feat: Real Latency Measurement (TCP) [PR #12]

src-tauri/src/latency.rs

@@ -0,0 +1,51 @@
+use std::net::TcpStream;
+use std::time::{Duration, Instant};
+use tracing::{debug, warn};
+
+/// Measure real TCP latency to a server endpoint.
+/// Performs a TCP connect to the given host:port and returns the round-trip time in ms.
+pub fn measure_tcp_latency(host: &str, port: u16, timeout_ms: u64) -> Result<f64, String> {
+    let addr = format!("{}:{}", host, port);
+    let timeout = Duration::from_millis(timeout_ms);
+
+    let start = Instant::now();
+    match TcpStream::connect_timeout(
+        &addr.parse().map_err(|e| format!("Invalid address {}: {}", addr, e))?,
+        timeout,
+    ) {
+        Ok(_stream) => {
+            let elapsed = start.elapsed().as_secs_f64() * 1000.0;
+            debug!(host, port, latency_ms = elapsed, "TCP latency measured");
+            Ok(elapsed)
+        }
+        Err(e) => {
+            warn!(host, port, error = %e, "TCP latency measurement failed");
+            Err(format!("Connection to {} failed: {}", addr, e))
+        }
+    }
+}
+
+/// Measure latency with multiple samples and return the median.
+pub fn measure_latency_median(host: &str, port: u16, samples: usize) -> Result<f64, String> {
+    let mut results = Vec::with_capacity(samples);
+
+    for i in 0..samples {
+        match measure_tcp_latency(host, port, 5000) {
+            Ok(ms) => results.push(ms),
+            Err(e) => {
+                if i == 0 {
+                    return Err(e); // Fail fast if first attempt fails
+                }
+                warn!("Sample {} failed: {}", i, e);
+            }
+        }
+    }
+
+    if results.is_empty() {
+        return Err("All latency samples failed".to_string());
+    }
+
+    results.sort_by(|a, b| a.partial_cmp(b).unwrap());
+    let median = results[results.len() / 2];
+    Ok(median)
+}