputPins;
+    public IReadOnlyList<int> OutputPins => outputPins;
+    public IReadOnlyList<MacroInstanceDef> MacroInstances => macroInstances;
+
+    public sealed record MacroInstanceDef(string Name, int[] Inputs, int[] Outputs);
+
+    public int AddGate(GateKind kind)
+    {
+        int id = gateKinds.Count;
+        gateKinds.Add(kind);
+        return id;
+    }
+
+    public int AddInputPin()
+    {
+        int id = AddGate(GateKind.Source);
+        inputPins.Add(id);
+        return id;
+    }
+
+    public int AddOutputPin()
+    {
+        int id = AddGate(GateKind.Sink);
+        outputPins.Add(id);
+        return id;
+    }
+
+    public MacroInstanceDef AddMacroInstance(string name, int inputCount, int outputCount)
+    {
+        if (string.IsNullOrWhiteSpace(name))
+        {
+            throw new ArgumentException("Macro name must not be empty.", nameof(name));
+        }
+
+        if (inputCount < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(inputCount));
+        }
+
+        if (outputCount < 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(outputCount));
+        }
+
+        int[] inputs = new int[inputCount];
+        for (int i = 0; i < inputCount; i++)
+        {
+            inputs[i] = AddGate(GateKind.Buffer);
+        }
+
+        int[] outputs = new int[outputCount];
+        for (int i = 0; i < outputCount; i++)
+        {
+            outputs[i] = AddGate(GateKind.Sink);
+        }
+
+        var instance = new MacroInstanceDef(name, inputs, outputs);
+        macroInstances.Add(instance);
+        return instance;
+    }
+
+    public void Connect(int fromGate, int toGate, int toInputBit)
+    {
+        if ((uint)fromGate >= (uint)gateKinds.Count) throw new ArgumentOutOfRangeException(nameof(fromGate));
+        if ((uint)toGate >= (uint)gateKinds.Count) throw new ArgumentOutOfRangeException(nameof(toGate));
+        if ((uint)toInputBit >= 32u) throw new ArgumentOutOfRangeException(nameof(toInputBit));
+
+        connections.Add((fromGate, toGate, (byte)toInputBit));
+    }
+
+    public void Validate()
+    {
+        // Minimal validation to prevent ambiguous semantics.
+        // - Input pins must be Source gates and must not have incoming connections.
+        // - Output pins must be Sink gates.
+        // - All Source gates must be listed as InputPins.
+        // - LUT gates are not allowed in definitions (they are generated by the simulator).
+        var hasIncoming = new bool[gateKinds.Count];
+        for (int i = 0; i < connections.Count; i++)
+        {
+            (_, int to, _) = connections[i];
+            hasIncoming[to] = true;
+        }
+
+        var inputPinSet = new HashSet<int>(inputPins);
+        for (int i = 0; i < gateKinds.Count; i++)
+        {
+            if (gateKinds[i] == GateKind.Source && !inputPinSet.Contains(i))
+            {
+                throw new InvalidOperationException("Only Source gates marked as InputPins are allowed inside a circuit definition.");
+            }
+
+            if (gateKinds[i] == GateKind.Lut)
+            {
+                throw new InvalidOperationException("Circuit definitions must not contain LUT gates.");
+            }
+        }
+
+        var macroPinGates = new HashSet<int>();
+        foreach (MacroInstanceDef instance in macroInstances)
+        {
+            if (string.IsNullOrWhiteSpace(instance.Name))
+            {
+                throw new InvalidOperationException("Macro instance name must not be empty.");
+            }
+
+            foreach (int pin in instance.Inputs)
+            {
+                if ((uint)pin >= (uint)gateKinds.Count) throw new InvalidOperationException("Macro instance input pin is out of range.");
+                if (gateKinds[pin] != GateKind.Buffer) throw new InvalidOperationException("Macro instance inputs must be Buffer gates.");
+                if (!macroPinGates.Add(pin)) throw new InvalidOperationException("Macro instance pin is used more than once.");
+            }
+
+            foreach (int pin in instance.Outputs)
+            {
+                if ((uint)pin >= (uint)gateKinds.Count) throw new InvalidOperationException("Macro instance output pin is out of range.");
+                if (gateKinds[pin] != GateKind.Sink) throw new InvalidOperationException("Macro instance outputs must be Sink gates.");
+                if (!macroPinGates.Add(pin)) throw new InvalidOperationException("Macro instance pin is used more than once.");
+            }
+        }
+
+        foreach (int pin in macroPinGates)
+        {
+            if (inputPinSet.Contains(pin) || outputPins.Contains(pin))
+            {
+                throw new InvalidOperationException("Macro instance pins must not be listed as InputPins/OutputPins.");
+            }
+        }
+
+        foreach (int input in inputPins)
+        {
+            if (gateKinds[input] != GateKind.Source)
+            {
+                throw new InvalidOperationException("Input pins must be Source gates.");
+            }
+
+            if (hasIncoming[input])
+            {
+                throw new InvalidOperationException("Input pins must not have incoming connections.");
+            }
+        }
+
+        foreach (int output in outputPins)
+        {
+            if (gateKinds[output] != GateKind.Sink)
+            {
+                throw new InvalidOperationException("Output pins must be Sink gates.");
+            }
+        }
+
+        if (inputPins.Distinct().Count() != inputPins.Count)
+        {
+            throw new InvalidOperationException("Input pins contain duplicates.");
+        }
+
+        if (outputPins.Distinct().Count() != outputPins.Count)
+        {
+            throw new InvalidOperationException("Output pins contain duplicates.");
+        }
+    }
+}
diff --git a/StoneRed.LogicSimulator.Test/ExprCircuitSimulator.cs b/StoneRed.LogicSimulator.Test/ExprCircuitSimulator.cs
new file mode 100644
index 0000000..10e6af3
--- /dev/null
+++ b/StoneRed.LogicSimulator.Test/ExprCircuitSimulator.cs
@@ -0,0 +1,618 @@
+using System.Linq.Expressions;
+
+namespace StoneRed.LogicSimulator.Test;
+
+public enum GateKind : byte
+{
+    Source,
+    Not,
+    And2,
+    Or2,
+    Buffer,
+    Sink,
+    Lut,
+}
+
+public sealed class ExprCircuitSimulator
+{
+    private readonly List<GateKind> gateKinds = [];
+    private readonly List<(int FromGate, int ToGate, byte ToInputBit)> connections = [];
+    private readonly List<int[]?> lutTableByGate = [];
+    private readonly Dictionary<string, MacroInfo> macroGates = new(StringComparer.Ordinal);
+
+    private int[] inputMasks = Array.Empty<int>();
+    private int[] nextInputMasks = Array.Empty<int>();
+    private int[] outputMasks = Array.Empty<int>();
+    private int[] sourceStates = Array.Empty<int>();
+
+    private int[] edgeStart = Array.Empty<int>();
+    private int[] edgeToGate = Array.Empty<int>();
+    private byte[] edgeToInputBit = Array.Empty<byte>();
+
+    private int[] lutOffsets = Array.Empty<int>();
+    private int[] lutMasks = Array.Empty<int>();
+    private int[] lutData = Array.Empty<int>();
+
+    private Action<int[], int[], int[]>? computeOutputs;
+    private bool compiled;
+
+    public int GateCount => gateKinds.Count;
+
+    public sealed record MacroInstance(string Name, int[] Inputs, int[] Outputs);
+
+    private sealed record MacroLut(int InputCount, int OutputCount, int[][] OutputTables);
+
+    private sealed record MacroInfo(CircuitDefinition Definition, MacroLut? Lut);
+
+    public int AddGate(GateKind kind)
+    {
+        if (kind == GateKind.Lut)
+        {
+            throw new InvalidOperationException("Use AddLutGate() to create LUT gates.");
+        }
+
+        int id = gateKinds.Count;
+        gateKinds.Add(kind);
+        lutTableByGate.Add(null);
+        compiled = false;
+        return id;
+    }
+
+    public int AddLutGate(int inputCount, int[] table)
+    {
+        if (inputCount is < 0 or > 30)
+        {
+            throw new ArgumentOutOfRangeException(nameof(inputCount), "LUT input count must be between 0 and 30.");
+        }
+
+        if (table is null)
+        {
+            throw new ArgumentNullException(nameof(table));
+        }
+
+        int expected = 1 << inputCount;
+        if (table.Length != expected)
+        {
+            throw new ArgumentException($"LUT table length must be {expected} for {inputCount} inputs.", nameof(table));
+        }
+
+        int id = gateKinds.Count;
+        gateKinds.Add(GateKind.Lut);
+        lutTableByGate.Add(table);
+        compiled = false;
+        return id;
+    }
+
+    public void ConnectGates(int fromGate, int toGate, int toInputBit)
+    {
+        if ((uint)fromGate >= (uint)gateKinds.Count)
+        {
+            throw new ArgumentOutOfRangeException(nameof(fromGate));
+        }
+
+        if ((uint)toGate >= (uint)gateKinds.Count)
+        {
+            throw new ArgumentOutOfRangeException(nameof(toGate));
+        }
+
+        if ((uint)toInputBit >= 32u)
+        {
+            throw new ArgumentOutOfRangeException(nameof(toInputBit));
+        }
+
+        connections.Add((fromGate, toGate, (byte)toInputBit));
+        compiled = false;
+    }
+
+    public void RegisterMacroGate(string name, CircuitDefinition definition)
+    {
+        if (string.IsNullOrWhiteSpace(name))
+        {
+            throw new ArgumentException("Macro name must not be empty.", nameof(name));
+        }
+
+        definition.Validate();
+        macroGates[name] = new MacroInfo(definition, Lut: null);
+        compiled = false;
+    }
+
+    public bool ComputeLut(string name, int maxSteps = 4096)
+    {
+        if (string.IsNullOrWhiteSpace(name))
+        {
+            throw new ArgumentException("Macro name must not be empty.", nameof(name));
+        }
+
+        if (maxSteps <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(maxSteps));
+        }
+
+        if (!macroGates.TryGetValue(name, out MacroInfo? macro))
+        {
+            throw new KeyNotFoundException($"Macro gate '{name}' is not registered.");
+        }
+
+        MacroLut? lut = TryBuildMacroLut(macro.Definition, maxSteps: maxSteps);
+        macroGates[name] = macro with { Lut = lut };
+        compiled = false;
+        return lut is not null;
+    }
+
+    public MacroInstance AddMacroGate(string name)
+    {
+        if (!macroGates.TryGetValue(name, out MacroInfo? macro))
+        {
+            throw new KeyNotFoundException($"Macro gate '{name}' is not registered.");
+        }
+
+        CircuitDefinition definition = macro.Definition;
+
+        if (macro.Lut is not null)
+        {
+            return AddMacroGateFromLut(name, macro.Lut);
+        }
+
+        // Inline (flatten) definition into this simulator by copying its gates and connections.
+        // Input pins are represented as Source gates in the definition, but Source gates cannot be driven by wires.
+        // For each input pin we substitute a Buffer gate, which can be driven externally and fans out internally.
+        int[] map = CopyDefinitionGatesAndConnections(
+            destination: this,
+            definition: definition,
+            mapKind: (gateId, kind) =>
+                kind == GateKind.Source
+                    ? (definition.InputPins.Contains(gateId) ? GateKind.Buffer : throw new InvalidOperationException("Only Source gates marked as InputPins are allowed inside a macro definition."))
+                    : kind);
+
+        int[] inputs = MapPins(definition.InputPins, map);
+        int[] outputs = MapPins(definition.OutputPins, map);
+
+        return new MacroInstance(name, inputs, outputs);
+    }
+
+    public void ClearSignals()
+    {
+        EnsureStorage();
+        Array.Clear(inputMasks);
+        Array.Clear(nextInputMasks);
+        Array.Clear(outputMasks);
+    }
+
+    public void SetSource(int gateId, bool value)
+    {
+        EnsureStorage();
+
+        if ((uint)gateId >= (uint)gateKinds.Count)
+        {
+            throw new ArgumentOutOfRangeException(nameof(gateId));
+        }
+
+        if (gateKinds[gateId] != GateKind.Source)
+        {
+            throw new InvalidOperationException("Gate is not a source.");
+        }
+
+        sourceStates[gateId] = value ? 1 : 0;
+    }
+
+    public bool GetOutput(int gateId)
+    {
+        EnsureStorage();
+        if ((uint)gateId >= (uint)gateKinds.Count)
+        {
+            throw new ArgumentOutOfRangeException(nameof(gateId));
+        }
+
+        return (outputMasks[gateId] & 1) != 0;
+    }
+
+    public void Step()
+    {
+        EnsureCompiled();
+        computeOutputs!(inputMasks, outputMasks, sourceStates);
+        Propagate();
+    }
+
+    public int RunUntilStable(int maxSteps = 1024)
+    {
+        if (!TryRunUntilStable(maxSteps, out int steps))
+        {
+            throw new InvalidOperationException($"Circuit did not stabilize within {maxSteps} steps.");
+        }
+
+        return steps;
+    }
+
+    private void Propagate()
+    {
+        Array.Clear(nextInputMasks);
+
+        for (int fromGate = 0; fromGate < gateKinds.Count; fromGate++)
+        {
+            if ((outputMasks[fromGate] & 1) == 0)
+            {
+                continue;
+            }
+
+            int start = edgeStart[fromGate];
+            int end = edgeStart[fromGate + 1];
+
+            for (int e = start; e < end; e++)
+            {
+                int toGate = edgeToGate[e];
+                nextInputMasks[toGate] |= 1 << edgeToInputBit[e];
+            }
+        }
+
+        (inputMasks, nextInputMasks) = (nextInputMasks, inputMasks);
+    }
+
+    private bool PropagateAndSwapDetectChange()
+    {
+        Array.Clear(nextInputMasks);
+
+        for (int fromGate = 0; fromGate < gateKinds.Count; fromGate++)
+        {
+            if ((outputMasks[fromGate] & 1) == 0)
+            {
+                continue;
+            }
+
+            int start = edgeStart[fromGate];
+            int end = edgeStart[fromGate + 1];
+
+            for (int e = start; e < end; e++)
+            {
+                int toGate = edgeToGate[e];
+                nextInputMasks[toGate] |= 1 << edgeToInputBit[e];
+            }
+        }
+
+        bool changed = false;
+        for (int i = 0; i < inputMasks.Length; i++)
+        {
+            if (inputMasks[i] != nextInputMasks[i])
+            {
+                changed = true;
+                break;
+            }
+        }
+
+        (inputMasks, nextInputMasks) = (nextInputMasks, inputMasks);
+        return changed;
+    }
+
+    private void EnsureStorage()
+    {
+        int n = gateKinds.Count;
+        if (inputMasks.Length == n)
+        {
+            return;
+        }
+
+        inputMasks = new int[n];
+        nextInputMasks = new int[n];
+        outputMasks = new int[n];
+        sourceStates = new int[n];
+    }
+
+    private void EnsureCompiled()
+    {
+        EnsureStorage();
+
+        if (compiled)
+        {
+            return;
+        }
+
+        CompileNetlist();
+        CompileLuts();
+        computeOutputs = CompileComputeOutputsExpr();
+        compiled = true;
+    }
+
+    private void CompileNetlist()
+    {
+        int n = gateKinds.Count;
+        edgeStart = new int[n + 1];
+
+        for (int i = 0; i < connections.Count; i++)
+        {
+            (int from, _, _) = connections[i];
+            edgeStart[from + 1]++;
+        }
+
+        for (int i = 1; i < edgeStart.Length; i++)
+        {
+            edgeStart[i] += edgeStart[i - 1];
+        }
+
+        edgeToGate = new int[connections.Count];
+        edgeToInputBit = new byte[connections.Count];
+
+        int[] cursor = (int[])edgeStart.Clone();
+        for (int i = 0; i < connections.Count; i++)
+        {
+            (int from, int to, byte bit) = connections[i];
+            int at = cursor[from]++;
+            edgeToGate[at] = to;
+            edgeToInputBit[at] = bit;
+        }
+    }
+
+    private void CompileLuts()
+    {
+        int n = gateKinds.Count;
+        lutOffsets = new int[n];
+        lutMasks = new int[n];
+
+        int total = 0;
+        for (int i = 0; i < n; i++)
+        {
+            if (gateKinds[i] != GateKind.Lut)
+            {
+                continue;
+            }
+
+            int[]? table = lutTableByGate[i];
+            if (table is null)
+            {
+                throw new InvalidOperationException("LUT gate is missing its truth table.");
+            }
+
+            if (!IsPowerOfTwo(table.Length))
+            {
+                throw new InvalidOperationException("LUT table length must be a power of two.");
+            }
+
+            lutOffsets[i] = total;
+            lutMasks[i] = table.Length - 1;
+            total += table.Length;
+        }
+
+        lutData = new int[total];
+        int cursor = 0;
+        for (int i = 0; i < n; i++)
+        {
+            if (gateKinds[i] != GateKind.Lut)
+            {
+                continue;
+            }
+
+            int[] table = lutTableByGate[i]!;
+            Array.Copy(table, 0, lutData, cursor, table.Length);
+            cursor += table.Length;
+        }
+    }
+
+    private Action<int[], int[], int[]> CompileComputeOutputsExpr()
+    {
+        ParameterExpression inputsParam = Expression.Parameter(typeof(int[]), "inputs");
+        ParameterExpression outputsParam = Expression.Parameter(typeof(int[]), "outputs");
+        ParameterExpression sourcesParam = Expression.Parameter(typeof(int[]), "sources");
+
+        ConstantExpression lutDataConst = Expression.Constant(lutData);
+
+        Expression[] block = new Expression[gateKinds.Count];
+
+        for (int i = 0; i < gateKinds.Count; i++)
+        {
+            ConstantExpression idx = Expression.Constant(i);
+            Expression inMask = Expression.ArrayIndex(inputsParam, idx);
+
+            Expression outExpr = gateKinds[i] switch
+            {
+                GateKind.Source => Expression.And(Expression.ArrayIndex(sourcesParam, idx), Expression.Constant(1)),
+
+                GateKind.Not => Expression.Condition(
+                    Expression.Equal(Expression.And(inMask, Expression.Constant(1)), Expression.Constant(0)),
+                    Expression.Constant(1),
+                    Expression.Constant(0)),
+
+                GateKind.And2 => Expression.Condition(
+                    Expression.Equal(Expression.And(inMask, Expression.Constant(0b11)), Expression.Constant(0b11)),
+                    Expression.Constant(1),
+                    Expression.Constant(0)),
+
+                GateKind.Or2 => Expression.Condition(
+                    Expression.NotEqual(Expression.And(inMask, Expression.Constant(0b11)), Expression.Constant(0)),
+                    Expression.Constant(1),
+                    Expression.Constant(0)),
+
+                GateKind.Buffer => Expression.Condition(
+                    Expression.NotEqual(Expression.And(inMask, Expression.Constant(1)), Expression.Constant(0)),
+                    Expression.Constant(1),
+                    Expression.Constant(0)),
+
+                GateKind.Sink => Expression.Condition(
+                    Expression.NotEqual(Expression.And(inMask, Expression.Constant(1)), Expression.Constant(0)),
+                    Expression.Constant(1),
+                    Expression.Constant(0)),
+
+                GateKind.Lut => Expression.ArrayIndex(
+                    lutDataConst,
+                    Expression.Add(
+                        Expression.Constant(lutOffsets[i]),
+                        Expression.And(inMask, Expression.Constant(lutMasks[i])))),
+
+                _ => throw new ArgumentOutOfRangeException(),
+            };
+
+            block[i] = Expression.Assign(Expression.ArrayAccess(outputsParam, idx), outExpr);
+        }
+
+        BlockExpression body = Expression.Block(block);
+        return Expression.Lambda<Action<int[], int[], int[]>>(body, inputsParam, outputsParam, sourcesParam).Compile();
+    }
+
+    private static bool IsPowerOfTwo(int value) => value > 0 && (value & (value - 1)) == 0;
+
+    private MacroLut? TryBuildMacroLut(CircuitDefinition definition, int maxSteps)
+    {
+        int inputCount = definition.InputPins.Count;
+        int outputCount = definition.OutputPins.Count;
+
+        if (inputCount < 0 || outputCount <= 0)
+        {
+            return null;
+        }
+
+        if (inputCount > 30)
+        {
+            throw new InvalidOperationException("Cannot build a LUT for macros with more than 30 inputs (would overflow 32-bit indexing).");
+        }
+
+        int patterns = 1 << inputCount;
+
+        var sim = new ExprCircuitSimulator();
+        foreach ((string macroName, MacroInfo macroInfo) in macroGates)
+        {
+            sim.macroGates[macroName] = macroInfo;
+        }
+        int[] map = CopyDefinitionGatesAndConnections(sim, definition, static (_, kind) => kind);
+        int[] inGates = MapPins(definition.InputPins, map);
+        int[] outGates = MapPins(definition.OutputPins, map);
+
+        int[][] outputTables = new int[outputCount][];
+        for (int o = 0; o < outputCount; o++)
+        {
+            outputTables[o] = new int[patterns];
+        }
+
+        for (int pattern = 0; pattern < patterns; pattern++)
+        {
+            sim.ClearSignals();
+
+            for (int i = 0; i < inputCount; i++)
+            {
+                bool bit = ((pattern >> i) & 1) != 0;
+                sim.SetSource(inGates[i], bit);
+            }
+
+            if (!sim.TryRunUntilStable(maxSteps: maxSteps, out _))
+            {
+                return null;
+            }
+
+            for (int o = 0; o < outputCount; o++)
+            {
+                outputTables[o][pattern] = sim.GetOutput(outGates[o]) ? 1 : 0;
+            }
+        }
+
+        return new MacroLut(inputCount, outputCount, outputTables);
+    }
+
+    private MacroInstance AddMacroGateFromLut(string name, MacroLut lut)
+    {
+        int[] inputs = new int[lut.InputCount];
+        for (int i = 0; i < inputs.Length; i++)
+        {
+            inputs[i] = AddGate(GateKind.Buffer);
+        }
+
+        int[] outputs = new int[lut.OutputCount];
+        for (int o = 0; o < outputs.Length; o++)
+        {
+            int lutGate = AddLutGate(lut.InputCount, lut.OutputTables[o]);
+            for (int i = 0; i < inputs.Length; i++)
+            {
+                ConnectGates(inputs[i], lutGate, toInputBit: i);
+            }
+
+            int sink = AddGate(GateKind.Sink);
+            ConnectGates(lutGate, sink, toInputBit: 0);
+            outputs[o] = sink;
+        }
+
+        return new MacroInstance(name, inputs, outputs);
+    }
+
+    public bool TryRunUntilStable(int maxSteps, out int steps)
+    {
+        steps = 0;
+        if (maxSteps <= 0)
+        {
+            return false;
+        }
+
+        EnsureCompiled();
+
+        while (steps < maxSteps)
+        {
+            steps++;
+            computeOutputs!(inputMasks, outputMasks, sourceStates);
+            if (!PropagateAndSwapDetectChange())
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    private static int[] CopyDefinitionGatesAndConnections(
+        ExprCircuitSimulator destination,
+        CircuitDefinition definition,
+        Func<int, GateKind, GateKind> mapKind)
+    {
+        int gateCount = definition.GateKinds.Count;
+        int[] map = new int[gateCount];
+        Array.Fill(map, -1);
+
+        // Expand nested macro instances by mapping their placeholder pin gates directly to the instantiated sub-macro pins.
+        for (int i = 0; i < definition.MacroInstances.Count; i++)
+        {
+            CircuitDefinition.MacroInstanceDef instanceDef = definition.MacroInstances[i];
+            MacroInstance instance = destination.AddMacroGate(instanceDef.Name);
+
+            if (instance.Inputs.Length != instanceDef.Inputs.Length || instance.Outputs.Length != instanceDef.Outputs.Length)
+            {
+                throw new InvalidOperationException($"Macro instance '{instanceDef.Name}' pin counts do not match the referenced macro definition.");
+            }
+
+            for (int p = 0; p < instanceDef.Inputs.Length; p++)
+            {
+                int gateId = instanceDef.Inputs[p];
+                if (map[gateId] != -1) throw new InvalidOperationException("Macro pin gate was mapped more than once.");
+                map[gateId] = instance.Inputs[p];
+            }
+
+            for (int p = 0; p < instanceDef.Outputs.Length; p++)
+            {
+                int gateId = instanceDef.Outputs[p];
+                if (map[gateId] != -1) throw new InvalidOperationException("Macro pin gate was mapped more than once.");
+                map[gateId] = instance.Outputs[p];
+            }
+        }
+
+        for (int i = 0; i < gateCount; i++)
+        {
+            if (map[i] != -1)
+            {
+                continue;
+            }
+
+            map[i] = destination.AddGate(mapKind(i, definition.GateKinds[i]));
+        }
+
+        for (int i = 0; i < definition.Connections.Count; i++)
+        {
+            (int from, int to, byte bit) = definition.Connections[i];
+            destination.ConnectGates(map[from], map[to], bit);
+        }
+
+        return map;
+    }
+
+    private static int[] MapPins(IReadOnlyList<int> pins, int[] map)
+    {
+        int[] result = new int[pins.Count];
+        for (int i = 0; i < result.Length; i++)
+        {
+            result[i] = map[pins[i]];
+        }
+
+        return result;
+    }
+}
diff --git a/StoneRed.LogicSimulator.Test/Program.cs b/StoneRed.LogicSimulator.Test/Program.cs
new file mode 100644
index 0000000..d9f4979
--- /dev/null
+++ b/StoneRed.LogicSimulator.Test/Program.cs
@@ -0,0 +1,46 @@
+namespace StoneRed.LogicSimulator.Test;
+
+internal static class Program
+{
+    public static void Main()
+    {
+        var sim = new ExprCircuitSimulator();
+
+        var inverter = new CircuitDefinition();
+        int invIn = inverter.AddInputPin();   // Source inside definition
+        int invNot = inverter.AddGate(GateKind.Not);
+        int invOut = inverter.AddOutputPin(); // Sink inside definition
+        inverter.Connect(invIn, invNot, toInputBit: 0);
+        inverter.Connect(invNot, invOut, toInputBit: 0);
+
+        sim.RegisterMacroGate("INV", inverter);
+        sim.ComputeLut("INV");
+
+        var doubleInverter = new CircuitDefinition();
+        int inv2In = doubleInverter.AddInputPin();
+        CircuitDefinition.MacroInstanceDef invA = doubleInverter.AddMacroInstance("INV", inputCount: 1, outputCount: 1);
+        CircuitDefinition.MacroInstanceDef invB = doubleInverter.AddMacroInstance("INV", inputCount: 1, outputCount: 1);
+        int inv2Out = doubleInverter.AddOutputPin();
+        doubleInverter.Connect(inv2In, invA.Inputs[0], toInputBit: 0);
+        doubleInverter.Connect(invA.Outputs[0], invB.Inputs[0], toInputBit: 0);
+        doubleInverter.Connect(invB.Outputs[0], inv2Out, toInputBit: 0);
+
+        sim.RegisterMacroGate("INV2", doubleInverter);
+        sim.ComputeLut("INV2");
+
+        int a = sim.AddGate(GateKind.Source);
+        ExprCircuitSimulator.MacroInstance inv2 = sim.AddMacroGate("INV2");
+        int lamp = sim.AddGate(GateKind.Sink);
+
+        sim.ConnectGates(a, inv2.Inputs[0], toInputBit: 0);
+        sim.ConnectGates(inv2.Outputs[0], lamp, toInputBit: 0);
+
+        sim.SetSource(a, value: false);
+        sim.RunUntilStable();
+        Console.WriteLine($"A=0 => Lamp={sim.GetOutput(lamp)} (expected False)");
+
+        sim.SetSource(a, value: true);
+        sim.RunUntilStable();
+        Console.WriteLine($"A=1 => Lamp={sim.GetOutput(lamp)} (expected True)");
+    }
+}
diff --git a/StoneRed.LogicSimulator.Test/StoneRed.LogicSimulator.Test.csproj b/StoneRed.LogicSimulator.Test/StoneRed.LogicSimulator.Test.csproj
new file mode 100644
index 0000000..129061d
--- /dev/null
+++ b/StoneRed.LogicSimulator.Test/StoneRed.LogicSimulator.Test.csproj
@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+  </ItemGroup>
+
+</Project>