//go:build linux

package host

import (
	"os"
	"path/filepath"
	"syscall"
	"time"
	"unsafe"

	"github.com/carlosframework/platform/internal/moves"
)

// measureDiskSize is how much T11's disk measurement writes and reads back.
const measureDiskSize = 64 << 20

// measureDiskRate times writing and reading measureDiskSize under dir with
// O_DIRECT, so the page cache neither absorbs the write nor answers the
// read (F12). A filesystem that refuses O_DIRECT (tmpfs) is measured with a
// plain write, fsync and read instead.
func measureDiskRate(dir string, now func() time.Time) (moves.MeasuredRate, error) {
	path := filepath.Join(dir, "disk")
	f, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_RDWR|syscall.O_DIRECT, 0o600)
	if err != nil {
		return measureDiskSynced(dir, now)
	}
	defer f.Close()
	buf := alignedBlock(1 << 20)
	start := now()
	for n := 0; n < measureDiskSize; n += len(buf) {
		if _, err := f.Write(buf); err != nil {
			return moves.MeasuredRate{}, err
		}
	}
	if err := f.Sync(); err != nil {
		return moves.MeasuredRate{}, err
	}
	if _, err := f.Seek(0, 0); err != nil {
		return moves.MeasuredRate{}, err
	}
	for n := 0; n < measureDiskSize; n += len(buf) {
		if _, err := f.Read(buf); err != nil {
			return moves.MeasuredRate{}, err
		}
	}
	return moves.RateOf(2*measureDiskSize, now().Sub(start), now().UTC()), nil
}

// alignedBlock is a size-byte buffer aligned to 4096 bytes, as O_DIRECT
// requires.
func alignedBlock(size int) []byte {
	const align = 4096
	b := make([]byte, size+align)
	off := 0
	if r := int(uintptr(unsafe.Pointer(&b[0])) & (align - 1)); r != 0 {
		off = align - r
	}
	return b[off : off+size]
}