package media

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"log"
	"sync"

	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/service/s3"
	"github.com/aws/aws-sdk-go-v2/service/s3/types"
)

// multipartUploadWriter is a Writer that uploads transparently to an S3 bucket
// in 5MB parts. It buffers data internally until a part is ready to send over
// the network. Parts are sent as soon as they exceed the minimum part size of
// 5MB.
//
// The caller must call either Complete() or Abort() after finishing writing.
// Failure to do so will leave S3 in an inconsistent state.
type multipartUploadWriter struct {
	ctx                      context.Context
	wg                       sync.WaitGroup
	s3                       S3Client
	buf                      *bytes.Buffer
	bucket, key, contentType string
	partNum                  int32
	uploadResults            chan uploadResult
	uploadID                 string
}

type uploadResult struct {
	completedPart types.CompletedPart
	size          int64
	err           error
}

const targetPartSizeBytes = 5 * 1024 * 1024 // 5MB

// newMultipartUploadWriter creates a new multipart upload writer, including
// creating the upload on S3. Either Complete or Abort must be called after
// calling this function.
func newMultipartUploadWriter(ctx context.Context, s3Client S3Client, bucket, key, contentType string) (*multipartUploadWriter, error) {
	input := s3.CreateMultipartUploadInput{
		Bucket:      aws.String(bucket),
		Key:         aws.String(key),
		ContentType: aws.String(contentType),
	}

	output, err := s3Client.CreateMultipartUpload(ctx, &input)
	if err != nil {
		return nil, fmt.Errorf("error creating multipart upload: %v", err)
	}

	const bufferOverflowSize = 16_384
	b := make([]byte, 0, targetPartSizeBytes+bufferOverflowSize)

	return &multipartUploadWriter{
		ctx:           ctx,
		s3:            s3Client,
		buf:           bytes.NewBuffer(b),
		bucket:        bucket,
		key:           key,
		contentType:   contentType,
		partNum:       1,
		uploadResults: make(chan uploadResult),
		uploadID:      *output.UploadId,
	}, nil
}

func (u *multipartUploadWriter) Write(p []byte) (int, error) {
	n, err := u.buf.Write(p)
	if err != nil {
		return n, fmt.Errorf("error writing to buffer: %v", err)
	}

	if u.buf.Len() >= targetPartSizeBytes {
		buf := make([]byte, u.buf.Len())
		copy(buf, u.buf.Bytes())
		u.buf.Truncate(0)

		u.wg.Add(1)
		go u.uploadPart(buf, u.partNum)

		u.partNum++
	}

	return n, err
}

func (u *multipartUploadWriter) uploadPart(buf []byte, partNum int32) {
	defer u.wg.Done()

	partLen := len(buf)
	log.Printf("uploading part num = %d, len = %d", partNum, partLen)

	input := s3.UploadPartInput{
		Body:          bytes.NewReader(buf),
		Bucket:        aws.String(u.bucket),
		Key:           aws.String(u.key),
		PartNumber:    partNum,
		UploadId:      aws.String(u.uploadID),
		ContentLength: int64(partLen),
	}

	output, uploadErr := u.s3.UploadPart(u.ctx, &input)
	if uploadErr != nil {
		// TODO: retry on failure
		u.uploadResults <- uploadResult{err: fmt.Errorf("error uploading part: %v", uploadErr)}
		return
	}

	log.Printf("uploaded part num = %d, etag = %s, bytes = %d", partNum, *output.ETag, partLen)

	u.uploadResults <- uploadResult{
		completedPart: types.CompletedPart{ETag: output.ETag, PartNumber: partNum},
		size:          int64(partLen),
	}
}

// Complete waits for all currently uploading parts to be uploaded, and
// finalizes the object in S3. Close() must be called first.
func (u *multipartUploadWriter) Complete() (int64, error) {
	var completedParts []types.CompletedPart
	var uploadedBytes int64

	// we wait for all parts to be completed before collecting the results:
	wgDone := make(chan struct{})
	go func() {
		u.wg.Wait()
		close(u.uploadResults)
		wgDone <- struct{}{}
	}()

outer:
	for {
		select {
		case uploadResult := <-u.uploadResults:
			if uploadResult.err != nil {
				return 0, uploadResult.err
			}

			completedParts = append(completedParts, uploadResult.completedPart)
			uploadedBytes += uploadResult.size
		case <-wgDone:
			break outer
		case <-u.ctx.Done():
			return 0, u.ctx.Err()
		}
	}

	if len(completedParts) == 0 {
		return 0, errors.New("no parts available to upload")
	}

	input := s3.CompleteMultipartUploadInput{
		Bucket:          aws.String(u.bucket),
		Key:             aws.String(u.key),
		UploadId:        aws.String(u.uploadID),
		MultipartUpload: &types.CompletedMultipartUpload{Parts: completedParts},
	}

	_, err := u.s3.CompleteMultipartUpload(u.ctx, &input)
	if err != nil {
		return 0, fmt.Errorf("error completing upload: %v", err)
	}

	log.Printf("completed upload, key = %s, bytesUploaded = %d", u.key, uploadedBytes)
	return uploadedBytes, nil
}

// Abort aborts the upload process, cancelling the upload on S3. It accepts a
// separate context to the associated writer in case it is called during
// cleanup after the original context was killed.
func (u *multipartUploadWriter) Abort(ctx context.Context) error {
	input := s3.AbortMultipartUploadInput{
		Bucket:   aws.String(u.bucket),
		Key:      aws.String(u.key),
		UploadId: aws.String(u.uploadID),
	}

	_, err := u.s3.AbortMultipartUpload(ctx, &input)
	if err != nil {
		return fmt.Errorf("error aborting upload: %v", err)
	}

	log.Printf("aborted upload, key = %s", u.key)

	return nil
}