package backup

import (
	"path/filepath"
	"strings"
)

// oneDrivePerSubtree is the test stand-in for system.SamePhysicalDevice (st_dev equality).
//
// Why it exists: the real predicate asks "are these two paths on the same physical disk?", and
// Tier-2's whole purpose is to refuse a target that is. On a host where every t.TempDir() lands on
// one filesystem — DooPlex, and any CI box with a single volume — a fixture's "usb" and "flash"
// dirs share one st_dev, so the guard correctly refuses them and the off-drive tests can never
// exercise their subject. This models what the fixture is actually depicting: one drive per
// directory subtree, so two paths share a device only when one contains the other (a path inside a
// drive IS on that drive). Unrelated subtrees are distinct devices, exactly as real mountpoints are.
//
// It does NOT relax any assertion — the guard still runs, still refuses same-device targets (see
// TestSharesTier2NeverTargetsItsOwnSourceDrive, which passes under this seam), and production keeps
// using the real st_dev check because the seam is nil there.
func oneDrivePerSubtree(a, b string) bool {
	a, b = filepath.Clean(a), filepath.Clean(b)
	if a == b {
		return true
	}
	sep := string(filepath.Separator)
	return strings.HasPrefix(a, b+sep) || strings.HasPrefix(b, a+sep)
}