diff --git a/tests/testCircuit.cpp b/tests/testCircuit.cpp
new file mode 100644
index 0000000..a033b94
--- /dev/null
+++ b/tests/testCircuit.cpp
@@ -0,0 +1,259 @@
+#include "testHarness.h"
+#include "vf/Circuit.h"
+
+using namespace vf;
+
+namespace {
+
+void wireLine(World& world, int x0, int x1, int y, int z) {
+    for (int x = x0; x <= x1; ++x) world.setBlock(x, y, z, Block::RedstoneWire);
+}
+
+} // namespace
+
+VF_TEST(wirePropagatesWithFalloffToZeroAfterFifteenBlocks) {
+    World world;
+    world.setBlock(0, 0, 0, Block::RedstoneBlock);
+    wireLine(world, 1, 16, 0, 0);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {17, 1, 1});
+    circuit.tick();
+
+    for (int x = 1; x <= 15; ++x) CHECK_EQ(circuit.powerAt(x, 0, 0), redstoneMaxPower - (x - 1));
+    CHECK_EQ(circuit.powerAt(16, 0, 0), 0);
+}
+
+VF_TEST(wireDoesNotCrossThroughSolidBlocks) {
+    World world;
+    world.setBlock(0, 0, 0, Block::RedstoneBlock);
+    wireLine(world, 1, 3, 0, 0);
+    world.setBlock(4, 0, 0, Block::Stone);
+    wireLine(world, 5, 7, 0, 0);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {8, 1, 1});
+    circuit.tick();
+
+    CHECK(circuit.powerAt(3, 0, 0) > 0);
+    CHECK_EQ(circuit.powerAt(5, 0, 0), 0);
+    CHECK_EQ(circuit.powerAt(6, 0, 0), 0);
+}
+
+VF_TEST(leverTogglesWireOnAndOff) {
+    World world;
+    world.setBlock(5, 0, 5, Block::Lever);
+    world.setBlock(6, 0, 5, Block::RedstoneWire);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {10, 3, 10});
+
+    circuit.tick();
+    CHECK_EQ(circuit.powerAt(6, 0, 5), 0);
+    CHECK(!circuit.leverOn({5, 0, 5}));
+
+    circuit.setLever({5, 0, 5}, true);
+    circuit.tick();
+    CHECK(circuit.leverOn({5, 0, 5}));
+    CHECK_EQ(circuit.powerAt(6, 0, 5), redstoneMaxPower);
+
+    circuit.setLever({5, 0, 5}, false);
+    circuit.tick();
+    CHECK_EQ(circuit.powerAt(6, 0, 5), 0);
+}
+
+VF_TEST(buttonPowersThenExpires) {
+    World world;
+    world.setBlock(5, 0, 5, Block::Button);
+    world.setBlock(6, 0, 5, Block::RedstoneWire);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {10, 3, 10});
+    circuit.pressButton({5, 0, 5}, 3);
+
+    circuit.tick();
+    CHECK(circuit.buttonActive({5, 0, 5}));
+    CHECK_EQ(circuit.powerAt(6, 0, 5), redstoneMaxPower);
+
+    circuit.tick();
+    CHECK(circuit.buttonActive({5, 0, 5}));
+    CHECK_EQ(circuit.powerAt(6, 0, 5), redstoneMaxPower);
+
+    circuit.tick();
+    CHECK(!circuit.buttonActive({5, 0, 5}));
+    CHECK_EQ(circuit.powerAt(6, 0, 5), 0);
+
+    circuit.tick();
+    CHECK(!circuit.buttonActive({5, 0, 5}));
+    CHECK_EQ(circuit.powerAt(6, 0, 5), 0);
+}
+
+VF_TEST(torchInvertsThePowerStateOfItsMountBlock) {
+    World world;
+    world.setBlock(4, 0, 5, Block::RedstoneBlock);
+    world.setBlock(5, 0, 5, Block::Stone);
+    world.setBlock(5, 1, 5, Block::RedstoneTorch);
+    world.setBlock(6, 1, 5, Block::RedstoneWire);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {10, 3, 10});
+
+    circuit.tick();
+    CHECK_EQ(circuit.powerAt(6, 1, 5), redstoneMaxPower);   // mount not yet observed as powered
+
+    circuit.tick();
+    CHECK_EQ(circuit.powerAt(6, 1, 5), 0);   // mount observed powered, torch inverted off
+}
+
+VF_TEST(torchWithUnpoweredMountStaysLitIndefinitely) {
+    World world;
+    world.setBlock(5, 0, 5, Block::Stone);
+    world.setBlock(5, 1, 5, Block::RedstoneTorch);
+    world.setBlock(6, 1, 5, Block::RedstoneWire);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {10, 3, 10});
+
+    for (int i = 0; i < 5; ++i) {
+        circuit.tick();
+        CHECK_EQ(circuit.powerAt(6, 1, 5), redstoneMaxPower);
+    }
+}
+
+VF_TEST(repeaterDelaysOutputByConfiguredTicksAndRefreshesToFull) {
+    World world;
+    world.setBlock(5, 0, 5, Block::Lever);
+    world.setBlock(6, 0, 5, Block::Repeater);
+    wireLine(world, 7, 20, 0, 5);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {21, 1, 10});
+    circuit.setRepeater({6, 0, 5}, Face::PosX, 4);
+    circuit.setLever({5, 0, 5}, true);
+
+    circuit.tick();   // edge not visible yet (fresh field)
+    CHECK(!circuit.repeaterOutputOn({6, 0, 5}));
+    circuit.tick();   // edge observed, countdown starts at 4 and ticks to 3
+    CHECK(!circuit.repeaterOutputOn({6, 0, 5}));
+    circuit.tick();
+    CHECK(!circuit.repeaterOutputOn({6, 0, 5}));
+    circuit.tick();
+    CHECK(!circuit.repeaterOutputOn({6, 0, 5}));
+    circuit.tick();
+    CHECK(circuit.repeaterOutputOn({6, 0, 5}));
+    CHECK_EQ(circuit.powerAt(7, 0, 5), redstoneMaxPower);   // output refreshed to full strength
+}
+
+VF_TEST(repeaterWithMinimumDelayRespondsOneTickAfterTheEdge) {
+    World world;
+    world.setBlock(5, 0, 5, Block::Lever);
+    world.setBlock(6, 0, 5, Block::Repeater);
+    world.setBlock(7, 0, 5, Block::RedstoneWire);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {10, 3, 10});
+    circuit.setRepeater({6, 0, 5}, Face::PosX, 1);
+    circuit.setLever({5, 0, 5}, true);
+
+    circuit.tick();
+    CHECK(!circuit.repeaterOutputOn({6, 0, 5}));
+    circuit.tick();
+    CHECK(circuit.repeaterOutputOn({6, 0, 5}));
+}
+
+VF_TEST(repeaterIgnoresInputPerpendicularToItsFacing) {
+    World world;
+    world.setBlock(5, 0, 6, Block::Lever);          // powers from the side, not behind
+    world.setBlock(5, 0, 5, Block::Repeater);
+    world.setBlock(6, 0, 5, Block::RedstoneWire);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {10, 3, 10});
+    circuit.setRepeater({5, 0, 5}, Face::PosX, 1);
+    circuit.setLever({5, 0, 6}, true);
+
+    for (int i = 0; i < 4; ++i) circuit.tick();
+    CHECK(!circuit.repeaterOutputOn({5, 0, 5}));
+    CHECK_EQ(circuit.powerAt(6, 0, 5), 0);
+}
+
+VF_TEST(comparatorCompareModePassesBackWhenSideIsLower) {
+    World world;
+    world.setBlock(5, 0, 5, Block::RedstoneBlock);
+    world.setBlock(6, 0, 5, Block::Comparator);
+    world.setBlock(7, 0, 5, Block::RedstoneWire);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {10, 3, 10});
+    circuit.setComparator({6, 0, 5}, Face::PosX, ComparatorMode::Compare);
+
+    circuit.tick();
+    circuit.tick();
+    CHECK_EQ(circuit.powerAt(7, 0, 5), redstoneMaxPower);
+}
+
+VF_TEST(comparatorCompareModeBlocksWhenSideExceedsBack) {
+    World world;
+    world.setBlock(3, 0, 5, Block::RedstoneBlock);
+    wireLine(world, 4, 5, 0, 5);                     // back input falls off to 14 by the comparator
+    world.setBlock(6, 0, 5, Block::Comparator);
+    world.setBlock(7, 0, 5, Block::RedstoneWire);
+    world.setBlock(6, 0, 6, Block::RedstoneBlock);   // full-strength side input
+    RedstoneCircuit circuit(world, {0, 0, 0}, {10, 3, 10});
+    circuit.setComparator({6, 0, 5}, Face::PosX, ComparatorMode::Compare);
+
+    circuit.tick();
+    circuit.tick();
+    CHECK_EQ(circuit.powerAt(7, 0, 5), 0);
+}
+
+VF_TEST(comparatorSubtractModeSubtractsSideFromBack) {
+    World world;
+    world.setBlock(4, 0, 5, Block::RedstoneBlock);
+    wireLine(world, 5, 5, 0, 5);
+    world.setBlock(6, 0, 5, Block::Comparator);
+    world.setBlock(7, 0, 5, Block::RedstoneWire);
+    world.setBlock(6, 0, 6, Block::Lever);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {10, 3, 10});
+    circuit.setComparator({6, 0, 5}, Face::PosX, ComparatorMode::Subtract);
+    circuit.setLever({6, 0, 6}, true);
+
+    circuit.tick();
+    circuit.tick();
+    CHECK_EQ(circuit.powerAt(7, 0, 5), 0);   // full back (15) minus full side (15) = 0
+}
+
+VF_TEST(redstoneBlockIsAConstantSourceRegardlessOfTicks) {
+    World world;
+    world.setBlock(5, 0, 5, Block::RedstoneBlock);
+    wireLine(world, 6, 8, 0, 5);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {9, 1, 10});
+
+    for (int i = 0; i < 4; ++i) {
+        circuit.tick();
+        CHECK_EQ(circuit.powerAt(6, 0, 5), redstoneMaxPower);
+        CHECK_EQ(circuit.powerAt(7, 0, 5), redstoneMaxPower - 1);
+    }
+}
+
+VF_TEST(lampLitsWhenAdjacentToPowerAndDarkensWhenNot) {
+    World world;
+    world.setBlock(5, 0, 5, Block::Lever);
+    world.setBlock(6, 0, 5, Block::RedstoneLamp);
+    RedstoneCircuit circuit(world, {0, 0, 0}, {10, 3, 10});
+
+    circuit.tick();
+    circuit.applyLamps();
+    CHECK(world.blockAt(6, 0, 5) == Block::RedstoneLamp);
+
+    circuit.setLever({5, 0, 5}, true);
+    circuit.tick();
+    circuit.applyLamps();
+    CHECK(world.blockAt(6, 0, 5) == Block::RedstoneLampLit);
+
+    circuit.setLever({5, 0, 5}, false);
+    circuit.tick();
+    circuit.applyLamps();
+    CHECK(world.blockAt(6, 0, 5) == Block::RedstoneLamp);
+}
+
+VF_TEST(circuitIsDeterministicAcrossIdenticalRuns) {
+    World worldA, worldB;
+    for (World* w : {&worldA, &worldB}) {
+        w->setBlock(4, 0, 5, Block::RedstoneBlock);
+        w->setBlock(5, 0, 5, Block::Stone);
+        w->setBlock(5, 1, 5, Block::RedstoneTorch);
+        wireLine(*w, 6, 12, 1, 5);
+        w->setBlock(6, 1, 6, Block::Lever);
+    }
+    RedstoneCircuit a(worldA, {0, 0, 0}, {14, 3, 10});
+    RedstoneCircuit b(worldB, {0, 0, 0}, {14, 3, 10});
+    a.setLever({6, 1, 6}, true);
+    b.setLever({6, 1, 6}, true);
+
+    for (int i = 0; i < 6; ++i) {
+        a.tick();
+        b.tick();
+        for (int x = 6; x <= 12; ++x) CHECK_EQ(a.powerAt(x, 1, 5), b.powerAt(x, 1, 5));
+    }
+}