package media

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"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"
)

type multipartUploader struct {
	s3 S3Client
}

type uploadResult struct {
	completedPart types.CompletedPart
	size          int64
}

type readResult struct {
	n   int
	err error
}

const (
	targetPartSizeBytes = 5 * 1024 * 1024 // 5MB
	bufferOverflowSize  = 16_384          // 16Kb
)

func newMultipartUploader(s3Client S3Client) *multipartUploader {
	return &multipartUploader{s3: s3Client}
}

// Upload uploads 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.
func (u *multipartUploader) Upload(ctx context.Context, r io.Reader, bucket, key, contentType string) (int64, error) {
	var uploaded bool

	input := s3.CreateMultipartUploadInput{
		Bucket:      aws.String(bucket),
		Key:         aws.String(key),
		ContentType: aws.String(contentType),
	}
	output, err := u.s3.CreateMultipartUpload(ctx, &input)
	if err != nil {
		return 0, fmt.Errorf("error creating multipart upload: %v", err)
	}

	// abort the upload if possible, logging any errors, on exit.
	defer func() {
		if uploaded {
			return
		}
		input := s3.AbortMultipartUploadInput{
			Bucket:   aws.String(bucket),
			Key:      aws.String(key),
			UploadId: output.UploadId,
		}

		_, abortErr := u.s3.AbortMultipartUpload(ctx, &input)
		if abortErr != nil {
			log.Printf("error aborting upload: %v", abortErr)
		} else {
			log.Printf("aborted upload, key = %s", key)
		}
	}()

	uploadResultChan := make(chan uploadResult)
	uploadErrorChan := make(chan error, 1)

	// uploadPart uploads an individual part.
	uploadPart := func(wg *sync.WaitGroup, buf []byte, partNum int32) {
		defer wg.Done()

		partLen := int64(len(buf))
		log.Printf("uploading part num = %d, len = %d", partNum, partLen)

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

		output, uploadErr := u.s3.UploadPart(ctx, &input)
		if uploadErr != nil {
			// TODO: retry on failure
			uploadErrorChan <- uploadErr
			return
		}

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

		uploadResultChan <- uploadResult{
			completedPart: types.CompletedPart{ETag: output.ETag, PartNumber: partNum},
			size:          partLen,
		}
	}

	wgDone := make(chan struct{})
	var wg sync.WaitGroup
	wg.Add(1) // done when the reader returns EOF
	go func() {
		wg.Wait()
		wgDone <- struct{}{}
	}()

	readChan := make(chan readResult)
	buf := make([]byte, 32_768)
	go func() {
		for {
			var rr readResult
			rr.n, rr.err = r.Read(buf)
			readChan <- rr

			if rr.err != nil {
				return
			}
		}
	}()

	var closing bool
	currPart := bytes.NewBuffer(make([]byte, 0, targetPartSizeBytes+bufferOverflowSize))
	partNum := int32(1)
	results := make([]uploadResult, 0, 64)

outer:
	for {
		select {
		case uploadResult := <-uploadResultChan:
			results = append(results, uploadResult)
		case uploadErr := <-uploadErrorChan:
			return 0, fmt.Errorf("error while uploading part: %v", uploadErr)
		case <-wgDone:
			break outer
		case <-ctx.Done():
			return 0, ctx.Err()
		case readResult := <-readChan:
			if readResult.err == io.EOF {
				wg.Done()
				closing = true
			} else if readResult.err != nil {
				return 0, fmt.Errorf("reader error: %v", readResult.err)
			}

			_, _ = currPart.Write(buf[:readResult.n])
			if closing || currPart.Len() >= targetPartSizeBytes {
				part := make([]byte, currPart.Len())
				copy(part, currPart.Bytes())
				currPart.Truncate(0)

				wg.Add(1)
				go uploadPart(&wg, part, partNum)
				partNum++
			}
		}
	}

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

	completedParts := make([]types.CompletedPart, 0, 64)
	var uploadedBytes int64
	for _, result := range results {
		completedParts = append(completedParts, result.completedPart)
		uploadedBytes += result.size
	}

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

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

	log.Printf("completed upload, key = %s, bytesUploaded = %d", key, uploadedBytes)
	uploaded = true

	return uploadedBytes, nil
}