service/graphql/connection/connection.go

package connection

import (
	"errors"
	"fmt"
	"github.com/samber/lo"
	"math"
)

type Connection[T Cursor] struct {
	Edges    []T       `json:"edges"`
	PageInfo *PageInfo `json:"page_info"`
}

func (c *Connection[T]) Reverse() {
	c.Edges = lo.Reverse[T](c.Edges)
}

func (c *Connection[T]) Grain(after, before *string) error {
	if after != nil && before != nil {
		return errors.New("illegal arguments: only one of `after` or `before` can be used at a time")
	}

	if len(c.Edges) == 0 {
		c.PageInfo = new(PageInfo)
		return nil
	}

	edges := lo.FlatMap[T, string](c.Edges, func(item T, _ int) []string {
		return []string{item.Cursor()}
	})

	c.PageInfo = &PageInfo{
		StartCursor: lo.ToPtr(edges[0]),
		EndCursor:   lo.ToPtr(edges[(len(edges) - 1)]),
	}

	if after != nil {
		if idx := lo.IndexOf(edges, *after); idx != -1 {
			c.Edges = lo.Slice[T](c.Edges, idx+1, math.MaxUint32)
		}
	}

	if before != nil {
		if idx := lo.IndexOf(edges, *before); idx != -1 {
			c.Edges = lo.Subset[T](c.Edges, -idx, math.MaxUint32)
		}
	}

	return nil
}

func (c *Connection[T]) Slice(first, last *int) error {
	if first != nil && last != nil {
		return fmt.Errorf("illegal agruments: only one of `first` or `last` can be used at a time")
	}

	if first != nil {
		c.Edges = lo.Slice[T](c.Edges, 0, *first)
	}

	if last != nil {
		c.Edges = lo.Subset[T](c.Edges, -*last, math.MaxUint32)
	}

	if len(c.Edges) < 1 {
		c.PageInfo.HasPreviousPage = c.PageInfo.StartCursor != nil
		c.PageInfo.HasNextPage = c.PageInfo.EndCursor != nil
		c.PageInfo.StartCursor = nil
		c.PageInfo.EndCursor = nil
	} else {
		c.PageInfo.HasPreviousPage = !(*c.PageInfo.StartCursor == c.Edges[0].Cursor())
		c.PageInfo.HasNextPage = !(*c.PageInfo.EndCursor == c.Edges[len(c.Edges)-1].Cursor())
		c.PageInfo.StartCursor = lo.ToPtr(c.Edges[0].Cursor())
		c.PageInfo.EndCursor = lo.ToPtr(c.Edges[len(c.Edges)-1].Cursor())
	}

	return nil
}