// Copyright (C) MongoDB, Inc. 2017-present.
//
// 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

package mongo

import (
	"context"
	"errors"

	"github.com/mongodb/mongo-go-driver/bson"
	"github.com/mongodb/mongo-go-driver/bson/bsoncodec"
	"github.com/mongodb/mongo-go-driver/x/mongo/driver"
)

// Cursor is used to iterate a stream of documents. Each document is decoded into the result
// according to the rules of the bson package.
//
// A typical usage of the Cursor type would be:
//
//		var cur *Cursor
//		ctx := context.Background()
//		defer cur.Close(ctx)
//
// 		for cur.Next(ctx) {
//			elem := &bson.D{}
//			if err := cur.Decode(elem); err != nil {
// 				log.Fatal(err)
// 			}
//
// 			// do something with elem....
//		}
//
// 		if err := cur.Err(); err != nil {
//			log.Fatal(err)
//		}
//
type Cursor struct {
	// Current is the BSON bytes of the current document. This property is only valid until the next
	// call to Next or Close. If continued access is required to the bson.Raw, you must make a copy
	// of it.
	Current bson.Raw

	bc       batchCursor
	pos      int
	batch    []byte
	registry *bsoncodec.Registry

	err error
}

func newCursor(bc batchCursor, registry *bsoncodec.Registry) (*Cursor, error) {
	if registry == nil {
		registry = bson.DefaultRegistry
	}
	if bc == nil {
		return nil, errors.New("batch cursor must not be nil")
	}
	return &Cursor{bc: bc, pos: 0, batch: make([]byte, 0, 256), registry: registry}, nil
}

func newEmptyCursor() *Cursor {
	return &Cursor{bc: driver.NewEmptyBatchCursor()}
}

// ID returns the ID of this cursor.
func (c *Cursor) ID() int64 { return c.bc.ID() }

func (c *Cursor) advanceCurrentDocument() bool {
	if len(c.batch[c.pos:]) < 4 {
		c.err = errors.New("could not read next document: insufficient bytes")
		return false
	}
	length := (int(c.batch[c.pos]) | int(c.batch[c.pos+1])<<8 | int(c.batch[c.pos+2])<<16 | int(c.batch[c.pos+3])<<24)
	if len(c.batch[c.pos:]) < length {
		c.err = errors.New("could not read next document: insufficient bytes")
		return false
	}
	if len(c.Current) > 4 {
		c.Current[0], c.Current[1], c.Current[2], c.Current[3] = 0x00, 0x00, 0x00, 0x00 // Invalidate the current document
	}
	c.Current = c.batch[c.pos : c.pos+length]
	c.pos += length
	return true
}

// Next gets the next result from this cursor. Returns true if there were no errors and the next
// result is available for decoding.
func (c *Cursor) Next(ctx context.Context) bool {
	if ctx == nil {
		ctx = context.Background()
	}
	if c.pos < len(c.batch) {
		return c.advanceCurrentDocument()
	}

	// clear the batch
	c.batch = c.batch[:0]
	c.pos = 0
	c.Current = c.Current[:0]

	// call the Next method in a loop until at least one document is returned in the next batch or
	// the context times out.
	for len(c.batch) == 0 {
		// If we don't have a next batch
		if !c.bc.Next(ctx) {
			// Do we have an error? If so we return false.
			c.err = c.bc.Err()
			if c.err != nil {
				return false
			}
			// Is the cursor ID zero?
			if c.bc.ID() == 0 {
				return false
			}
			// empty batch, but cursor is still valid, so continue.
			continue
		}

		c.batch = c.bc.Batch(c.batch[:0])
	}

	return c.advanceCurrentDocument()
}

// Decode will decode the current document into val.
func (c *Cursor) Decode(val interface{}) error {
	return bson.UnmarshalWithRegistry(c.registry, c.Current, val)
}

// Err returns the current error.
func (c *Cursor) Err() error { return c.err }

// Close closes this cursor.
func (c *Cursor) Close(ctx context.Context) error { return c.bc.Close(ctx) }