package pages

import (
	"errors"
	"fmt"
	"hash/fnv"
	"math"
	"os"
	"runtime"
	"sync"
	"time"
	"unsafe"
)

// These errors can be returned when opening or calling methods on a DB.
var (
	// ErrDatabaseNotOpen is returned when a DB instance is accessed before it
	// is opened or after it is closed.
	ErrDatabaseNotOpen = errors.New("database not open")

	// ErrDatabaseOpen is returned when opening a database that is
	// already open.
	ErrDatabaseOpen = errors.New("database already open")

	// ErrInvalid is returned when both meta pages on a database are invalid.
	// This typically occurs when a file is not a bolt database.
	ErrInvalid = errors.New("invalid database")

	// ErrVersionMismatch is returned when the data file was created with a
	// different version of Bolt.
	ErrVersionMismatch = errors.New("version mismatch")

	// ErrChecksum is returned when either meta page checksum does not match.
	ErrChecksum = errors.New("checksum error")

	// ErrTimeout is returned when a database cannot obtain an exclusive lock
	// on the data file after the timeout passed to Open().
	ErrTimeout = errors.New("timeout")

	// ErrNotFound is returned when a user page for an ID could not be found.
	ErrNotFound = errors.New("not found")

	ErrTxClosed = errors.New("transaction closed")

	ErrTxNotWritable = errors.New("transaction not writable")
)

// Marker value that indicates that a file is a pagebuf file.
const magic uint32 = 0xAFFEAFFE

// The data file version.
const version = 1

// The largest step that can be taken when remapping the mmap.
const maxMmapStep = 1 << 30 // 1GB

// defaultPageSize of the underlying buffers is set to the OS page size.
var defaultPageSize = os.Getpagesize()

// DB is an interface providing access to persistent byte chunks that
// are backed by memory-mapped pages.
type DB struct {
	// If you want to read the entire database fast, you can set MmapFlag to
	// syscall.MAP_POPULATE on Linux 2.6.23+ for sequential read-ahead.
	MmapFlags int

	// AllocSize is the amount of space allocated when the database
	// needs to create new pages. This is done to amortize the cost
	// of truncate() and fsync() when growing the data file.
	AllocSize int

	path     string   // location of the pagebuf file
	file     *os.File // the opened file of path
	opened   bool
	data     *[maxMapSize]byte
	dataref  []byte // mmap'ed readonly, write throws SEGV
	datasz   int
	filesz   int // current on disk file size
	pageSize int
	meta0    *meta
	meta1    *meta
	freelist *freelist
	rwtx     *Tx
	txs      []*Tx

	pagePool sync.Pool

	rwlock   sync.Mutex   // Allows only one writer at a time.
	metalock sync.Mutex   // Protects meta page access.
	mmaplock sync.RWMutex // Protects mmap access during remapping

	ops struct {
		writeAt func(b []byte, off int64) (n int, err error)
	}
}

// Options defines configuration parameters with which a PageBuf is initialized.
type Options struct {
	// Timeout is the amount of time to wait to obtain a file lock.
	// When set to zero it will wait indefinitely. This option is only
	// available on Darwin and Linux.
	Timeout time.Duration

	// Sets the DB.MmapFlags flag before memory mapping the file.
	MmapFlags int

	// XXX(fabxc): potentially allow setting different allocation strategies
	// to fit different use cases.

	// InitialMmapSize is the initial mmap size of the database
	// in bytes.
	//
	// If <=0, the initial map size is 0.
	// If initialMmapSize is smaller than the previous database size,
	// it takes no effect.
	InitialMmapSize int

	// PageSize defines a custom page size used. It cannot be changed later.
	// Must be a multiple of the operating system's default page size.
	PageSize int
}

// DefaultOptions specifies a set of default parameters used when a pagebuf
// is opened without explicit options.
var DefaultOptions = Options{
	// Use the OS's default page size.
	PageSize: defaultPageSize,
}

// Default values if not set in a DB instance.
const (
	DefaultAllocSize = 16 * 1024 * 1024
)

// Open and create a new database under the given path.
func Open(path string, mode os.FileMode, o *Options) (*DB, error) {
	db := &DB{
		opened: true,
	}

	// Set default options if no options are provided.
	if o == nil {
		o = &DefaultOptions
	}
	db.MmapFlags = o.MmapFlags

	db.AllocSize = DefaultAllocSize

	flag := os.O_RDWR

	// Open data file and separate sync handler for metadata writes.
	db.path = path
	var err error
	if db.file, err = os.OpenFile(db.path, flag|os.O_CREATE, mode); err != nil {
		_ = db.close()
		return nil, err
	}

	// Lock file so that other processes using pagebuf in read-write mode cannot
	// use the underlying data at the same time.
	if err := flock(db, mode, true, o.Timeout); err != nil {
		_ = db.close()
		return nil, err
	}

	// Default values for test hooks
	db.ops.writeAt = db.file.WriteAt

	// Initialize the database if it doesn't exist.
	if info, err := db.file.Stat(); err != nil {
		return nil, err
	} else if info.Size() == 0 {
		// Initialize new files with meta pages.
		if err := db.init(o.PageSize); err != nil {
			return nil, err
		}
	} else {
		// Read the first meta page to determine the page size.
		var buf [0x1000]byte
		if _, err := db.file.ReadAt(buf[:], 0); err == nil {
			m := db.pageInBuffer(buf[:], 0).meta()
			if err := m.validate(); err != nil {
				// We cannot verify which page sizes are used.
				return nil, fmt.Errorf("cannot read page size: %s", err)
			} else {
				db.pageSize = int(m.pageSize)
			}
		} else {
			return nil, fmt.Errorf("reading first meta page failed: %s", err)
		}
	}

	// Initialize page pool.
	db.pagePool = sync.Pool{
		New: func() interface{} {
			return make([]byte, db.pageSize)
		},
	}

	// Memory map the data file.
	if err := db.mmap(o.InitialMmapSize); err != nil {
		_ = db.close()
		return nil, err
	}

	// Read in the freelist.
	db.freelist = newFreelist()
	db.freelist.read(db.page(db.meta().freelist))

	// Mark the database as opened and return.
	return db, nil
}

func validatePageSize(psz int) error {
	// Max value the content length can hold.
	if defaultPageSize > math.MaxUint16 {
		return fmt.Errorf("invalid page size %d", psz)
	}
	// Page size must be a multiple of OS page size so we stay
	// page aligned.
	if psz < defaultPageSize {
		if defaultPageSize%psz != 0 {
			return fmt.Errorf("invalid page size %d", psz)
		}
	} else if psz > defaultPageSize {
		if psz%defaultPageSize != 0 {
			return fmt.Errorf("invalid page size %d", psz)
		}
	}
	return nil
}

// init creates a new database file and initializes its meta pages.
func (db *DB) init(psz int) error {
	if err := validatePageSize(psz); err != nil {
		return err
	}
	// Set the page size to the OS page size.
	db.pageSize = psz

	// Create two meta pages on a buffer.
	buf := make([]byte, db.pageSize*4)
	for i := 0; i < 2; i++ {
		p := db.pageInBuffer(buf[:], pgid(i))
		p.id = pgid(i)
		p.flags = pageFlagMeta

		// Initialize the meta page.
		m := p.meta()
		m.magic = magic
		m.version = version
		m.pageSize = uint32(db.pageSize)
		m.freelist = 2
		m.txid = txid(i)
		m.pgid = 4 // TODO(fabxc): we initialize with zero pages, what to do here?
		m.checksum = m.sum64()
	}

	// Write an empty freelist at page 3.
	p := db.pageInBuffer(buf[:], pgid(2))
	p.id = pgid(2)
	p.flags = pageFlagFreelist
	p.count = 0

	// Write the first empty page.
	p = db.pageInBuffer(buf[:], pgid(3))
	p.id = pgid(3)
	p.flags = pageFlagData
	p.count = 0

	// Write the buffer to our data file.
	if _, err := db.ops.writeAt(buf, 0); err != nil {
		return err
	}
	if err := fdatasync(db); err != nil {
		return err
	}

	return nil
}

// Sync executes fdatasync() against the database file handle.
func (db *DB) Sync() error { return fdatasync(db) }

// Close synchronizes and closes the memory-mapped pagebuf file.
func (db *DB) Close() error {
	db.rwlock.Lock()
	defer db.rwlock.Unlock()

	db.metalock.Lock()
	defer db.metalock.Unlock()

	db.mmaplock.RLock()
	defer db.mmaplock.RUnlock()

	return db.close()
}

func (db *DB) close() error {
	if !db.opened {
		return nil
	}

	db.opened = false
	db.freelist = nil
	db.ops.writeAt = nil

	// Close the mmap.
	if err := db.munmap(); err != nil {
		return err
	}

	// Close file handles.
	if db.file != nil {
		// Close the file descriptor.
		if err := db.file.Close(); err != nil {
			return fmt.Errorf("db file close: %s", err)
		}
		db.file = nil
	}

	db.path = ""
	return nil
}

// Update executes a function within the context of a read-write managed transaction.
// If no error is returned from the function then the transaction is committed.
// If an error is returned then the entire transaction is rolled back.
// Any error that is returned from the function or returned from the commit is
// returned from the Update() method.
//
// Attempting to manually commit or rollback within the function will cause a panic.
func (db *DB) Update(fn func(*Tx) error) error {
	t, err := db.Begin(true)
	if err != nil {
		return err
	}

	// Make sure the transaction rolls back in the event of a panic.
	defer func() {
		if t.db != nil {
			t.rollback()
		}
	}()

	// Mark as a managed tx so that the inner function cannot manually commit.
	t.managed = true

	// If an error is returned from the function then rollback and return error.
	err = fn(t)
	t.managed = false
	if err != nil {
		_ = t.Rollback()
		return err
	}

	return t.Commit()
}

// View executes a function within the context of a managed read-only transaction.
// Any error that is returned from the function is returned from the View() method.
//
// Attempting to manually rollback within the function will cause a panic.
func (db *DB) View(fn func(*Tx) error) error {
	t, err := db.Begin(false)
	if err != nil {
		return err
	}

	// Make sure the transaction rolls back in the event of a panic.
	defer func() {
		if t.db != nil {
			t.rollback()
		}
	}()

	// Mark as a managed tx so that the inner function cannot manually rollback.
	t.managed = true

	// If an error is returned from the function then pass it through.
	err = fn(t)
	t.managed = false
	if err != nil {
		_ = t.Rollback()
		return err
	}

	if err := t.Rollback(); err != nil {
		return err
	}

	return nil
}

// pageExists checks whether the page with the given id exists.
func (db *DB) pageExists(id pgid) bool {
	// The page exists if it is not in the freelist or out of the data range.
	return !db.freelist.cache[pgid(id)] && int(id+1)*db.pageSize < db.datasz
}

// page retrieves a page reference from the mmap based on the current page size.
func (db *DB) page(id pgid) *page {
	pos := id * pgid(db.pageSize)
	return (*page)(unsafe.Pointer(&db.data[pos]))
}

// pageInBuffer retrieves a page reference from a given byte array based on the current
// page size.
func (db *DB) pageInBuffer(b []byte, id pgid) *page {
	pos := id * pgid(db.pageSize)
	return (*page)(unsafe.Pointer(&b[pos]))
}

// meta retrieves the current meta page reference.
func (db *DB) meta() *meta {
	// We have to return the meta with the highest txid which doesn't fail
	// validation. Otherwise, we can cause errors when in fact the database is
	// in a consistent state. metaA is the one with the higher txid.
	metaA := db.meta0
	metaB := db.meta1
	if db.meta1.txid > db.meta0.txid {
		metaA = db.meta1
		metaB = db.meta0
	}

	// Use higher meta page if valid. Otherwise fallback to previous, if valid.
	if err := metaA.validate(); err == nil {
		return metaA
	} else if err := metaB.validate(); err == nil {
		return metaB
	}

	// This should never be reached, because both meta1 and meta0 were validated
	// on mmap() and we do fsync() on every write.
	panic("pagebuf.PageBuf.meta(): invalid meta pages")
}

// allocate returns a contiguous block of memory starting at a given page.
func (db *DB) allocate(count int) (*page, error) {
	// Allocate a temporary buffer for the page.
	var buf []byte
	if count == 1 {
		buf = db.pagePool.Get().([]byte)
	} else {
		buf = make([]byte, count*db.pageSize)
	}
	p := (*page)(unsafe.Pointer(&buf[0]))
	p.overflow = uint32(count - 1)

	// Use pages from the freelist if they are available.
	if p.id = db.freelist.allocate(count); p.id != 0 {
		return p, nil
	}

	// Resize mmap() if we're at the end.
	p.id = db.rwtx.meta.pgid
	var minsz = int((p.id+pgid(count))+1) * db.pageSize
	if minsz >= db.datasz {
		if err := db.mmap(minsz); err != nil {
			return nil, fmt.Errorf("mmap allocate error: %s", err)
		}
	}

	// Move the page id high water mark.
	db.rwtx.meta.pgid += pgid(count)
	return p, nil
}

// grow grows the size of the database to the given sz.
func (db *DB) grow(sz int) error {
	// Ignore if the new size is less than available file size.
	if sz <= db.filesz {
		return nil
	}

	// If the data is smaller than the alloc size then only allocate what's needed.
	// Once it goes over the allocation size then allocate in chunks.
	if db.datasz < db.AllocSize {
		sz = db.datasz
	} else {
		sz += db.AllocSize
	}

	// Truncate and fsync to ensure file size metadata is flushed.
	// https://github.com/boltdb/bolt/issues/284
	if runtime.GOOS != "windows" {
		if err := db.file.Truncate(int64(sz)); err != nil {
			return fmt.Errorf("file resize error: %s", err)
		}
	}
	if err := db.file.Sync(); err != nil {
		return fmt.Errorf("file sync error: %s", err)
	}

	db.filesz = sz
	return nil
}

// mmap opens the underlying memory-mapped file and initializes it.
// minsz is the minimum size that the mmap can be.
func (db *DB) mmap(minsz int) error {
	db.mmaplock.Lock()
	defer db.mmaplock.Unlock()

	info, err := db.file.Stat()
	if err != nil {
		return fmt.Errorf("mmap stat error: %s", err)
	} else if int(info.Size()) < db.pageSize*2 {
		return fmt.Errorf("file size too small")
	}

	// Ensure the size is at least the minimum size.
	var size = int(info.Size())
	if size < minsz {
		size = minsz
	}
	size, err = db.mmapSize(size)
	if err != nil {
		return err
	}

	// Unmap existing data before continuing.
	if err := db.munmap(); err != nil {
		return err
	}
	// Memory-map the data file as a byte slice.
	if err := mmap(db, size); err != nil {
		return err
	}

	// Save references to the meta pages.
	db.meta0 = db.page(0).meta()
	db.meta1 = db.page(1).meta()

	// Validate the meta pages. We only return an error if both meta pages fail
	// validation, since meta0 failing validation means that it wasn't saved
	// properly -- but we can recover using meta1. And vice-versa.
	err0 := db.meta0.validate()
	err1 := db.meta1.validate()
	if err0 != nil && err1 != nil {
		return err0
	}
	return nil
}

// munmap unmaps the data file from memory.
func (db *DB) munmap() error {
	if err := munmap(db); err != nil {
		return fmt.Errorf("unmap error: %s", err)
	}
	return nil
}

// mmapSize determines the appropriate size for the mmap given the current size
// of the database. The minimum size is 32KB and doubles until it reaches 1GB.
// Returns an error if the new mmap size is greater than the max allowed.
func (db *DB) mmapSize(size int) (int, error) {
	// Double the size from 32KB until 1GB.
	for i := uint(15); i <= 30; i++ {
		if size <= 1<<i {
			return 1 << i, nil
		}
	}

	// Verify the requested size is not above the maximum allowed.
	if size > maxMapSize {
		return 0, fmt.Errorf("mmap too large")
	}

	// If larger than 1GB then grow by 1GB at a time.
	sz := int64(size)
	if remainder := sz % int64(maxMmapStep); remainder > 0 {
		sz += int64(maxMmapStep) - remainder
	}

	// Ensure that the mmap size is a multiple of the page size.
	// This should always be true since we're incrementing in MBs.
	pageSize := int64(db.pageSize)
	if (sz % pageSize) != 0 {
		sz = ((sz / pageSize) + 1) * pageSize
	}

	// If we've exceeded the max size then only grow up to the max size.
	if sz > maxMapSize {
		sz = maxMapSize
	}

	return int(sz), nil
}

func (db *DB) String() string {
	return fmt.Sprintf("PageBuf<%s>", db.path)
}

// Path returns the path to the currently opened pagebuf file.
func (db *DB) Path() string {
	return db.path
}

// Begin starts a new transaction.
// Multiple read-only transactions can be used concurrently but only one
// write transaction can be used at a time. Starting multiple write transactions
// will cause the calls to block and be serialized until the current write
// transaction finishes.
//
// Transactions should not be dependent on one another. Opening a read
// transaction and a write transaction in the same goroutine can cause the
// writer to deadlock because the database periodically needs to re-mmap itself
// as it grows and it cannot do that while a read transaction is open.
//
// If a long running read transaction (for example, a snapshot transaction) is
// needed, you might want to set PageBuf.InitialMmapSize to a large enough value
// to avoid potential blocking of write transaction.
//
// IMPORTANT: You must close read-only transactions after you are finished or
// else the database will not reclaim old pages.
func (db *DB) Begin(writable bool) (*Tx, error) {
	if writable {
		return db.beginRWTx()
	}
	return db.beginTx()
}

func (db *DB) beginTx() (*Tx, error) {
	// Lock the meta pages while we initialize the transaction. We obtain
	// the meta lock before the mmap lock because that's the order that the
	// write transaction will obtain them.
	db.metalock.Lock()

	// Obtain a read-only lock on the mmap. When the mmap is remapped it will
	// obtain a write lock so all transactions must finish before it can be
	// remapped.
	db.mmaplock.RLock()

	// Exit if the database is not open yet.
	if !db.opened {
		db.mmaplock.RUnlock()
		db.metalock.Unlock()
		return nil, ErrDatabaseNotOpen
	}

	// Create a transaction associated with the database.
	t := &Tx{}
	t.init(db)

	// Keep track of transaction until it closes.
	db.txs = append(db.txs, t)

	// Unlock the meta pages.
	db.metalock.Unlock()

	return t, nil
}

func (db *DB) beginRWTx() (*Tx, error) {
	// Obtain writer lock. This is released by the transaction when it closes.
	// This enforces only one writer transaction at a time.
	db.rwlock.Lock()

	// Once we have the writer lock then we can lock the meta pages so that
	// we can set up the transaction.
	db.metalock.Lock()
	defer db.metalock.Unlock()

	// Exit if the database is not open yet.
	if !db.opened {
		db.rwlock.Unlock()
		return nil, ErrDatabaseNotOpen
	}

	// Create a transaction associated with the database.
	t := &Tx{writable: true}
	t.init(db)
	db.rwtx = t

	// Free any pages associated with closed read-only transactions.
	var minid txid = 0xFFFFFFFFFFFFFFFF
	for _, t := range db.txs {
		if t.meta.txid < minid {
			minid = t.meta.txid
		}
	}
	if minid > 0 {
		db.freelist.release(minid - 1)
	}

	return t, nil
}

// removeTx removes a transaction from the database.
func (db *DB) removeTx(tx *Tx) {
	// Release the read lock on the mmap.
	db.mmaplock.RUnlock()

	// Use the meta lock to restrict access to the DB object.
	db.metalock.Lock()

	// Remove the transaction.
	for i, t := range db.txs {
		if t == tx {
			db.txs = append(db.txs[:i], db.txs[i+1:]...)
			break
		}
	}
	// Unlock the meta pages.
	db.metalock.Unlock()
}

// Size represents a valid page size.
type Size int8

// The valid sizes for allocated pages.
const (
	Size512  Size = -3
	Size1024      = -2
	Size2048      = -1
	Size4096      = 0
	Size8192      = 1
)

const (
	upageSizeMin = Size512
	upageSizeMax = Size8192
)

type meta struct {
	magic    uint32
	version  uint32
	pageSize uint32
	flags    uint32
	freelist pgid
	txid     txid
	pgid     pgid
	checksum uint64
}

// validate checks the marker bytes and version of the meta page to ensure it matches this binary.
func (m *meta) validate() error {
	if m.magic != magic {
		return ErrInvalid
	} else if m.version != version {
		return ErrVersionMismatch
	} else if m.checksum != 0 && m.checksum != m.sum64() {
		return ErrChecksum
	}
	return nil
}

// copy copies one meta object to another.
func (m *meta) copy(dest *meta) {
	*dest = *m
}

// write writes the meta onto a page.
func (m *meta) write(p *page) {
	if m.freelist >= m.pgid {
		panic(fmt.Sprintf("freelist pgid (%d) above high water mark (%d)", m.freelist, m.pgid))
	}

	// Page id is either going to be 0 or 1 which we can determine by the transaction ID.
	p.id = pgid(m.txid % 2)
	p.flags |= pageFlagMeta

	// Calculate the checksum.
	m.checksum = m.sum64()

	m.copy(p.meta())
}

// generates the checksum for the meta.
func (m *meta) sum64() uint64 {
	var h = fnv.New64a()
	_, _ = h.Write((*[unsafe.Offsetof(meta{}.checksum)]byte)(unsafe.Pointer(m))[:])
	return h.Sum64()
}

// _assert will panic with a given formatted message if the given condition is false.
func _assert(condition bool, msg string, v ...interface{}) {
	if !condition {
		panic(fmt.Sprintf("assertion failed: "+msg, v...))
	}
}