// Copyright 2015 The Vanadium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package watchable provides a Syncbase-specific store.Store wrapper that
// provides versioned storage for specified prefixes and maintains a watchable
// log of operations performed on versioned records. This log forms the basis
// for the implementation of client-facing watch as well as the sync module's
// internal watching of store updates.
//
// LogEntry records are stored chronologically, using keys of the form
// "$log:<seq>". Sequence numbers are zero-padded to ensure that the
// lexicographic order matches the numeric order.
//
// Version number records are stored using keys of the form "$version:<key>",
// where <key> is the client-specified key.
package watchable

// TODO(sadovsky): Write unit tests. (As of May 2015 we're still iterating on
// the design for how to expose a "watch" API from the storage engine, and we
// don't want to write lots of tests prematurely.)
// TODO(sadovsky): Expose helper functions for constructing LogEntry keys.
// TODO(sadovsky): Allow clients to subscribe via Go channel.

import (
	"fmt"
	"strconv"
	"strings"
	"sync"

	pubutil "v.io/syncbase/v23/syncbase/util"
	"v.io/syncbase/x/ref/services/syncbase/clock"
	"v.io/syncbase/x/ref/services/syncbase/server/util"
	"v.io/syncbase/x/ref/services/syncbase/store"
)

// Store is a store.Store that provides versioned storage and a watchable oplog.
// TODO(sadovsky): Extend interface.
type Store interface {
	store.Store
}

// Options configures a watchable.Store.
type Options struct {
	// Key prefixes to version and log. If nil, all keys are managed.
	ManagedPrefixes []string
}

// Wrap returns a watchable.Store that wraps the given store.Store.
func Wrap(st store.Store, opts *Options) (Store, error) {
	// Determine initial value for seq.
	// TODO(sadovsky): Consider using a bigger seq.
	var seq uint64 = 0
	// TODO(sadovsky): Perform a binary search to determine seq, or persist the
	// current sequence number on every nth write so that at startup time we can
	// start our scan from there instead scanning over all log entries just to
	// find the latest one.
	it := st.Scan(util.ScanPrefixArgs(util.LogPrefix, ""))
	advanced := false
	keybuf := []byte{}
	for it.Advance() {
		advanced = true
		keybuf = it.Key(keybuf)
	}
	if err := it.Err(); err != nil {
		return nil, err
	}
	if advanced {
		key := string(keybuf)
		parts := split(key)
		if len(parts) != 2 {
			panic("wrong number of parts: " + key)
		}
		var err error
		seq, err = strconv.ParseUint(parts[1], 10, 64)
		// Current value of seq points to the last transaction committed.
		// Increment this value by 1.
		seq++
		if err != nil {
			panic("failed to parse seq: " + key)
		}
	}
	vclock := clock.NewVClock()
	return &wstore{ist: st, opts: opts, seq: seq, clock: vclock}, nil
}

type wstore struct {
	ist   store.Store
	opts  *Options
	mu    sync.Mutex    // held during transaction commits; protects seq
	seq   uint64        // the next sequence number to be used for a new commit
	clock *clock.VClock // used to provide write timestamps
}

var _ Store = (*wstore)(nil)

// Close implements the store.Store interface.
func (st *wstore) Close() error {
	return st.ist.Close()
}

// Get implements the store.StoreReader interface.
func (st *wstore) Get(key, valbuf []byte) ([]byte, error) {
	if !st.managesKey(key) {
		return st.ist.Get(key, valbuf)
	}
	sn := newSnapshot(st)
	defer sn.Close()
	return sn.Get(key, valbuf)
}

// Scan implements the store.StoreReader interface.
func (st *wstore) Scan(start, limit []byte) store.Stream {
	if !st.managesRange(start, limit) {
		return st.ist.Scan(start, limit)
	}
	// TODO(sadovsky): Close snapshot once stream is finished or canceled.
	return newSnapshot(st).Scan(start, limit)
}

// Put implements the store.StoreWriter interface.
func (st *wstore) Put(key, value []byte) error {
	// Use watchable.Store transaction so this op gets logged.
	return store.RunInTransaction(st, func(st store.StoreReadWriter) error {
		return st.Put(key, value)
	})
}

// Delete implements the store.StoreWriter interface.
func (st *wstore) Delete(key []byte) error {
	// Use watchable.Store transaction so this op gets logged.
	return store.RunInTransaction(st, func(st store.StoreReadWriter) error {
		return st.Delete(key)
	})
}

// NewTransaction implements the store.Store interface.
func (st *wstore) NewTransaction() store.Transaction {
	return newTransaction(st)
}

// NewSnapshot implements the store.Store interface.
func (st *wstore) NewSnapshot() store.Snapshot {
	return newSnapshot(st)
}

// GetOptions returns the options configured on a watchable.Store.
// TODO(rdaoud): expose watchable store through an interface and change this
// function to be a method on the store.
func GetOptions(st store.Store) (*Options, error) {
	wst := st.(*wstore)
	return wst.opts, nil
}

////////////////////////////////////////
// Internal helpers

func (st *wstore) managesKey(key []byte) bool {
	if st.opts.ManagedPrefixes == nil {
		return true
	}
	ikey := string(key)
	// TODO(sadovsky): Optimize, e.g. use binary search (here and below).
	for _, p := range st.opts.ManagedPrefixes {
		if strings.HasPrefix(ikey, p) {
			return true
		}
	}
	return false
}

func (st *wstore) managesRange(start, limit []byte) bool {
	if st.opts.ManagedPrefixes == nil {
		return true
	}
	istart, ilimit := string(start), string(limit)
	for _, p := range st.opts.ManagedPrefixes {
		pstart, plimit := pubutil.PrefixRangeStart(p), pubutil.PrefixRangeLimit(p)
		if pstart <= istart && ilimit <= plimit {
			return true
		}
		if !(plimit <= istart || ilimit <= pstart) {
			// If this happens, there's a bug in the Syncbase server implementation.
			panic(fmt.Sprintf("partial overlap: %q %q %q", p, start, limit))
		}
	}
	return false
}