command: implement help builder

Signed-off-by: Ophestra <cat@gensokyo.uk>

command/command.go

@@ -1,7 +1,10 @@
 // Package command implements generic nested command parsing.
 package command
 
-import "flag"
+import (
+	"flag"
+	"strings"
+)
 
 type (
 	// HandlerFunc is called when matching a directly handled subcommand tree.
@@ -13,7 +16,7 @@ type (
 	// FlagDefiner is a deferred flag definer value, usually encapsulating the default value.
 	FlagDefiner interface {
 		// Define defines the flag in set.
-		Define(set *flag.FlagSet, p any, name, usage string)
+		Define(b *strings.Builder, set *flag.FlagSet, p any, name, usage string)
 	}
 
 	Command interface {

command/help.go

@@ -0,0 +1,49 @@
+package command
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+	"text/tabwriter"
+)
+
+var ErrHelp = errors.New("help requested")
+
+func (n *node) writeHelp() error {
+	if _, err := fmt.Fprintf(n.out,
+		"\nUsage:\t%s [-h | --help]%s COMMAND [OPTIONS]\n",
+		strings.Join(append(n.prefix, n.name), " "), &n.suffix,
+	); err != nil {
+		return err
+	}
+	if n.child != nil {
+		if _, err := fmt.Fprint(n.out, "\nCommands:\n"); err != nil {
+			return err
+		}
+	}
+
+	tw := tabwriter.NewWriter(n.out, 0, 1, 4, ' ', 0)
+	if err := n.child.writeCommands(tw); err != nil {
+		return err
+	}
+	if err := tw.Flush(); err != nil {
+		return err
+	}
+
+	_, err := n.out.Write([]byte{'\n'})
+	if err == nil {
+		err = ErrHelp
+	}
+	return err
+}
+
+func (n *node) writeCommands(w io.Writer) error {
+	if n == nil {
+		return nil
+	}
+	if _, err := fmt.Fprintf(w, "\t%s\t%s\n", n.name, n.usage); err != nil {
+		return err
+	}
+	return n.next.writeCommands(w)
+}

command/node.go

@@ -3,6 +3,7 @@ package command
 import (
 	"flag"
 	"io"
+	"strings"
 )
 
 type node struct {
@@ -12,6 +13,9 @@ type node struct {
 	out  io.Writer
 	logf LogFunc
 
+	prefix []string
+	suffix strings.Builder
+
 	f   HandlerFunc
 	set *flag.FlagSet
 }