Add Tree Selector

tree.go

@@ -0,0 +1,85 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+	"os"
+	"sort"
+	"strings"
+)
+
+type treeEntry struct {
+	Name string `json:"name"`
+	Kind string `json:"kind"`
+}
+
+type treeResponse struct {
+	Path    string      `json:"path"`
+	Entries []treeEntry `json:"entries"`
+}
+
+// handleTree responds with a JSON listing of the immediate children of the
+// folder at fsPath. Hidden entries and `index.md` are filtered. Files are not
+// descended — the client lazy-loads children on expand.
+func (h *handler) handleTree(w http.ResponseWriter, r *http.Request, urlPath, fsPath string) {
+	info, err := os.Stat(fsPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			http.NotFound(w, r)
+			return
+		}
+		http.Error(w, "stat failed", http.StatusInternalServerError)
+		return
+	}
+	if !info.IsDir() {
+		http.Error(w, "not a folder", http.StatusBadRequest)
+		return
+	}
+
+	entries, err := listTreeEntries(fsPath)
+	if err != nil {
+		http.Error(w, "read failed", http.StatusInternalServerError)
+		return
+	}
+
+	resp := treeResponse{Path: canonicalTreePath(urlPath), Entries: entries}
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	_ = json.NewEncoder(w).Encode(resp)
+}
+
+// canonicalTreePath returns the URL path in the form used by the picker:
+// "/" for root, otherwise stripped of any trailing slash.
+func canonicalTreePath(urlPath string) string {
+	if urlPath == "" || urlPath == "/" {
+		return "/"
+	}
+	return "/" + strings.Trim(urlPath, "/")
+}
+
+// listTreeEntries returns the immediate children of fsPath, filtering hidden
+// entries and index.md. Folders are listed before files; both groups are
+// sorted alphabetically.
+func listTreeEntries(fsPath string) ([]treeEntry, error) {
+	raw, err := os.ReadDir(fsPath)
+	if err != nil {
+		return nil, err
+	}
+	var folders, files []treeEntry
+	for _, e := range raw {
+		name := e.Name()
+		if strings.HasPrefix(name, ".") {
+			continue
+		}
+		if e.IsDir() {
+			folders = append(folders, treeEntry{Name: name, Kind: "folder"})
+		} else {
+			if name == "index.md" {
+				continue
+			}
+			files = append(files, treeEntry{Name: name, Kind: "file"})
+		}
+	}
+	sort.Slice(folders, func(i, j int) bool { return folders[i].Name < folders[j].Name })
+	sort.Slice(files, func(i, j int) bool { return files[i].Name < files[j].Name })
+	return append(folders, files...), nil
+}