diff --git a/src/fusion.cpp b/src/fusion.cpp
new file mode 100644
index 0000000..457fed2
--- /dev/null
+++ b/src/fusion.cpp
@@ -0,0 +1,23 @@
+#include "fusion.hpp"
+#include <cmath>
+
+FusionResult fuse(const ChannelAssessment& primary, const ChannelAssessment& secondary) {
+    bool a = primary.state == ChannelState::Healthy;
+    bool b = secondary.state == ChannelState::Healthy;
+    if (!a && !b) return {FusionState::Unavailable, std::nullopt, "none"};
+    if (a && !b) return {FusionState::Degraded, primary.reading.celsius, primary.reading.channel};
+    if (!a && b) return {FusionState::Degraded, secondary.reading.celsius, secondary.reading.channel};
+    if (std::abs(primary.reading.celsius - secondary.reading.celsius) > 3.0)
+        return {FusionState::Conflict, std::nullopt, "conflict"};
+    return {FusionState::Nominal, (primary.reading.celsius + secondary.reading.celsius) / 2.0, "dual"};
+}
+
+const char* state_name(FusionState state) {
+    switch (state) {
+        case FusionState::Nominal: return "nominal";
+        case FusionState::Degraded: return "degraded";
+        case FusionState::Conflict: return "conflict";
+        case FusionState::Unavailable: return "unavailable";
+    }
+    return "unknown";
+}