fdroidcl: add `list categories`

Useful to know what can go in `search -c`, either for humans or for
completion scripts.

Fixes #12.

cmd/fdroidcl/list.go

@@ -0,0 +1,47 @@
+// Copyright (c) 2015, Daniel Martí <mvdan@mvdan.cc>
+// See LICENSE for licensing information
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"sort"
+)
+
+var cmdList = &Command{
+	UsageLine: "list (categories)",
+	Short:     "List all known values of a kind",
+}
+
+func init() {
+	cmdList.Run = runList
+}
+
+func runList(args []string) {
+	if len(args) != 1 {
+		fmt.Fprintf(os.Stderr, "need exactly one argument")
+		cmdList.Flag.Usage()
+	}
+	apps := mustLoadIndexes()
+	values := make(map[string]struct{})
+	switch args[0] {
+	case "categories":
+		for _, app := range apps {
+			for _, c := range app.Categs {
+				values[c] = struct{}{}
+			}
+		}
+	default:
+		fmt.Fprintf(os.Stderr, "invalid argument")
+		cmdList.Flag.Usage()
+	}
+	result := make([]string, 0, len(values))
+	for s := range values {
+		result = append(result, s)
+	}
+	sort.Strings(result)
+	for _, s := range result {
+		fmt.Println(s)
+	}
+}

cmd/fdroidcl/main.go

@@ -148,6 +148,7 @@ var commands = []*Command{
 	cmdUpdate,
 	cmdSearch,
 	cmdShow,
+	cmdList,
 	cmdDevices,
 	cmdDownload,
 	cmdInstall,