pagination/pagination.go

package pagination

import (
	"encoding/base64"
	"fmt"
	"slices"
)

func Validate(first *int, after *string, last *int, before *string) error {
	if first != nil && last != nil {
		return fmt.Errorf("only one of first and last can be provided")
	}
	if first != nil && *first < 0 {
		return fmt.Errorf("first must be greater than 0")
	}
	if last != nil && *last < 0 {
		return fmt.Errorf("last must be greater than 0")
	}
	if after != nil && len(*after) > 0 && before != nil && len(*before) > 0 {
		return fmt.Errorf("only one of after and before can be provided")
	}
	if ValidateCursor(after) != nil {
		return fmt.Errorf("after is not a valid cursor")
	}
	if ValidateCursor(before) != nil {
		return fmt.Errorf("before is not a valid cursor")
	}
	return nil
}

func ValidateCursor(cursor *string) error {
	_, err := DecodeCursor(cursor)
	if err != nil {
		return err
	}
	return nil
}

func DecodeCursor(cursor *string) (string, error) {
	if cursor == nil {
		return "", nil
	}
	b64, err := base64.StdEncoding.DecodeString(*cursor)
	if err != nil {
		return "", err
	}
	return string(b64), nil
}

func EncodeCursor(cursor string) string {
	return base64.StdEncoding.EncodeToString([]byte(cursor))
}

func GetPage[T any](items []T, first *int, after *string, last *int, before *string, max int, fn func(T) string) ([]T, PageInfo) {
	if len(items) == 0 {
		return nil, PageInfo{}
	}
	tmp := min(max, len(items))
	sIx := 0
	eIx := sIx + tmp
	if first != nil {
		tmp = *first
		eIx = sIx + tmp
	} else if last != nil {
		tmp = *last
		sIx = len(items) - tmp
		eIx = len(items)
	}
	if cursor, err := DecodeCursor(after); err == nil && cursor != "" {
		idx := slices.IndexFunc(items, func(item T) bool {
			return fn(item) == cursor
		})
		idx = idx + 1
		if idx+tmp >= len(items) {
			tmp = len(items) - idx
		}
		sIx = idx
		eIx = idx + tmp
	} else if cursor, err := DecodeCursor(before); err == nil && cursor != "" {
		idx := slices.IndexFunc(items, func(item T) bool {
			return fn(item) == cursor
		})
		f := idx - tmp
		if f < 0 {
			f = 0
		}
		sIx = f
		eIx = idx
	}
	page := items[sIx:min(eIx, len(items))]
	if len(page) == 0 {
		return nil, PageInfo{}
	}
	return page, PageInfo{
		StartCursor:     ptr(EncodeCursor(fn(page[0]))),
		HasNextPage:     eIx < len(items),
		HasPreviousPage: sIx > 0,
		EndCursor:       ptr(EncodeCursor(fn(page[len(page)-1]))),
	}
}

func ptr[T any](v T) *T {
	return &v
}

type PageInfo struct {
	StartCursor     *string
	HasNextPage     bool
	HasPreviousPage bool
	EndCursor       *string
}