select

select/racer.go

@@ -0,0 +1,24 @@
+package racer
+
+import (
+	"net/http"
+)
+
+func Racer(a, b string) (winner string) {
+  select {
+  case <-ping(a):
+    return a
+  case <-ping(b):
+    return b
+  }
+}
+
+
+func ping(url string) chan struct{} {
+  ch := make(chan struct{})
+  go func() {
+    http.Get(url)
+    close(ch)
+  }()
+  return ch
+}

select/racer_test.go

@@ -0,0 +1,34 @@
+package racer
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestRacer(t *testing.T) {
+  slowServer := makeDelayedServer(20 * time.Millisecond)
+  fastServer := makeDelayedServer(0 * time.Millisecond)
+
+  defer slowServer.Close()
+  defer fastServer.Close()
+
+  slowURL := slowServer.URL
+	fastURL := fastServer.URL
+
+	want := fastURL
+	got := Racer(slowURL, fastURL)
+
+  if got != want {
+    t.Errorf("got %q, want %q", got, want)
+  }
+}
+
+func makeDelayedServer(delay time.Duration) *httptest.Server {
+  return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+    time.Sleep(delay)
+    w.WriteHeader(http.StatusOK)
+  }))
+
+}