// Package reportbuf accumulates telemetry reports in memory
// and periodically flushes them to zstd-compressed JSONL files.
package reportbuf

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io/fs"
	"log/slog"
	"os"
	"path/filepath"
	"strings"
	"sync"
	"time"

	"sneak.berlin/go/netwatch/internal/config"
	"sneak.berlin/go/netwatch/internal/logger"

	"github.com/klauspost/compress/zstd"
	"go.uber.org/fx"
)

const (
	flushSizeThreshold             = 10 << 20 // 10 MiB
	flushInterval                  = 1 * time.Minute
	defaultDataDir                 = "./data/reports"
	dirPerms           fs.FileMode = 0o750
	filePerms          fs.FileMode = 0o640

	// Report files are named filePrefix + timestamp + fileSuffix.
	filePrefix = "reports-"
	fileSuffix = ".jsonl.zst"
)

// ErrFull is returned by Append when storing the report would
// take the report files past the configured maximum size.
var ErrFull = errors.New("report files at their size cap")

// Params defines the dependencies for Buffer.
type Params struct {
	fx.In

	Config *config.Config
	Logger *logger.Logger
}

// Buffer accumulates JSON lines in memory and flushes them
// to zstd-compressed files on disk.
type Buffer struct {
	buf      bytes.Buffer
	dataDir  string
	done     chan struct{}
	log      *slog.Logger
	maxBytes int64
	mu       sync.Mutex
	stopOnce sync.Once
	// usedBytes is what Append checks against maxBytes: the size
	// of the report files in dataDir, plus the reports not yet
	// written to one at their uncompressed size.
	usedBytes int64
}

// New creates a Buffer and registers lifecycle hooks to
// manage the data directory and flush goroutine.
func New(
	lc fx.Lifecycle,
	params Params,
) (*Buffer, error) {
	dir := params.Config.DataDir
	if dir == "" {
		dir = defaultDataDir
	}

	b := &Buffer{
		dataDir:  dir,
		done:     make(chan struct{}),
		log:      params.Logger.Get(),
		maxBytes: params.Config.DataDirMaxBytes,
	}

	lc.Append(fx.Hook{
		OnStart: func(_ context.Context) error {
			err := os.MkdirAll(b.dataDir, dirPerms)
			if err != nil {
				return fmt.Errorf("create data dir: %w", err)
			}

			// Report files left by earlier runs count too.
			b.usedBytes, err = reportFilesSize(b.dataDir)
			if err != nil {
				return err
			}

			go b.flushLoop()

			return nil
		},
		OnStop: func(_ context.Context) error {
			// stopOnce makes OnStop idempotent: a second
			// invocation must not close an already-closed channel
			// (which would panic) or flush again.
			var err error

			b.stopOnce.Do(func() {
				close(b.done)
				err = b.flushLocked()
			})

			// A failed final flush fails the stop, so the process
			// exits non-zero.
			return err
		},
	})

	return b, nil
}

// Append marshals v as a single JSON line and appends it to
// the buffer. It stores nothing and returns ErrFull if the line
// would take usedBytes past maxBytes. If the buffer reaches the
// size threshold, it is drained and written to disk
// asynchronously.
func (b *Buffer) Append(v any) error {
	line, err := json.Marshal(v)
	if err != nil {
		return fmt.Errorf("marshal report: %w", err)
	}

	lineBytes := int64(len(line)) + 1 // with its newline

	b.mu.Lock()

	if b.usedBytes+lineBytes > b.maxBytes {
		b.mu.Unlock()

		return ErrFull
	}

	b.usedBytes += lineBytes
	b.buf.Write(line)
	b.buf.WriteByte('\n')

	if b.buf.Len() >= flushSizeThreshold {
		data := b.drainBuf()
		b.mu.Unlock()

		go func() {
			writeErr := b.writeFile(data)
			if writeErr != nil {
				b.log.Error("flush reports failed", "error", writeErr)
			}
		}()

		return nil
	}

	b.mu.Unlock()

	return nil
}

// flushLoop runs a ticker that periodically flushes buffered
// data to disk until the done channel is closed.
func (b *Buffer) flushLoop() {
	ticker := time.NewTicker(flushInterval)
	defer ticker.Stop()

	for {
		select {
		case <-ticker.C:
			err := b.flushLocked()
			if err != nil {
				b.log.Error("flush reports failed", "error", err)
			}
		case <-b.done:
			return
		}
	}
}

// flushLocked acquires the lock, drains the buffer, and
// writes the data to a compressed file.
func (b *Buffer) flushLocked() error {
	b.mu.Lock()

	if b.buf.Len() == 0 {
		b.mu.Unlock()

		return nil
	}

	data := b.drainBuf()
	b.mu.Unlock()

	return b.writeFile(data)
}

// drainBuf copies the buffer contents and resets it.
// The caller must hold b.mu.
func (b *Buffer) drainBuf() []byte {
	data := make([]byte, b.buf.Len())
	copy(data, b.buf.Bytes())
	b.buf.Reset()

	return data
}

// writeFile creates a timestamped zstd-compressed JSONL file
// in the data directory.
func (b *Buffer) writeFile(data []byte) error {
	ts := time.Now().UTC().Format("2006-01-02T15-04-05.000Z")
	path := filepath.Join(b.dataDir, filePrefix+ts+fileSuffix)

	// path is built from the operator-supplied dataDir plus a
	// generated timestamp, so it carries no external input.
	f, err := os.OpenFile( //nolint:gosec // see comment above
		path,
		os.O_WRONLY|os.O_CREATE|os.O_EXCL,
		filePerms,
	)
	if err != nil {
		return fmt.Errorf("create report file: %w", err)
	}

	// Closes the file on the early returns below. The success
	// path closes it explicitly to check the error; closing it
	// a second time here is harmless.
	defer func() { _ = f.Close() }()

	enc, err := zstd.NewWriter(f)
	if err != nil {
		return fmt.Errorf("create zstd encoder: %w", err)
	}

	_, err = enc.Write(data)
	if err != nil {
		_ = enc.Close()

		return fmt.Errorf("write compressed data: %w", err)
	}

	err = enc.Close()
	if err != nil {
		return fmt.Errorf("close zstd encoder: %w", err)
	}

	info, err := f.Stat()
	if err != nil {
		return fmt.Errorf("stat report file: %w", err)
	}

	err = f.Close()
	if err != nil {
		return fmt.Errorf("close report file: %w", err)
	}

	// The reports counted at their uncompressed size while they
	// waited; now they count as the file. After a failed write they
	// stay counted as they were, which errs toward refusing reports
	// early rather than letting the files pass the cap.
	b.mu.Lock()
	b.usedBytes += info.Size() - int64(len(data))
	b.mu.Unlock()

	return nil
}

// reportFilesSize returns the total size of the report files in
// dir.
func reportFilesSize(dir string) (int64, error) {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return 0, fmt.Errorf("read data dir: %w", err)
	}

	var total int64

	for _, entry := range entries {
		name := entry.Name()
		if !strings.HasPrefix(name, filePrefix) ||
			!strings.HasSuffix(name, fileSuffix) {
			continue
		}

		info, err := entry.Info()
		if err != nil {
			return 0, fmt.Errorf("stat report file: %w", err)
		}

		total += info.Size()
	}

	return total, nil
}