test: add tests for ensure_config function

Co-authored-by: aider (gemini/gemini-2.5-pro-preview-05-06) <aider@aider.chat>

src/main.rs

@@ -1,81 +1,127 @@
-// src/main.rs
-
-mod config;
-mod sync_logic;
-mod serial_input;
-mod ui;
-
-use crate::config::watch_config;
-use crate::sync_logic::LtcState;
-use crate::serial_input::start_serial_thread;
-use crate::ui::start_ui;
-
-use std::{
-    fs,
-    path::Path,
-    sync::{Arc, Mutex, mpsc},
-    thread,
-};
-
-/// Embed the default config.json at compile time.
-const DEFAULT_CONFIG: &str = include_str!("../config.json");
-
-/// If no `config.json` exists alongside the binary, write out the default.
-fn ensure_config() {
-    let p = Path::new("config.json");
-    if !p.exists() {
-        fs::write(p, DEFAULT_CONFIG)
-            .expect("Failed to write default config.json");
-        eprintln!("⚙️  Emitted default config.json");
-    }
-}
-
-fn main() {
-    // 🔄 Ensure there's always a config.json present
-    ensure_config();
-
-    // 1️⃣ Start watching config.json for changes
-    let hw_offset = watch_config("config.json");
-    println!("🔧 Watching config.json (hardware_offset_ms)...");
-
-    // 2️⃣ Channel for raw LTC frames
-    let (tx, rx) = mpsc::channel();
-    println!("✅ Channel created");
-
-    // 3️⃣ Shared state for UI and serial reader
-    let ltc_state = Arc::new(Mutex::new(LtcState::new()));
-    println!("✅ State initialised");
-
-    // 4️⃣ Spawn the serial reader thread (no offset here)
-    {
-        let tx_clone    = tx.clone();
-        let state_clone = ltc_state.clone();
-        thread::spawn(move || {
-            println!("🚀 Serial thread launched");
-            start_serial_thread(
-                "/dev/ttyACM0",
-                115200,
-                tx_clone,
-                state_clone,
-                0, // ignored in serial path
-            );
-        });
-    }
-
-    // 5️⃣ Spawn the UI renderer thread, passing the live offset Arc
-    {
-        let ui_state     = ltc_state.clone();
-        let offset_clone = hw_offset.clone();
-        let port         = "/dev/ttyACM0".to_string();
-        thread::spawn(move || {
-            println!("🖥️ UI thread launched");
-            start_ui(ui_state, port, offset_clone);
-        });
-    }
-
-    // 6️⃣ Keep main thread alive
-    println!("📡 Main thread entering loop...");
-    for _frame in rx {
-        // no-op
-    }
-}
+// src/main.rs
+
+mod config;
+mod sync_logic;
+mod serial_input;
+mod ui;
+
+use crate::config::watch_config;
+use crate::sync_logic::LtcState;
+use crate::serial_input::start_serial_thread;
+use crate::ui::start_ui;
+
+use std::{
+    fs,
+    path::Path,
+    sync::{Arc, Mutex, mpsc},
+    thread,
+};
+
+/// Embed the default config.json at compile time.
+const DEFAULT_CONFIG: &str = include_str!("../config.json");
+
+/// If no `config.json` exists alongside the binary, write out the default.
+fn ensure_config() {
+    let p = Path::new("config.json");
+    if !p.exists() {
+        fs::write(p, DEFAULT_CONFIG)
+            .expect("Failed to write default config.json");
+        eprintln!("⚙️  Emitted default config.json");
+    }
+}
+
+fn main() {
+    // 🔄 Ensure there's always a config.json present
+    ensure_config();
+
+    // 1️⃣ Start watching config.json for changes
+    let hw_offset = watch_config("config.json");
+    println!("🔧 Watching config.json (hardware_offset_ms)...");
+
+    // 2️⃣ Channel for raw LTC frames
+    let (tx, rx) = mpsc::channel();
+    println!("✅ Channel created");
+
+    // 3️⃣ Shared state for UI and serial reader
+    let ltc_state = Arc::new(Mutex::new(LtcState::new()));
+    println!("✅ State initialised");
+
+    // 4️⃣ Spawn the serial reader thread (no offset here)
+    {
+        let tx_clone    = tx.clone();
+        let state_clone = ltc_state.clone();
+        thread::spawn(move || {
+            println!("🚀 Serial thread launched");
+            start_serial_thread(
+                "/dev/ttyACM0",
+                115200,
+                tx_clone,
+                state_clone,
+                0, // ignored in serial path
+            );
+        });
+    }
+
+    // 5️⃣ Spawn the UI renderer thread, passing the live offset Arc
+    {
+        let ui_state     = ltc_state.clone();
+        let offset_clone = hw_offset.clone();
+        let port         = "/dev/ttyACM0".to_string();
+        thread::spawn(move || {
+            println!("🖥️ UI thread launched");
+            start_ui(ui_state, port, offset_clone);
+        });
+    }
+
+    // 6️⃣ Keep main thread alive
+    println!("📡 Main thread entering loop...");
+    for _frame in rx {
+        // no-op
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs;
+    use std::path::Path;
+
+    /// RAII guard to ensure config file is cleaned up after test.
+    struct ConfigGuard;
+
+    impl Drop for ConfigGuard {
+        fn drop(&mut self) {
+            let _ = fs::remove_file("config.json");
+        }
+    }
+
+    #[test]
+    fn test_ensure_config() {
+        let _guard = ConfigGuard; // Cleanup when _guard goes out of scope.
+
+        // --- Test 1: File creation ---
+        // Pre-condition: config.json does not exist.
+        let _ = fs::remove_file("config.json");
+
+        ensure_config();
+
+        // Post-condition: config.json exists and has default content.
+        let p = Path::new("config.json");
+        assert!(p.exists(), "config.json should have been created");
+        let contents = fs::read_to_string(p).expect("Failed to read created config.json");
+        assert_eq!(contents, DEFAULT_CONFIG, "config.json content should match default");
+
+        // --- Test 2: File is not overwritten ---
+        // Pre-condition: config.json exists with different content.
+        let custom_content = "{\"hardware_offset_ms\": 999}";
+        fs::write("config.json", custom_content)
+            .expect("Failed to write custom config.json for test");
+
+        ensure_config();
+
+        // Post-condition: config.json still has the custom content.
+        let contents_after = fs::read_to_string("config.json")
+            .expect("Failed to read config.json after second ensure_config call");
+        assert_eq!(contents_after, custom_content, "config.json should not be overwritten");
+    }
+}