package main

import (
	"context"
	"encoding/json"
	"flag"
	"fmt"
	"io"
	"io/fs"
	"os"
	"os/signal"
	"path/filepath"
	"runtime"
	"sync"
	"sync/atomic"
	"syscall"

	"github.com/sourcegraph/conc/pool"
)

var (
	sourceDir   = flag.String("source", ".", "source data directory")
	tempDir     = flag.String("temp", "/tmp", "temporary storage directory")
	dbPath      = flag.String("db", "datashake.json", "database file path")
	minimumSize = flag.Int64("min-size", 1024*1024, "minimum size in bytes")
	concurrency = flag.Int("concurrency", 1, "concurrent processing limit")
)

func main() {
	ctx, stop := signal.NotifyContext(
		context.Background(),
		syscall.SIGINT, syscall.SIGTERM,
	)
	defer stop()

	flag.Parse()

	concurrency := *concurrency
	if concurrency < 1 {
		concurrency = runtime.GOMAXPROCS(0)
	}
	tasks = pool.New().WithMaxGoroutines(concurrency)

	if err := loadDb(); err != nil {
		fmt.Println("error", err)
		os.Exit(1)
	}

	running.Store(true)
	go func() {
		if err := filepath.WalkDir(*sourceDir, process); err != nil {
			errors <- err
		}
		pending.Wait()
		close(errors)
	}()

Loop:
	for {
		select {
		case err, ok := <-errors:
			if ok {
				fmt.Println("error:", err)
			} else {
				break Loop
			}
		case <-ctx.Done():
			running.Store(false)
		}
	}

	if err := saveDb(); err != nil {
		fmt.Println("error", err)
		os.Exit(1)
	}
}

var (
	running atomic.Bool

	tasks   *pool.Pool
	pending sync.WaitGroup
	errors  = make(chan error)

	db = DB{
		Processed: make(map[string]struct{}),
	}
	dbLock sync.Mutex
)

// process is a visitor for `filepath.WalkDir` that performs the rebalancing
// algorithm against regular files.
//
// This function normally never returns an error, since that would stop the
// directory walk. Instead, any errors are sent to the `errors` channel.
func process(path string, d fs.DirEntry, err error) (typicallyNil error) {
	if !running.Load() {
		return fs.SkipAll
	}
	if err != nil || d.IsDir() || !d.Type().IsRegular() {
		return
	}
	pending.Add(1)
	tasks.Go(func() {
		defer pending.Done()
		if running.Load() {
			work(path, d)
		}
	})
	return
}

// work rebalances a single file.
func work(path string, d fs.DirEntry) {
	var err error
	defer func() {
		if err != nil {
			errors <- err
		}
	}()

	srcFileName := d.Name()
	srcFilePath, err := filepath.Abs(path)
	if err != nil {
		return
	}
	if db.Contains(srcFilePath) {
		return
	}
	srcStat, err := os.Stat(srcFilePath)
	if err != nil {
		return
	}
	if srcStat.Size() < *minimumSize {
		return
	}

	tempDirPath, err := os.MkdirTemp(*tempDir, "*")
	if err != nil {
		return
	}
	tempFilePath := filepath.Join(tempDirPath, srcFileName)
	safeToRemoveTemp := true
	defer func() {
		if !safeToRemoveTemp {
			err := fmt.Errorf(
				"%s may be lost in %s",
				srcFilePath, tempDirPath,
			)
			errors <- err
			return
		}
		if err := os.RemoveAll(tempDirPath); err != nil {
			errors <- err
		}
	}()

	err = copy(srcFilePath, tempFilePath)
	if err != nil {
		return
	}

	safeToRemoveTemp = false
	err = os.Remove(srcFilePath)
	if err != nil {
		return
	}

	err = copy(tempFilePath, srcFilePath)
	if err != nil {
		return
	}
	safeToRemoveTemp = true
	db.Remember(srcFilePath)
}

// copy opens the file from the source path, then creates a copy of it at the
// destination path. The mode, uid and gid bits from the source file are
// replicated in the copy.
func copy(srcPath, dstPath string) error {
	fmt.Println("copying", srcPath, "to", dstPath)
	srcFile, err := os.Open(srcPath)
	if err != nil {
		return err
	}
	defer func() {
		_ = srcFile.Close()
	}()

	dstFile, err := os.Create(dstPath)
	if err != nil {
		return err
	}
	defer func() {
		_ = dstFile.Close()
	}()

	srcStat, err := os.Stat(srcPath)
	if err != nil {
		return err
	}
	err = os.Chmod(dstPath, srcStat.Mode())
	if err != nil {
		return err
	}
	if sysStat, ok := srcStat.Sys().(*syscall.Stat_t); ok {
		uid := int(sysStat.Uid)
		gid := int(sysStat.Gid)
		err = os.Chown(dstPath, uid, gid)
		if err != nil {
			return err
		}
	}

	_, err = io.Copy(dstFile, srcFile)
	return err
}

// DB holds a set of files which have been rebalanced.
//
// These files are skipped on future runs of the program.
//
// The database is loaded from a JSON file when the program starts and saved
// back to that JSON file as the program finishes.
type DB struct {
	Processed map[string]struct{}
}

func (db *DB) Contains(path string) bool {
	dbLock.Lock()
	defer dbLock.Unlock()
	_, ok := db.Processed[path]
	return ok
}

func (db *DB) Remember(path string) {
	dbLock.Lock()
	defer dbLock.Unlock()
	db.Processed[path] = struct{}{}
}

func loadDb() error {
	if *dbPath == "" {
		return nil
	}
	dbFile, err := os.Open(*dbPath)
	if err != nil {
		return nil
	}
	defer func() {
		_ = dbFile.Close()
	}()
	d := json.NewDecoder(dbFile)
	err = d.Decode(&db)
	return err
}

func saveDb() error {
	if *dbPath == "" {
		return nil
	}
	dbFile, err := os.Create(*dbPath)
	if err != nil {
		return err
	}
	defer func() {
		_ = dbFile.Close()
	}()
	e := json.NewEncoder(dbFile)
	err = e.Encode(&db)
	return err
}