Add simple CLI helper

Signed-off-by: Knut Ahlers <knut@ahlers.me>

cli/cli.go

@@ -0,0 +1,89 @@
+package cli
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"sort"
+	"strings"
+	"sync"
+)
+
+type (
+	// Registry contains a collection of commands to be executed
+	Registry struct {
+		cmds map[string]RegistryEntry
+		sync.Mutex
+	}
+
+	// Entry defines a sub-command with its parameters, description and
+	// run function to be called when this command is executed
+	RegistryEntry struct {
+		Description string
+		Name        string
+		Params      []string
+		Run         func([]string) error
+	}
+)
+
+// ErrHelpCalled is returned from the Call function if the given
+// command is not found and the help function was executed
+var ErrHelpCalled = errors.New("help called")
+
+// New creates a new Registry
+func New() *Registry {
+	return &Registry{
+		cmds: make(map[string]RegistryEntry),
+	}
+}
+
+// Add adds a new command to the Registry
+func (c *Registry) Add(e RegistryEntry) {
+	c.Lock()
+	defer c.Unlock()
+
+	c.cmds[e.Name] = e
+}
+
+// Call executes the matchign command from the given arguments
+func (c *Registry) Call(args []string) error {
+	c.Lock()
+	defer c.Unlock()
+
+	cmdEntry := c.cmds[args[0]]
+	if cmdEntry.Name != args[0] {
+		c.help()
+		return ErrHelpCalled
+	}
+
+	return cmdEntry.Run(args)
+}
+
+func (c *Registry) help() {
+	// Called from Call, does not need lock
+
+	var (
+		maxCmdLen int
+		cmds      []RegistryEntry
+	)
+
+	for name := range c.cmds {
+		entry := c.cmds[name]
+		if l := len(entry.commandDisplay()); l > maxCmdLen {
+			maxCmdLen = l
+		}
+		cmds = append(cmds, entry)
+	}
+
+	sort.Slice(cmds, func(i, j int) bool { return cmds[i].Name < cmds[j].Name })
+
+	tpl := fmt.Sprintf("  %%-%ds  %%s\n", maxCmdLen)
+	fmt.Fprintln(os.Stdout, "Supported sub-commands are:")
+	for _, cmd := range cmds {
+		fmt.Fprintf(os.Stdout, tpl, cmd.commandDisplay(), cmd.Description)
+	}
+}
+
+func (c RegistryEntry) commandDisplay() string {
+	return strings.Join(append([]string{c.Name}, c.Params...), " ")
+}