package partition

import (
	"fmt"
	"syscall"
)

// FreeBytes returns the number of free bytes available on the filesystem
// containing `path`. Uses statfs(2); path must exist and be readable.
func FreeBytes(path string) (uint64, error) {
	var stat syscall.Statfs_t
	if err := syscall.Statfs(path, &stat); err != nil {
		return 0, fmt.Errorf("statfs %s: %w", path, err)
	}
	// Bavail is the count of free blocks available to non-root users —
	// matches what `df` reports. Bsize is the block size in bytes.
	//nolint:unconvert // Bavail is uint64 on most platforms but int64 on darwin/freebsd
	return uint64(stat.Bavail) * uint64(stat.Bsize), nil
}

// HasFreeSpaceFor reports whether `path`'s filesystem has at least `wantBytes`
// of free space, with `headroomPct` reserved (e.g. 10 = require 110% of want).
// Returns the available bytes alongside, so callers can render a useful error.
func HasFreeSpaceFor(path string, wantBytes int64, headroomPct int) (avail uint64, ok bool, err error) {
	avail, err = FreeBytes(path)
	if err != nil {
		return 0, false, err
	}
	if wantBytes < 0 {
		return avail, false, fmt.Errorf("invalid wantBytes %d", wantBytes)
	}
	required := uint64(wantBytes) * uint64(100+headroomPct) / 100
	return avail, avail >= required, nil
}