// SPDX-License-Identifier: GPL-3.0-or-later

package plan

import (
	"fmt"
	"strconv"
	"strings"
)

// MaxIndex returns the highest {N} used in s, 0 when none. It reports the
// same errors Expand does for a malformed placeholder: an unclosed
// placeholder, or {0} (capture groups are numbered from 1). It shares
// Expand's scanner shape: "{{" and "}}" each emit one literal brace, and any
// other "{" opens a placeholder that runs to the next "}".
func MaxIndex(s string) (int, error) {
	max := 0
	for i := 0; i < len(s); {
		switch {
		case s[i] == '{' && i+1 < len(s) && s[i+1] == '{':
			i += 2
		case s[i] == '}' && i+1 < len(s) && s[i+1] == '}':
			i += 2
		case s[i] == '{':
			end := strings.IndexByte(s[i+1:], '}')
			if end < 0 {
				return 0, fmt.Errorf("unclosed placeholder")
			}
			body := s[i+1 : i+1+end]
			if n, err := strconv.Atoi(body); err == nil {
				if n == 0 {
					return 0, fmt.Errorf("capture groups are numbered from 1")
				}
				if n >= 1 && n <= 9 && n > max {
					max = n
				}
			}
			i += end + 2
		default:
			i++
		}
	}
	return max, nil
}