command: implement builder and parser

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

command/builder.go

@@ -0,0 +1,58 @@
+package command
+
+import (
+	"flag"
+	"fmt"
+	"io"
+)
+
+// New initialises a root Node.
+func New(output io.Writer, logf LogFunc, name string) Command {
+	return rootNode{newNode(output, logf, name, "")}
+}
+
+func newNode(output io.Writer, logf LogFunc, name, usage string) *node {
+	n := &node{
+		name: name, usage: usage,
+		out: output, logf: logf,
+		set: flag.NewFlagSet(name, flag.ContinueOnError),
+	}
+	n.set.SetOutput(output)
+	n.set.Usage = func() {
+		_ = n.writeHelp()
+		if n.suffix.Len() > 0 {
+			_, _ = fmt.Fprintln(output, "Flags:")
+			n.set.PrintDefaults()
+			_, _ = fmt.Fprintln(output)
+		}
+	}
+
+	return n
+}
+
+func (n *node) Command(name, usage string, f HandlerFunc) Node {
+	if f == nil {
+		panic("invalid handler")
+	}
+	if name == "" || usage == "" {
+		panic("invalid subcommand")
+	}
+
+	s := newNode(n.out, n.logf, name, usage)
+	s.f = f
+	if !n.adopt(s) {
+		panic("attempted to initialise subcommand with non-unique name")
+	}
+	return n
+}
+
+func (n *node) New(name, usage string) Node {
+	if name == "" || usage == "" {
+		panic("invalid subcommand tree")
+	}
+	s := newNode(n.out, n.logf, name, usage)
+	if !n.adopt(s) {
+		panic("attempted to initialise subcommand tree with non-unique name")
+	}
+	return s
+}