// SPDX-FileCopyrightText: (C) Copyright 2026 Pim van Pelt <pim@ipng.ch>
// SPDX-License-Identifier: Apache-2.0

package cli

import (
	"errors"
	"fmt"
	"strings"
)

// Validate reports common authoring faults in a command tree. Walk tolerates
// all of them, so Validate is optional — but calling it once at startup, or
// from a unit test, catches mistakes that otherwise cause silent misdispatch.
// It returns all problems found, joined, or nil if the tree is clean.
//
// It reports:
//   - a node with more than one slot child (Dynamic != nil): only the first is
//     ever reachable (FR-1.7);
//   - a non-root node with an empty Word: it can never be matched or displayed;
//   - duplicate fixed (non-slot) words among a node's children: the second is
//     shadowed by the first;
//   - a node that neither runs nor has children: a dead end that can only print
//     "<no completions>".
//
// Circular slots (a slot that is its own descendant, e.g. a watch-options node)
// are traversed once, not looped.
func Validate[C any](root *Node[C]) error {
	var errs []error
	validateNode(root, "", true, make(map[*Node[C]]bool), &errs)
	return errors.Join(errs...)
}

func validateNode[C any](n *Node[C], path string, isRoot bool, seen map[*Node[C]]bool, errs *[]error) {
	if seen[n] {
		return
	}
	seen[n] = true

	where := path
	if where == "" {
		where = "<root>"
	}

	if !isRoot && n.Word == "" {
		*errs = append(*errs, fmt.Errorf("%s: node has an empty Word", where))
	}
	if !isRoot && n.Run == nil && len(n.Children) == 0 {
		*errs = append(*errs, fmt.Errorf("%s: node neither runs nor has children (dead end)", where))
	}

	slots := 0
	fixedWords := make(map[string]bool)
	for _, c := range n.Children {
		if c.Dynamic != nil {
			slots++
			continue
		}
		if c.Word != "" {
			if fixedWords[c.Word] {
				*errs = append(*errs, fmt.Errorf("%s: duplicate child word %q", where, c.Word))
			}
			fixedWords[c.Word] = true
		}
	}
	if slots > 1 {
		*errs = append(*errs, fmt.Errorf("%s: %d slot children, but only the first is reachable", where, slots))
	}

	for _, c := range n.Children {
		cp := c.Word
		if path != "" {
			cp = path + " " + c.Word
		}
		validateNode(c, strings.TrimSpace(cp), false, seen, errs)
	}
}