// Copyright (c) 2026 Lark Technologies Pte. Ltd.
// SPDX-License-Identifier: MIT

package binding

import (
	"fmt"
	"strings"
)

// ReadJSONPointer navigates a parsed JSON value (typically the result of
// json.Unmarshal into interface{}) using an RFC 6901 JSON Pointer string.
//
// Supported pointer format: "/key/subkey/subsubkey".
// An empty pointer ("") returns data as-is.
// RFC 6901 escape sequences: ~1 → /, ~0 → ~.
//
// Limitation: only object (map) traversal is supported. Array index segments
// (e.g., "/channels/0/appId") are not implemented because OpenClaw's
// SecretRef file provider uses object-only paths in practice.
func ReadJSONPointer(data interface{}, pointer string) (interface{}, error) {
	if pointer == "" {
		return data, nil
	}

	if !strings.HasPrefix(pointer, "/") {
		return nil, fmt.Errorf("json pointer must start with '/' or be empty, got %q", pointer)
	}

	// Split after the leading "/" and decode each segment.
	segments := strings.Split(pointer[1:], "/")
	current := data

	for i, raw := range segments {
		// RFC 6901 unescaping: ~1 → /, ~0 → ~ (order matters).
		key, err := decodeJSONPointerSegment(raw)
		if err != nil {
			return nil, fmt.Errorf("json pointer %q: segment %q: %w", pointer, raw, err)
		}

		m, ok := current.(map[string]interface{})
		if !ok {
			traversed := "/" + strings.Join(segments[:i], "/")
			return nil, fmt.Errorf("json pointer %q: value at %q is %T, not an object",
				pointer, traversed, current)
		}

		val, exists := m[key]
		if !exists {
			return nil, fmt.Errorf("json pointer %q: key %q not found", pointer, key)
		}

		current = val
	}

	return current, nil
}

func decodeJSONPointerSegment(raw string) (string, error) {
	var out strings.Builder
	for i := 0; i < len(raw); i++ {
		if raw[i] != '~' {
			out.WriteByte(raw[i])
			continue
		}
		if i+1 >= len(raw) {
			return "", fmt.Errorf("invalid escape: ~ must be followed by 0 or 1")
		}
		switch raw[i+1] {
		case '0':
			out.WriteByte('~')
		case '1':
			out.WriteByte('/')
		default:
			return "", fmt.Errorf("invalid escape: ~%c must be ~0 or ~1", raw[i+1])
		}
		i++
	}
	return out.String(), nil
}