From d6cbf0f4aef0d5bc7428f261673d61068e642c96 Mon Sep 17 00:00:00 2001
From: Lucien Cartier-Tilet <lucien@phundrak.com>
Date: Sat, 3 Jan 2026 22:41:28 +0100
Subject: [PATCH] test(domain/relay): write failing tests for RelayState
 serialization

Tests verify serialization and deserialization of RelayState enum with
"on" and "off" states. Red phase of TDD - tests define expected behavior
before implementation.

Ref: T019 (specs/001-modbus-relay-control/tasks.md)
---
 backend/src/domain/relay/types/mod.rs        |  3 ++
 backend/src/domain/relay/types/relaystate.rs | 52 ++++++++++++++++++++
 specs/001-modbus-relay-control/tasks.md      |  2 +-
 3 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 backend/src/domain/relay/types/relaystate.rs

diff --git a/backend/src/domain/relay/types/mod.rs b/backend/src/domain/relay/types/mod.rs
index d5467b3..2edc683 100644
--- a/backend/src/domain/relay/types/mod.rs
+++ b/backend/src/domain/relay/types/mod.rs
@@ -1,2 +1,5 @@
 mod relayid;
+mod relaystate;
+
 pub use relayid::RelayId;
+pub use relaystate::RelayState;
diff --git a/backend/src/domain/relay/types/relaystate.rs b/backend/src/domain/relay/types/relaystate.rs
new file mode 100644
index 0000000..e651d27
--- /dev/null
+++ b/backend/src/domain/relay/types/relaystate.rs
@@ -0,0 +1,52 @@
+/// State of a relay (on or off).
+///
+/// Represents the binary state of a relay channel in the Modbus controller.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum RelayState {
+    /// Relay is energized (circuit closed).
+    On,
+    /// Relay is de-energized (circuit open).
+    Off,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_relay_state_on_serializes_to_on() {
+        // Test: RelayState::On → serializes to "on"
+        let state = RelayState::On;
+        let json = serde_json::to_string(&state).unwrap();
+        assert_eq!(json, r#""on""#);
+    }
+
+    #[test]
+    fn test_relay_state_off_serializes_to_off() {
+        // Test: RelayState::Off → serializes to "off"
+        let state = RelayState::Off;
+        let json = serde_json::to_string(&state).unwrap();
+        assert_eq!(json, r#""off""#);
+    }
+
+    #[test]
+    fn test_parse_on_from_string() {
+        // Test: Parse "on" from string
+        let state: RelayState = serde_json::from_str(r#""on""#).unwrap();
+        assert_eq!(state, RelayState::On);
+    }
+
+    #[test]
+    fn test_parse_off_from_string() {
+        // Test: Parse "off" from string
+        let state: RelayState = serde_json::from_str(r#""off""#).unwrap();
+        assert_eq!(state, RelayState::Off);
+    }
+
+    #[test]
+    fn test_parse_invalid_string_fails() {
+        // Test: Parse invalid string fails
+        let result: Result<RelayState, _> = serde_json::from_str(r#""invalid""#);
+        assert!(result.is_err());
+    }
+}
diff --git a/specs/001-modbus-relay-control/tasks.md b/specs/001-modbus-relay-control/tasks.md
index bddaa4c..ae5f6aa 100644
--- a/specs/001-modbus-relay-control/tasks.md
+++ b/specs/001-modbus-relay-control/tasks.md
@@ -218,7 +218,7 @@
   - **File**: src/domain/relay.rs
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T019** [P] [US1] [TDD] Write tests for RelayState enum
+- [x] **T019** [P] [US1] [TDD] Write tests for RelayState enum
   - Test: RelayState::On → serializes to "on"
   - Test: RelayState::Off → serializes to "off"
   - Test: Parse "on"/"off" from strings