package main

import (
	"bytes"
	"context"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"syscall"
	"time"
)

// CommandTimeout bounds the one command this module runs (the container runtime's list of names).
const CommandTimeout = 5 * time.Second

// StatfsTimeout bounds the look at one mount: a network filesystem whose server is gone blocks statfs
// for as long as it likes, and must cost the answer one line, never the whole call.
const StatfsTimeout = 2 * time.Second

// Runner runs one command and answers its standard output. It is injected so that the tools are tested
// against recorded answers rather than this machine's daemons.
type Runner func(ctx context.Context, name string, args ...string) (string, error)

// ExecRunner runs a command on the machine, bounded by CommandTimeout.
func ExecRunner(ctx context.Context, name string, args ...string) (string, error) {
	ctx, cancel := context.WithTimeout(ctx, CommandTimeout)
	defer cancel()
	cmd := exec.CommandContext(ctx, name, args...)
	var stdout, stderr bytes.Buffer
	cmd.Stdout, cmd.Stderr = &stdout, &stderr
	if err := cmd.Run(); err != nil {
		said := strings.TrimSpace(stderr.String())
		if said != "" {
			return "", fmt.Errorf("%s: %w: %s", name, err, said)
		}
		return "", fmt.Errorf("%s: %w", name, err)
	}
	return stdout.String(), nil
}

// FSStat is what statfs says of one mounted filesystem, in bytes and inodes.
type FSStat struct {
	Size, Free, Avail uint64
	Files, FilesFree  uint64
}

// Machine is what the module reads: a filesystem root (the real one, or a test's tree standing in for
// /proc and /sys), a way to run a command, a way to wait out the sampling window, and statfs.
type Machine struct {
	Root   string
	Run    Runner
	Wait   func(time.Duration)
	Statfs func(path string) (FSStat, error)
}

// Here is the machine this process runs on.
func Here() *Machine {
	return &Machine{Root: "/", Run: ExecRunner, Wait: time.Sleep, Statfs: statfs}
}

func statfs(path string) (FSStat, error) {
	type answer struct {
		s   FSStat
		err error
	}
	done := make(chan answer, 1)
	go func() {
		var s syscall.Statfs_t
		if err := syscall.Statfs(path, &s); err != nil {
			done <- answer{err: err}
			return
		}
		bs := uint64(s.Bsize)
		done <- answer{s: FSStat{Size: s.Blocks * bs, Free: s.Bfree * bs, Avail: s.Bavail * bs, Files: s.Files, FilesFree: s.Ffree}}
	}()
	select {
	case a := <-done:
		return a.s, a.err
	case <-time.After(StatfsTimeout):
		return FSStat{}, fmt.Errorf("statfs did not answer within %s", StatfsTimeout)
	}
}

func (m *Machine) path(p string) string { return filepath.Join(m.Root, p) }

// read is a file's content; "" when it cannot be read.
func (m *Machine) read(p string) string {
	b, err := os.ReadFile(m.path(p))
	if err != nil {
		return ""
	}
	return string(b)
}

func (m *Machine) exists(p string) bool {
	_, err := os.Stat(m.path(p))
	return err == nil
}

// names lists the entries of a directory, sorted; none when it cannot be read.
func (m *Machine) names(dir string) []string {
	entries, err := os.ReadDir(m.path(dir))
	if err != nil {
		return nil
	}
	out := make([]string, 0, len(entries))
	for _, e := range entries {
		out = append(out, e.Name())
	}
	return out
}

// round1 rounds to one decimal, for what a person reads.
func round1(f float64) float64 {
	if f < 0 {
		return -float64(int64(-f*10+0.5)) / 10
	}
	return float64(int64(f*10+0.5)) / 10
}

const mib = 1 << 20
const gib = 1 << 30