/*
   Copyright 2022 CAcert Inc.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

package ocspsource

import (
	"bufio"
	"context"
	"errors"
	"fmt"
	"io"
	"math/big"
	"os"
	"path"
	"path/filepath"
	"strings"
	"time"

	"github.com/fsnotify/fsnotify"
	"github.com/sirupsen/logrus"
	"golang.org/x/crypto/ocsp"
)

// OpenSSLCertDB implements the needed glue code to interface with the index.txt format of openssl ca
type OpenSSLCertDB struct {
	fileName string
	content  map[string]*ocsp.Response
}

// NewCertDB creates a new certificate database for the given index.txt file
func NewCertDB(fileName string) (*OpenSSLCertDB, error) {
	absFile, err := filepath.Abs(fileName)
	if err != nil {
		return nil, fmt.Errorf("could not determine absolute file name of %s: %w", fileName, err)
	}

	certDb := &OpenSSLCertDB{fileName: absFile}

	err = certDb.update()
	if err != nil {
		return nil, err
	}

	return certDb, nil
}

// The update method reads the content of index.txt and replaces the current content of the OpenSSLCertDB
func (o *OpenSSLCertDB) update() error {
	f, err := os.Open(o.fileName)
	if err != nil {
		return fmt.Errorf("could not open %s: %w", o.fileName, err)
	}
	defer func(f *os.File) {
		_ = f.Close()
	}(f)

	newContent := make(map[string]*ocsp.Response)
	b := bufio.NewReader(f)
	lastLine := false
	for {
		line, err := b.ReadString('\n')
		if err != nil {
			if !errors.Is(err, io.EOF) {
				return fmt.Errorf("could not read next line: %w", err)
			}
			lastLine = true
		}

		serial, response := parseLine(strings.TrimSpace(line))
		if serial != "" {
			newContent[serial] = response
		}

		if lastLine {
			break
		}
	}

	logrus.Infof("parsed certificate database '%s', found information for %d certificates", o.fileName, len(newContent))

	o.content = newContent

	return nil
}

// LookupResponseTemplate retrieves an OCSP response template for the given certificate serial number.
func (o *OpenSSLCertDB) LookupResponseTemplate(number *big.Int) *ocsp.Response {
	serial := number.Text(16)
	if response, ok := o.content[serial]; ok {
		return response
	}
	return nil
}

// WatchChanges creates file system monitoring for the index.txt file of the OpenSSLCertDB
func (o *OpenSSLCertDB) WatchChanges(ctx context.Context) {
	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		logrus.Fatalf("could not create file watcher: %v", err)
	}
	defer func(watcher *fsnotify.Watcher) {
		_ = watcher.Close()
		logrus.Infof("stopped watching for %s changes", o.fileName)
	}(watcher)

	go func(watcher *fsnotify.Watcher, filename string) {
		for {
			select {
			case event, ok := <-watcher.Events:
				if !ok {
					return
				}
				if event.Op&fsnotify.Write == fsnotify.Write {
					if event.Name == filename {
						logrus.Infof("modified: %s", event.Name)
						err := o.update()
						if err != nil {
							logrus.Error(err)
						}
					}
				}
			case err, ok := <-watcher.Errors:
				if !ok {
					return
				}
				logrus.Errorf("error from watcher: %v", err)
			}
		}
	}(watcher, o.fileName)

	err = watcher.Add(path.Dir(o.fileName))
	if err != nil {
		logrus.Fatalf("could not watch %s: %v", o.fileName, err)
	}
	logrus.Infof("watching for changes on %s", o.fileName)

	<-ctx.Done()
	logrus.Infof("ending background tasks for %s", o.fileName)
}

// The parseLine function parses a line of index.txt.
func parseLine(line string) (string, *ocsp.Response) {
	if line == "" {
		return "", nil
	}

	parts := strings.Split(line, "\t")
	if len(parts) != 6 {
		logrus.Warnf("found invalid line '%s'", line)
		return "", nil
	}

	serial := parts[3]
	serialNumber := new(big.Int)
	_, ok := serialNumber.SetString(serial, 16)
	if !ok {
		logrus.Warnf("could not parse %s as serial number", serial)
		return "", nil
	}

	response := ocsp.Response{
		Status:       0,
		SerialNumber: serialNumber,
	}

	switch parts[0] {
	case "V":
		response.Status = ocsp.Good
	case "R":
		response.Status = ocsp.Revoked
		response.RevocationReason = ocsp.Unspecified
	default:
		response.Status = ocsp.Unknown
	}

	if response.Status == ocsp.Revoked && parts[2] == "" {
		logrus.Warnf("inconsistency detected certificate with serial %s is marked as revoked but has no revocation timestamp", serial)
		return "", nil
	}

	if response.Status == ocsp.Revoked {
		revocation := []string{parts[2]}
		if strings.Contains(revocation[0], ",") {
			revocation = strings.SplitN(revocation[0], ",", 2)
		}
		if len(revocation) == 2 {
			response.RevocationReason = mapRevocationReason(revocation[1])
		}
		revocationTimeStamp, err := time.Parse("20060102150405Z", revocation[0])
		if err != nil {
			logrus.Warnf("could not parse %s as revocation timestamp for serial %s: %v", revocation[0], serial, err)
			return "", nil
		}
		response.RevokedAt = revocationTimeStamp.UTC()
	}

	return serialNumber.Text(16), &response
}

// mapRevocationReason takes a OCSP revocation reason string and converts it to the numerical representation required
// for OCSP responses.
func mapRevocationReason(reason string) int {
	switch reason {
	case "keyCompromise":
		return ocsp.KeyCompromise
	case "CACompromise":
		return ocsp.CACompromise
	case "affiliationChanged":
		return ocsp.AffiliationChanged
	case "superseded":
		return ocsp.Superseded
	case "cessationOfOperation":
		return ocsp.CessationOfOperation
	case "certificateHold":
		return ocsp.CertificateHold
	case "removeFromCRL":
		return ocsp.RemoveFromCRL
	case "privilegeWithdrawn":
		return ocsp.PrivilegeWithdrawn
	case "AACompromise":
		return ocsp.AACompromise
	default:
		return ocsp.Unspecified
	}
}