package openssl

import (
	"bufio"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/asn1"
	"errors"
	"fmt"
	"math/big"
	"os"
	"path"
	"strings"
	"sync"
	"time"

	"git.cacert.org/cacert-gosigner/pkg/x509/revoking"
)

const TimeSpec = "060102030405Z"

type indexStatus string

const (
	certificateValid   indexStatus = "V"
	certificateRevoked indexStatus = "R"
	certificateExpired indexStatus = "E"
)

// An indexEntry represents a line in an openssl ca compatible index.txt file
// a format specification is available at https://pki-tutorial.readthedocs.io/en/latest/cadb.html
type indexEntry struct {
	statusFlag           indexStatus
	expiresAt            time.Time
	revokedAt            *time.Time
	revocationReason     revoking.CRLReason
	serialNumber         *big.Int
	fileName             string
	certificateSubjectDN string
}

func (ie *indexEntry) String() string {
	var revoked, fileName string

	if ie.revokedAt != nil {
		revoked = fmt.Sprintf("%s,%s", ie.revokedAt.Format(TimeSpec), ie.revocationReason)
	}

	if ie.fileName == "" {
		fileName = "unknown"
	}

	return strings.Join([]string{
		string(ie.statusFlag),
		ie.expiresAt.Format(TimeSpec),
		revoked,
		strings.ToUpper(ie.serialNumber.Text(16)),
		fileName,
		ie.certificateSubjectDN, // this is not 100% compatible with openssl that uses a non-RFC syntax
	}, "\t")
}

// The Repository stores information about signed and revoked certificates
// in an openssl index.txt compatible file.
//
// A reference for the file format can be found at
// https://pki-tutorial.readthedocs.io/en/latest/cadb.html.
type Repository struct {
	indexFileName string
	lock          sync.Locker
	entries       []indexEntry
}

func (ie *indexEntry) markRevoked(revocationTime time.Time, reason revoking.CRLReason) {
	if ie.statusFlag == certificateValid {
		ie.statusFlag = certificateRevoked
		ie.revokedAt = &revocationTime
		ie.revocationReason = reason
	}
}

func (r *Repository) findEntry(number *big.Int) (*indexEntry, error) {
	if number == nil {
		return nil, errors.New("serial number must not be nil")
	}

	for _, entry := range r.entries {
		if entry.serialNumber.Cmp(number) == 0 {
			return &entry, nil
		}
	}

	return nil, nil
}

type CannotRevokeUnknown struct {
	Serial *big.Int
}

func (c CannotRevokeUnknown) Error() string {
	return fmt.Sprintf("cannot revoke unknown certificate with serial number %s", c.Serial)
}

// StoreRevocation records information about a revoked certificate.
func (r *Repository) StoreRevocation(revoked *pkix.RevokedCertificate) error {
	r.lock.Lock()
	defer r.lock.Unlock()

	err := r.loadIndex()
	if err != nil {
		return err
	}

	entry, err := r.findEntry(revoked.SerialNumber)
	if err != nil {
		return err
	}

	if entry == nil {
		return CannotRevokeUnknown{Serial: revoked.SerialNumber}
	}

	reason := revoking.CRLReasonUnspecified

	for _, ext := range revoked.Extensions {
		if ext.Id.Equal(revoking.OidCRLReason) {
			_, err := asn1.Unmarshal(ext.Value, &reason)
			if err != nil {
				return fmt.Errorf("could not unmarshal ")
			}
		}
	}

	entry.markRevoked(revoked.RevocationTime, reason)

	err = r.writeIndex()
	return err
}

// StoreCertificate records information about a signed certificate.
func (r *Repository) StoreCertificate(signed *x509.Certificate) error {
	var err error

	r.lock.Lock()
	defer r.lock.Unlock()

	err = r.loadIndex()
	if err != nil {
		return err
	}

	entry, err := r.findEntry(signed.SerialNumber)
	if err != nil {
		return err
	}

	if entry != nil {
		return fmt.Errorf("certificate with serial %s is already in the index", signed.SerialNumber)
	}

	status := certificateValid

	if signed.NotAfter.Before(time.Now().UTC()) {
		status = certificateExpired
	}

	err = r.addIndexEntry(&indexEntry{
		statusFlag:           status,
		expiresAt:            signed.NotAfter,
		serialNumber:         signed.SerialNumber,
		certificateSubjectDN: signed.Subject.String(), // not openssl compatible
	})
	if err != nil {
		return err
	}

	return nil
}

func (r *Repository) RevokedCertificates() ([]pkix.RevokedCertificate, error) {
	var err error

	r.lock.Lock()
	defer r.lock.Unlock()

	err = r.loadIndex()
	if err != nil {
		return nil, err
	}

	result := make([]pkix.RevokedCertificate, 0)
	for _, entry := range r.entries {
		if entry.revokedAt != nil {
			result = append(result, pkix.RevokedCertificate{
				SerialNumber:   entry.serialNumber,
				RevocationTime: *entry.revokedAt,
				Extensions:     []pkix.Extension{entry.revocationReason.BuildExtension()},
			})
		}
	}

	return result, nil
}

func (r *Repository) loadIndex() error {
	entries := make([]indexEntry, 0, 100)

	f, err := os.Open(r.indexFileName)
	if err != nil && !os.IsNotExist(err) {
		return fmt.Errorf("could not load index from %s: %w", r.indexFileName, err)
	}

	if f == nil {
		r.entries = entries

		return nil
	}

	defer func() { _ = f.Close() }()

	indexScanner := bufio.NewScanner(f)
	for indexScanner.Scan() {
		indexEntry, err := r.newIndexEntryFromLine(indexScanner.Text())
		if err != nil {
			return err
		}
		entries = append(entries, *indexEntry)
	}
	if err := indexScanner.Err(); err != nil {
		return err
	}

	r.entries = entries

	return nil
}

func (r *Repository) writeIndex() error {
	f, err := os.OpenFile(r.indexFileName, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o600)
	if err != nil {
		return fmt.Errorf("could not create index file %s: %w", r.indexFileName, err)
	}
	defer func(f *os.File) {
		_ = f.Close()
	}(f)

	w := bufio.NewWriter(f)

	for i, entry := range r.entries {
		_, err = w.WriteString(entry.String())
		if err != nil {
			return fmt.Errorf("could not write entry for serial %s: %w", entry.serialNumber, err)
		}
		if i < len(r.entries)-1 {
			_, err = w.WriteString("\n")
			if err != nil {
				return fmt.Errorf("could not write linebreak: %w", err)
			}
		}
	}
	err = w.Flush()
	if err != nil {
		return fmt.Errorf("could not write to %s: %w", r.indexFileName, err)
	}

	return nil
}

func (r *Repository) addIndexEntry(ie *indexEntry) error {
	r.entries = append(r.entries, *ie)

	err := r.writeIndex()
	if err != nil {
		return err
	}

	return nil
}

func (r *Repository) newIndexEntryFromLine(text string) (*indexEntry, error) {
	var err error

	fields := strings.Split(text, "\t")

	const expectedFieldNumber = 6
	if len(fields) != expectedFieldNumber {
		return nil, fmt.Errorf(
			"unexpected number of fields %d instead of %d",
			len(fields),
			expectedFieldNumber,
		)
	}

	expirationParsed, err := time.Parse(TimeSpec, fields[1])
	if err != nil {
		return nil, err
	}

	var revocationTimeParsed time.Time
	var revocationReason revoking.CRLReason

	if fields[2] != "" {
		var timeString string
		if strings.Contains(fields[2], ",") {
			parts := strings.SplitN(fields[2], ",", 2)
			timeString = parts[0]
			revocationReason = revoking.ParseReason(parts[1])
		} else {
			timeString = fields[2]
		}
		revocationTimeParsed, err = time.Parse(TimeSpec, timeString)
		if err != nil {
			return nil, err
		}
	}

	serialParsed := new(big.Int)
	if _, ok := serialParsed.SetString(fields[3], 16); !ok {
		return nil, fmt.Errorf("could not parse serial number %s", fields[3])
	}

	fileNameParsed := "unknown"
	if fields[4] != "" {
		_, err = os.Stat(path.Join(path.Dir(r.indexFileName), fields[4]))
		if err != nil && !os.IsNotExist(err) {
			return nil, err
		}
		fileNameParsed = fields[4]
	}

	return &indexEntry{
		statusFlag:           indexStatus(fields[0]),
		expiresAt:            expirationParsed,
		revokedAt:            &revocationTimeParsed,
		revocationReason:     revocationReason,
		serialNumber:         serialParsed,
		fileName:             fileNameParsed,
		certificateSubjectDN: fields[5],
	}, nil
}

func NewFileRepository(baseDirectory string) (*Repository, error) {
	err := os.Chdir(baseDirectory)
	if err != nil {
		return nil, err
	}
	return &Repository{
		indexFileName: path.Join(baseDirectory, "index.txt"),
		lock:          &sync.Mutex{},
	}, nil
}