//go:build linux

package httpdl

import (
	"os"

	"golang.org/x/sys/unix"
)

// allocate reserves disk space for the output file according to the
// --file-allocation mode: none does nothing (sparse), trunc sets the size,
// prealloc/falloc reserve real blocks with fallocate (falling back to a plain
// truncate if the filesystem does not support it).
func allocate(f *os.File, mode string, size int64) error {
	switch mode {
	case "none":
		return nil
	case "trunc":
		return f.Truncate(size)
	default: // prealloc, falloc
		if err := unix.Fallocate(int(f.Fd()), 0, 0, size); err != nil {
			return f.Truncate(size)
		}
		return nil
	}
}