start work on porting completion to Go

tools/completion/parse-args.go

@@ -0,0 +1,82 @@
+// License: GPLv3 Copyright: 2022, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package completion
+
+import (
+	"strings"
+)
+
+func complete_word(word string, completions *Completions, only_args_allowed bool, expecting_arg_for *option) {
+	cmd := Completions.current_cmd
+	if expecting_arg_for != nil {
+		if expecting_arg_for.completion_for_arg != nil {
+			expecting_arg_for.completion_for_arg(completions, word)
+		}
+		return
+	}
+	if !only_args_allowed && strings.HasPrefix(word, "-") {
+		// handle single letter multiple options -abcd
+		// handle standalone --long-opt
+		// handle long opt ends with =
+		// handle long opt containing =
+		completions.add_options_group(cmd.options, word)
+		return
+	}
+	if cmd.completion_for_arg != nil {
+		cmd.completion_for_arg(completions, word)
+	}
+	return
+}
+
+func parse_args(cmd *command, words []string, completions *Completions) {
+	completions.current_cmd = cmd
+	if len(words) == 0 {
+		complete_word("", completions, false, nil)
+		return
+	}
+
+	var expecting_arg_for *option
+	only_args_allowed := false
+	arg_num := 0
+
+	for i, word := range words {
+		cmd = completions.current_cmd
+		is_last_word := i == len(words)-1
+		if is_last_word {
+			complete_word(word, completions, only_args_allowed, expecting_arg_for)
+		} else {
+			if expecting_arg_for != nil {
+				expecting_arg_for = nil
+				continue
+			}
+			if word == "--" {
+				only_args_allowed = true
+				continue
+			}
+			arg_num++
+			if !only_args_allowed && strings.HasPrefix(word, "-") {
+				// handle single letter multiple options -abcd
+				// handle standalone --long-opt
+				// handle long opt ends with =
+				// handle long opt containing =
+				continue
+			}
+			if len(cmd.subcommands) > 0 && arg_num == 1 {
+				sc := cmd.find_subcommand(word)
+				if sc == nil {
+					only_args_allowed = true
+					continue
+				}
+				completions.current_cmd = sc
+				cmd = sc
+				arg_num = 0
+				only_args_allowed = false
+			} else if cmd.stop_processing_at_arg > 0 && arg_num >= cmd.stop_processing_at_arg {
+				return
+			} else {
+				only_args_allowed = true
+				continue
+			}
+		}
+	}
+}