/*
   GoToSocial
   Copyright (C) 2021-2023 GoToSocial Authors admin@gotosocial.org

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU Affero General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Affero General Public License for more details.

   You should have received a copy of the GNU Affero General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

package timeline

import (
	"container/list"
	"context"
	"errors"
	"fmt"
	"time"

	"codeberg.org/gruf/go-kv"
	"github.com/superseriousbusiness/gotosocial/internal/log"
)

const retries = 5

func (t *timeline) LastGot() time.Time {
	t.Lock()
	defer t.Unlock()
	return t.lastGot
}

func (t *timeline) Get(ctx context.Context, amount int, maxID string, sinceID string, minID string, prepareNext bool) ([]Preparable, error) {
	l := log.WithFields(kv.Fields{
		{"accountID", t.accountID},
		{"amount", amount},
		{"maxID", maxID},
		{"sinceID", sinceID},
		{"minID", minID},
	}...)
	l.Debug("entering get and updating t.lastGot")

	// regardless of what happens below, update the
	// last time Get was called for this timeline
	t.Lock()
	t.lastGot = time.Now()
	t.Unlock()

	var items []Preparable
	var err error

	// no params are defined to just fetch from the top
	// this is equivalent to a user asking for the top x items from their timeline
	if maxID == "" && sinceID == "" && minID == "" {
		items, err = t.getXFromTop(ctx, amount)
		// aysnchronously prepare the next predicted query so it's ready when the user asks for it
		if len(items) != 0 {
			nextMaxID := items[len(items)-1].GetID()
			if prepareNext {
				// already cache the next query to speed up scrolling
				go func() {
					// use context.Background() because we don't want the query to abort when the request finishes
					if err := t.prepareNextQuery(context.Background(), amount, nextMaxID, "", ""); err != nil {
						l.Errorf("error preparing next query: %s", err)
					}
				}()
			}
		}
	}

	// maxID is defined but sinceID isn't so take from behind
	// this is equivalent to a user asking for the next x items from their timeline, starting from maxID
	if maxID != "" && sinceID == "" {
		attempts := 0
		items, err = t.getXBehindID(ctx, amount, maxID, &attempts)
		// aysnchronously prepare the next predicted query so it's ready when the user asks for it
		if len(items) != 0 {
			nextMaxID := items[len(items)-1].GetID()
			if prepareNext {
				// already cache the next query to speed up scrolling
				go func() {
					// use context.Background() because we don't want the query to abort when the request finishes
					if err := t.prepareNextQuery(context.Background(), amount, nextMaxID, "", ""); err != nil {
						l.Errorf("error preparing next query: %s", err)
					}
				}()
			}
		}
	}

	// maxID is defined and sinceID || minID are as well, so take a slice between them
	// this is equivalent to a user asking for items older than x but newer than y
	if maxID != "" && sinceID != "" {
		items, err = t.getXBetweenID(ctx, amount, maxID, minID)
	}
	if maxID != "" && minID != "" {
		items, err = t.getXBetweenID(ctx, amount, maxID, minID)
	}

	// maxID isn't defined, but sinceID || minID are, so take x before
	// this is equivalent to a user asking for items newer than x (eg., refreshing the top of their timeline)
	if maxID == "" && sinceID != "" {
		items, err = t.getXBeforeID(ctx, amount, sinceID, true)
	}
	if maxID == "" && minID != "" {
		items, err = t.getXBeforeID(ctx, amount, minID, true)
	}

	return items, err
}

// getXFromTop returns x amount of items from the top of the timeline, from newest to oldest.
func (t *timeline) getXFromTop(ctx context.Context, amount int) ([]Preparable, error) {
	// make a slice of preparedItems with the length we need to return
	preparedItems := make([]Preparable, 0, amount)

	if t.preparedItems.data == nil {
		t.preparedItems.data = &list.List{}
	}

	// make sure we have enough items prepared to return
	if t.preparedItems.data.Len() < amount {
		if err := t.PrepareFromTop(ctx, amount); err != nil {
			return nil, err
		}
	}

	// work through the prepared items from the top and return
	var served int
	for e := t.preparedItems.data.Front(); e != nil; e = e.Next() {
		entry, ok := e.Value.(*preparedItemsEntry)
		if !ok {
			return nil, errors.New("getXFromTop: could not parse e as a preparedItemsEntry")
		}
		preparedItems = append(preparedItems, entry.prepared)
		served++
		if served >= amount {
			break
		}
	}

	return preparedItems, nil
}

// getXBehindID returns x amount of items from the given id onwards, from newest to oldest.
// This will NOT include the item with the given ID.
//
// This corresponds to an api call to /timelines/home?max_id=WHATEVER
func (t *timeline) getXBehindID(ctx context.Context, amount int, behindID string, attempts *int) ([]Preparable, error) {
	l := log.WithFields(kv.Fields{
		{"amount", amount},
		{"behindID", behindID},
		{"attempts", attempts},
	}...)

	newAttempts := *attempts
	newAttempts++
	attempts = &newAttempts

	// make a slice of items with the length we need to return
	items := make([]Preparable, 0, amount)

	if t.preparedItems.data == nil {
		t.preparedItems.data = &list.List{}
	}

	// iterate through the modified list until we hit the mark we're looking for
	var position int
	var behindIDMark *list.Element

findMarkLoop:
	for e := t.preparedItems.data.Front(); e != nil; e = e.Next() {
		position++
		entry, ok := e.Value.(*preparedItemsEntry)
		if !ok {
			return nil, errors.New("getXBehindID: could not parse e as a preparedPostsEntry")
		}

		if entry.itemID <= behindID {
			l.Trace("found behindID mark")
			behindIDMark = e
			break findMarkLoop
		}
	}

	// we didn't find it, so we need to make sure it's indexed and prepared and then try again
	// this can happen when a user asks for really old items
	if behindIDMark == nil {
		if err := t.prepareBehind(ctx, behindID, amount); err != nil {
			return nil, fmt.Errorf("getXBehindID: error preparing behind and including ID %s", behindID)
		}
		oldestID, err := t.oldestPreparedItemID(ctx)
		if err != nil {
			return nil, err
		}
		if oldestID == "" {
			l.Tracef("oldestID is empty so we can't return behindID %s", behindID)
			return items, nil
		}
		if oldestID == behindID {
			l.Tracef("given behindID %s is the same as oldestID %s so there's nothing to return behind it", behindID, oldestID)
			return items, nil
		}
		if *attempts > retries {
			l.Tracef("exceeded retries looking for behindID %s", behindID)
			return items, nil
		}
		l.Trace("trying getXBehindID again")
		return t.getXBehindID(ctx, amount, behindID, attempts)
	}

	// make sure we have enough items prepared behind it to return what we're being asked for
	if t.preparedItems.data.Len() < amount+position {
		if err := t.prepareBehind(ctx, behindID, amount); err != nil {
			return nil, err
		}
	}

	// start serving from the entry right after the mark
	var served int
serveloop:
	for e := behindIDMark.Next(); e != nil; e = e.Next() {
		entry, ok := e.Value.(*preparedItemsEntry)
		if !ok {
			return nil, errors.New("getXBehindID: could not parse e as a preparedPostsEntry")
		}

		// serve up to the amount requested
		items = append(items, entry.prepared)
		served++
		if served >= amount {
			break serveloop
		}
	}

	return items, nil
}

// getXBeforeID returns x amount of items up to the given id, from newest to oldest.
// This will NOT include the item with the given ID.
//
// This corresponds to an api call to /timelines/home?since_id=WHATEVER
func (t *timeline) getXBeforeID(ctx context.Context, amount int, beforeID string, startFromTop bool) ([]Preparable, error) {
	// make a slice of items with the length we need to return
	items := make([]Preparable, 0, amount)

	if t.preparedItems.data == nil {
		t.preparedItems.data = &list.List{}
	}

	// iterate through the modified list until we hit the mark we're looking for, or as close as possible to it
	var beforeIDMark *list.Element
findMarkLoop:
	for e := t.preparedItems.data.Front(); e != nil; e = e.Next() {
		entry, ok := e.Value.(*preparedItemsEntry)
		if !ok {
			return nil, errors.New("getXBeforeID: could not parse e as a preparedPostsEntry")
		}

		if entry.itemID >= beforeID {
			beforeIDMark = e
		} else {
			break findMarkLoop
		}
	}

	if beforeIDMark == nil {
		return items, nil
	}

	var served int

	if startFromTop {
		// start serving from the front/top and keep going until we hit mark or get x amount items
	serveloopFromTop:
		for e := t.preparedItems.data.Front(); e != nil; e = e.Next() {
			entry, ok := e.Value.(*preparedItemsEntry)
			if !ok {
				return nil, errors.New("getXBeforeID: could not parse e as a preparedPostsEntry")
			}

			if entry.itemID == beforeID {
				break serveloopFromTop
			}

			// serve up to the amount requested
			items = append(items, entry.prepared)
			served++
			if served >= amount {
				break serveloopFromTop
			}
		}
	} else if !startFromTop {
		// start serving from the entry right before the mark
	serveloopFromBottom:
		for e := beforeIDMark.Prev(); e != nil; e = e.Prev() {
			entry, ok := e.Value.(*preparedItemsEntry)
			if !ok {
				return nil, errors.New("getXBeforeID: could not parse e as a preparedPostsEntry")
			}

			// serve up to the amount requested
			items = append(items, entry.prepared)
			served++
			if served >= amount {
				break serveloopFromBottom
			}
		}
	}

	return items, nil
}

// getXBetweenID returns x amount of items from the given maxID, up to the given id, from newest to oldest.
// This will NOT include the item with the given IDs.
//
// This corresponds to an api call to /timelines/home?since_id=WHATEVER&max_id=WHATEVER_ELSE
func (t *timeline) getXBetweenID(ctx context.Context, amount int, behindID string, beforeID string) ([]Preparable, error) {
	// make a slice of items with the length we need to return
	items := make([]Preparable, 0, amount)

	if t.preparedItems.data == nil {
		t.preparedItems.data = &list.List{}
	}

	// iterate through the modified list until we hit the mark we're looking for
	var position int
	var behindIDMark *list.Element
findMarkLoop:
	for e := t.preparedItems.data.Front(); e != nil; e = e.Next() {
		position++
		entry, ok := e.Value.(*preparedItemsEntry)
		if !ok {
			return nil, errors.New("getXBetweenID: could not parse e as a preparedPostsEntry")
		}

		if entry.itemID == behindID {
			behindIDMark = e
			break findMarkLoop
		}
	}

	// we didn't find it
	if behindIDMark == nil {
		return nil, fmt.Errorf("getXBetweenID: couldn't find item with ID %s", behindID)
	}

	// make sure we have enough items prepared behind it to return what we're being asked for
	if t.preparedItems.data.Len() < amount+position {
		if err := t.prepareBehind(ctx, behindID, amount); err != nil {
			return nil, err
		}
	}

	// start serving from the entry right after the mark
	var served int
serveloop:
	for e := behindIDMark.Next(); e != nil; e = e.Next() {
		entry, ok := e.Value.(*preparedItemsEntry)
		if !ok {
			return nil, errors.New("getXBetweenID: could not parse e as a preparedPostsEntry")
		}

		if entry.itemID == beforeID {
			break serveloop
		}

		// serve up to the amount requested
		items = append(items, entry.prepared)
		served++
		if served >= amount {
			break serveloop
		}
	}

	return items, nil
}