package util

import (
	"bufio"
	"errors"
	"os"
	"slices"
	"strings"
)

// SupportsFilesystem checks whether a given filesystem is already supported
// by the kernel. Note that if the filesystem is a module, you may need to
// load it first.
func SupportsFilesystem(filesystem string) bool {
	file, err := os.Open("/proc/filesystems")
	if err != nil {
		return false
	}

	defer func() { _ = file.Close() }()

	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		fields := strings.Fields(strings.TrimSpace(scanner.Text()))
		entry := fields[len(fields)-1]

		if entry == filesystem {
			return true
		}
	}

	return false
}

// HugepagesPath attempts to locate the mount point of the hugepages filesystem.
func HugepagesPath() (string, error) {
	// Find the source mount of the path
	file, err := os.Open("/proc/mounts")
	if err != nil {
		return "", err
	}

	defer func() { _ = file.Close() }()

	scanner := bufio.NewScanner(file)
	matches := []string{}
	for scanner.Scan() {
		line := scanner.Text()
		cols := strings.Fields(line)
		if len(cols) < 3 {
			continue
		}

		if cols[2] == "hugetlbfs" {
			matches = append(matches, cols[1])
		}
	}

	if len(matches) == 0 {
		return "", errors.New("No hugetlbfs mount found, can't use hugepages")
	}

	if len(matches) > 1 {
		if slices.Contains(matches, "/dev/hugepages") {
			return "/dev/hugepages", nil
		}

		return "", errors.New("More than one hugetlbfs instance found and none at standard /dev/hugepages")
	}

	return matches[0], nil
}
