package cursor

import (
	"errors"
)

const (
	// DefaultPageSize 默认页面大小
	DefaultPageSize = 20
	// MaxPageSize 最大页面大小
	MaxPageSize = 100
)

// Direction 分页方向
type Direction string

const (
	// Forward 向前分页（下一页）
	Forward Direction = "forward"
	// Backward 向后分页（上一页）
	Backward Direction = "backward"
)

// IsValid 检查分页方向是否有效
func (d Direction) IsValid() bool {
	return d == Forward || d == Backward
}

// PageRequest 游标分页请求参数
type PageRequest struct {
	Cursor    string    // 游标字符串
	Direction Direction // 分页方向
	PageSize  int       // 每页数量
}

// NewPageRequest 创建新的游标请求
func NewPageRequest(cursor string, direction Direction, pageSize int) *PageRequest {
	req := &PageRequest{
		Cursor:    cursor,
		Direction: direction,
		PageSize:  pageSize,
	}
	req.Normalize()
	return req
}

// Normalize 规范化分页参数
func (r *PageRequest) Normalize() {
	if r.Direction == "" {
		r.Direction = Forward
	}
	if r.PageSize <= 0 {
		r.PageSize = DefaultPageSize
	}
	if r.PageSize > MaxPageSize {
		r.PageSize = MaxPageSize
	}
}

// GetPageSize 获取有效的页面大小（默认20，最大100）
func (r *PageRequest) GetPageSize() int {
	if r.PageSize <= 0 {
		return DefaultPageSize
	}
	if r.PageSize > MaxPageSize {
		return MaxPageSize
	}
	return r.PageSize
}

// Validate 验证分页请求
func (r *PageRequest) Validate() error {
	if r.Direction != "" && !r.Direction.IsValid() {
		return errors.New("invalid direction, must be 'forward' or 'backward'")
	}
	return nil
}

// GetCursor 解码游标
func (r *PageRequest) GetCursor() (*Cursor, error) {
	return DecodeCursor(r.Cursor)
}

// IsForward 是否向前分页
func (r *PageRequest) IsForward() bool {
	return r.Direction == Forward || r.Direction == ""
}

// IsBackward 是否向后分页
func (r *PageRequest) IsBackward() bool {
	return r.Direction == Backward
}

// PageResponse 游标分页响应结构
type PageResponse struct {
	Items      any    `json:"items"`
	NextCursor string `json:"next_cursor"`
	PrevCursor string `json:"prev_cursor,omitempty"`
	HasMore    bool   `json:"has_more"`
}


// CursorPageResult 游标分页结果（泛型版本，供内部使用）
type CursorPageResult[T any] struct {
	Items      []T
	NextCursor string
	PrevCursor string
	HasMore    bool
}

// NewCursorPageResult 创建游标分页结果
func NewCursorPageResult[T any](items []T, nextCursor, prevCursor string, hasMore bool) *CursorPageResult[T] {
	return &CursorPageResult[T]{
		Items:      items,
		NextCursor: nextCursor,
		PrevCursor: prevCursor,
		HasMore:    hasMore,
	}
}

// ToPageResponse 转换为通用分页响应
func (r *CursorPageResult[T]) ToPageResponse() *PageResponse {
	return &PageResponse{
		Items:      r.Items,
		NextCursor: r.NextCursor,
		PrevCursor: r.PrevCursor,
		HasMore:    r.HasMore,
	}
}