package urlenc

import (
	"net/url"
	"encoding/json"
	"strings"
	"fmt"
)

func Unmarshal(bts []byte, v any) error {
	unesc, err := url.QueryUnescape(string(bts))
	if err != nil {
		return err
	}

	mp := map[string] any{}
	err = parseStr(unesc, mp)
	if err != nil {
		return err
	}

	js, err := json.Marshal(mp)
	if err != nil {
		return err
	}

	err = json.Unmarshal(js, v)
	if err != nil {
		return err
	}

	return nil
}

func parseStr(encodedString string, result map[string]any) error {
	// build nested map.
	var build func(map[string]any, []string, any) error

	build = func(result map[string]any, keys []string, value any) error {
		length := len(keys)
		// trim ',"
		key := strings.Trim(keys[0], "'\"")
		if length == 1 {
			result[key] = value
			return nil
		}

		// The end is slice. like f[], f[a][]
		if keys[1] == "" && length == 2 {
			// todo nested slice
			if key == "" {
				return nil
			}
			val, ok := result[key]
			if !ok {
				result[key] = []any{value}
				return nil
			}
			children, ok := val.([]any)
			if !ok {
				return fmt.Errorf("expected type '[]any' for key '%s', but got '%T'", key, val)
			}
			result[key] = append(children, value)
			return nil
		}

		// The end is slice + map. like f[][a]
		if keys[1] == "" && length > 2 && keys[2] != "" {
			val, ok := result[key]
			if !ok {
				result[key] = []any{}
				val = result[key]
			}
			children, ok := val.([]any)
			if !ok {
				return fmt.Errorf("expected type '[]any' for key '%s', but got '%T'", key, val)
			}
			if l := len(children); l > 0 {
				if child, ok := children[l-1].(map[string]any); ok {
					if _, ok := child[keys[2]]; !ok {
						_ = build(child, keys[2:], value)
						return nil
					}
				}
			}
			child := map[string]any{}
			_ = build(child, keys[2:], value)
			result[key] = append(children, child)

			return nil
		}

		// map. like f[a], f[a][b]
		val, ok := result[key]
		if !ok {
			result[key] = map[string]any{}
			val = result[key]
		}
		children, ok := val.(map[string]any)
		if !ok {
			return fmt.Errorf("expected type 'map[string]any' for key '%s', but got '%T'", key, val)
		}

		return build(children, keys[1:], value)
	}

	// split encodedString.
	parts := strings.Split(encodedString, "&")
	for _, part := range parts {
		pos := strings.Index(part, "=")
		if pos <= 0 {
			continue
		}
		key, err := url.QueryUnescape(part[:pos])
		if err != nil {
			return err
		}
		for key[0] == ' ' {
			key = key[1:]
		}
		if key == "" || key[0] == '[' {
			continue
		}
		value, err := url.QueryUnescape(part[pos+1:])
		if err != nil {
			return err
		}

		// split into multiple keys
		var keys []string
		left := 0
		for i, k := range key {
			if k == '[' && left == 0 {
				left = i
			} else if k == ']' {
				if left > 0 {
					if len(keys) == 0 {
						keys = append(keys, key[:left])
					}
					keys = append(keys, key[left+1:i])
					left = 0
					if i+1 < len(key) && key[i+1] != '[' {
						break
					}
				}
			}
		}
		if len(keys) == 0 {
			keys = append(keys, key)
		}
		// first key
		first := ""
		for i, chr := range keys[0] {
			if chr == ' ' || chr == '.' || chr == '[' {
				first += "_"
			} else {
				first += string(chr)
			}
			if chr == '[' {
				first += keys[0][i+1:]
				break
			}
		}
		keys[0] = first

		// build nested map
		if err := build(result, keys, value); err != nil {
			return err
		}
	}

	return nil
}