package assetcache

import "errors"

// FS is the store's view of the filesystem's free space. The Linux
// implementation is statfs; tests may supply a fake.
type FS interface {
	// Space reports the size of the filesystem holding dir and the bytes
	// available to an unprivileged writer. The store may call it with its
	// own lock held, so it must not call back into the Store.
	Space(dir string) (total, free uint64, err error)
}

// ErrUnsupported is returned by Statfs on systems where free space is not
// measured.
var ErrUnsupported = errors.New("assetcache: free space unknown on this system")

const (
	gib = 1 << 30

	// maxDefaultBudget is the largest budget DefaultBudget returns.
	maxDefaultBudget = 4 * gib
)

// DefaultBudget is the cache budget used when none is configured: the smaller
// of 4 GiB and a tenth of the filesystem's size.
func DefaultBudget(total uint64) int64 {
	if total/10 >= maxDefaultBudget {
		return maxDefaultBudget
	}
	return int64(total / 10)
}