// Package pagination defines bounded offset pagination with camelCase fields.
package pagination

import (
	"errors"
	"net/url"
	"strconv"
)

type Config struct {
	DefaultSize int   `json:"defaultSize"`
	MaxSize     int   `json:"maxSize"`
	MaxOffset   int64 `json:"maxOffset"`
}
type Request struct {
	Page     int64 `json:"page"`
	PageSize int   `json:"pageSize"`
}

func (r Request) Offset() int64 { return (r.Page - 1) * int64(r.PageSize) }
func Parse(query url.Values, cfg Config) (Request, error) {
	if cfg.DefaultSize == 0 {
		cfg.DefaultSize = 20
	}
	if cfg.MaxSize == 0 {
		cfg.MaxSize = 100
	}
	if cfg.MaxOffset == 0 {
		cfg.MaxOffset = 1000000
	}
	if cfg.DefaultSize < 1 || cfg.MaxSize < cfg.DefaultSize || cfg.MaxSize > 10000 || cfg.MaxOffset < 0 {
		return Request{}, errors.New("pagination: invalid configuration")
	}
	page, size := int64(1), int64(cfg.DefaultSize)
	for key, target := range map[string]*int64{"page": &page, "pageSize": &size} {
		if v, ok := query[key]; ok {
			if len(v) != 1 {
				return Request{}, errors.New("pagination: duplicate parameter")
			}
			n, err := strconv.ParseInt(v[0], 10, 64)
			if err != nil || n < 1 {
				return Request{}, errors.New("pagination: positive integer required")
			}
			*target = n
		}
	}
	if size > int64(cfg.MaxSize) || (page-1) > cfg.MaxOffset/size {
		return Request{}, errors.New("pagination: size or offset exceeds limit")
	}
	return Request{Page: page, PageSize: int(size)}, nil
}

type Result[T any] struct {
	Items      []T   `json:"items"`
	Page       int64 `json:"page"`
	PageSize   int   `json:"pageSize"`
	Total      int64 `json:"total"`
	TotalPages int64 `json:"totalPages"`
}

func NewResult[T any](r Request, items []T, total int64) (Result[T], error) {
	if r.Page < 1 || r.PageSize < 1 || total < 0 || len(items) > r.PageSize || int64(len(items)) > total {
		return Result[T]{}, errors.New("pagination: invalid result")
	}
	if items == nil {
		items = []T{}
	}
	pages := total / int64(r.PageSize)
	if total%int64(r.PageSize) != 0 {
		pages++
	}
	return Result[T]{Items: items, Page: r.Page, PageSize: r.PageSize, Total: total, TotalPages: pages}, nil
}