Add --latest flag

Allow using the latest version of the Unifi controller with `--latest`.

fields/version.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/hashicorp/go-version"
+)
+
+var uiDownloadUrl = "https://www.ui.com/download/?platform=unifi"
+
+func latestUnifiVersion() (*version.Version, error) {
+	client := &http.Client{}
+
+	req, err := http.NewRequest("GET", uiDownloadUrl, nil)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Add("X-Requested-With", "XMLHttpRequest")
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+
+	var respData struct {
+		Downloads []struct {
+			Id           int    `json:"id"`
+			CategorySlug string `json:"category__slug"`
+			Filename     string `json:"filename"`
+			Version      string `json:"version"`
+		} `json:"downloads"`
+	}
+	var latestVersion *version.Version
+
+	err = json.NewDecoder(resp.Body).Decode(&respData)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, download := range respData.Downloads {
+		if download.CategorySlug != "software" {
+			continue
+		}
+
+		if download.Filename != "unifi_sysvinit_all.deb" {
+			continue
+		}
+
+		downloadVersion, err := version.NewVersion(download.Version)
+		if err != nil {
+			// Skip this entry if the version isn't valid.
+			continue
+		}
+
+		if latestVersion == nil || downloadVersion.GreaterThan(latestVersion) {
+			latestVersion = downloadVersion
+		}
+	}
+
+	return latestVersion, nil
+}