feat: initial version

pagination.go

@@ -0,0 +1,108 @@
+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 items == nil {
+		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:eIx]
+	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
+}