package observability

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strconv"
	"sync"
	"time"
)

// RotatingFile is an append-only daemon log with bounded numbered archives.
// It is deliberately a single-process writer: deployment supplies one daemon
// process per configured path, and a second writer must use a distinct file.
// Archive .1 is newest; .maxFiles is oldest.
type RotatingFile struct {
	mu       sync.Mutex
	path     string
	maxBytes uint64
	maxFiles uint32
	file     *os.File
	size     uint64
}

// OpenRotatingFile opens path for append, creating its parent directories with
// conservative permissions. A blank path disables file logging and returns a
// no-op closer. Both rotation controls must be zero (unbounded) or positive.
func OpenRotatingFile(path string, maxBytes uint64, maxFiles uint32) (io.WriteCloser, error) {
	if path == "" {
		return nopWriteCloser{Writer: io.Discard}, nil
	}
	if (maxBytes == 0) != (maxFiles == 0) {
		return nil, errors.New("log rotation byte/file limits must both be zero or positive")
	}
	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
		return nil, fmt.Errorf("create log directory: %w", err)
	}
	result := &RotatingFile{path: path, maxBytes: maxBytes, maxFiles: maxFiles}
	if err := result.open(); err != nil {
		return nil, err
	}
	return result, nil
}

func (file *RotatingFile) open() error {
	opened, err := os.OpenFile(file.path, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o600)
	if err != nil {
		return fmt.Errorf("open log file: %w", err)
	}
	info, err := opened.Stat()
	if err != nil {
		_ = opened.Close()
		return fmt.Errorf("stat log file: %w", err)
	}
	file.file = opened
	file.size = uint64(info.Size())
	return nil
}

func (file *RotatingFile) Write(data []byte) (int, error) {
	file.mu.Lock()
	defer file.mu.Unlock()
	if file.file == nil {
		return 0, os.ErrClosed
	}
	if file.maxBytes != 0 && file.size != 0 && (file.size >= file.maxBytes || uint64(len(data)) > file.maxBytes-file.size) {
		if err := file.rotate(); err != nil {
			return 0, err
		}
	}
	written, err := file.file.Write(data)
	file.size += uint64(written)
	return written, err
}

func (file *RotatingFile) Close() error {
	file.mu.Lock()
	defer file.mu.Unlock()
	if file.file == nil {
		return nil
	}
	err := file.file.Close()
	file.file = nil
	return err
}

func (file *RotatingFile) rotate() error {
	if err := file.file.Close(); err != nil {
		return fmt.Errorf("close log before rotation: %w", err)
	}
	file.file = nil
	oldest := file.archivePath(file.maxFiles)
	if err := os.Remove(oldest); err != nil && !errors.Is(err, os.ErrNotExist) {
		return fmt.Errorf("remove oldest log archive: %w", err)
	}
	for index := file.maxFiles; index > 1; index-- {
		from := file.archivePath(index - 1)
		to := file.archivePath(index)
		if err := os.Rename(from, to); err != nil && !errors.Is(err, os.ErrNotExist) {
			return fmt.Errorf("rotate log archive: %w", err)
		}
	}
	if err := os.Rename(file.path, file.archivePath(1)); err != nil && !errors.Is(err, os.ErrNotExist) {
		return fmt.Errorf("seal current log: %w", err)
	}
	return file.open()
}

func (file *RotatingFile) archivePath(index uint32) string {
	return file.path + "." + strconv.FormatUint(uint64(index), 10)
}

type nopWriteCloser struct{ io.Writer }

func (nopWriteCloser) Close() error { return nil }

// FormatLog makes file records either newline-delimited JSON or plain text.
// It is intentionally applied only to daemon diagnostics, never command
// output, stdin, environment values, or other payload-bearing data.
func FormatLog(destination io.Writer, format string) io.Writer {
	if destination == nil || format != "json" {
		return destination
	}
	return &jsonLogWriter{destination: destination}
}

type jsonLogWriter struct {
	mu          sync.Mutex
	destination io.Writer
}

func (writer *jsonLogWriter) Write(data []byte) (int, error) {
	writer.mu.Lock()
	defer writer.mu.Unlock()
	for _, line := range bytes.Split(data, []byte{'\n'}) {
		if len(line) == 0 {
			continue
		}
		record, err := json.Marshal(struct {
			Time    string `json:"time"`
			Level   string `json:"level"`
			Message string `json:"message"`
		}{Time: time.Now().UTC().Format(time.RFC3339Nano), Level: "info", Message: string(line)})
		if err != nil {
			return 0, fmt.Errorf("encode structured log: %w", err)
		}
		if _, err := writer.destination.Write(append(record, '\n')); err != nil {
			return 0, err
		}
	}
	return len(data), nil
}