package races

import (
	"encoding/base64"
	"fmt"
	"strconv"
	"strings"
)

// Cursor is a keyset cursor: opaque to the client, encoded as
// base64url("<ms>:<id>") by Encode. Order: descending by ms, then by id.
//
// Empty cursor = "start from the newest".
type Cursor struct {
	Ms int64
	ID string
}

// String returns a human-readable representation (for debug logs).
func (c Cursor) String() string {
	return fmt.Sprintf("%d:%s", c.Ms, c.ID)
}

// IsZero reports whether the cursor is the zero value.
func (c Cursor) IsZero() bool {
	return c.Ms == 0 && c.ID == ""
}

// Encode renders the cursor for an HTTP query string.
func (c Cursor) Encode() string {
	if c.IsZero() {
		return ""
	}
	raw := c.String()
	return base64.RawURLEncoding.EncodeToString([]byte(raw))
}

// DecodeCursor parses a cursor produced by Cursor.Encode.
// Returns the zero cursor on empty input.
func DecodeCursor(s string) (Cursor, error) {
	if s == "" {
		return Cursor{}, nil
	}
	raw, err := base64.RawURLEncoding.DecodeString(s)
	if err != nil {
		return Cursor{}, fmt.Errorf("%w: bad cursor encoding: %v", ErrInvalidInput, err)
	}
	parts := strings.SplitN(string(raw), ":", 2)
	if len(parts) != 2 {
		return Cursor{}, fmt.Errorf("%w: cursor must be <ms>:<id>", ErrInvalidInput)
	}
	ms, err := strconv.ParseInt(parts[0], 10, 64)
	if err != nil {
		return Cursor{}, fmt.Errorf("%w: cursor ms: %v", ErrInvalidInput, err)
	}
	return Cursor{Ms: ms, ID: parts[1]}, nil
}