package r2

import (
	"context"
	"fmt"
	"io"
	"time"

	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/credentials"
	"github.com/aws/aws-sdk-go-v2/service/s3"
	"github.com/aws/aws-sdk-go-v2/service/s3/types"
	"github.com/jeffemmett/upload-service/internal/config"
)

const (
	multipartThreshold = 100 * 1024 * 1024  // 100MB
	partSize           = 64 * 1024 * 1024   // 64MB chunks
	presignExpiry      = 5 * time.Minute
)

type Client struct {
	s3     *s3.Client
	presign *s3.PresignClient
	bucket string
}

func NewClient(cfg *config.Config) *Client {
	s3Client := s3.New(s3.Options{
		BaseEndpoint: aws.String(cfg.R2Endpoint),
		Region:       "auto",
		Credentials:  credentials.NewStaticCredentialsProvider(cfg.R2AccessKeyID, cfg.R2SecretAccessKey, ""),
	})

	return &Client{
		s3:     s3Client,
		presign: s3.NewPresignClient(s3Client),
		bucket: cfg.R2BucketName,
	}
}

// Upload streams the reader to R2. For files > 100MB, uses multipart upload.
func (c *Client) Upload(ctx context.Context, key, contentType string, size int64, body io.Reader) error {
	if size > multipartThreshold {
		return c.uploadMultipart(ctx, key, contentType, body)
	}
	return c.uploadSimple(ctx, key, contentType, body)
}

func (c *Client) uploadSimple(ctx context.Context, key, contentType string, body io.Reader) error {
	_, err := c.s3.PutObject(ctx, &s3.PutObjectInput{
		Bucket:      &c.bucket,
		Key:         &key,
		Body:        body,
		ContentType: &contentType,
	})
	return err
}

func (c *Client) uploadMultipart(ctx context.Context, key, contentType string, body io.Reader) error {
	create, err := c.s3.CreateMultipartUpload(ctx, &s3.CreateMultipartUploadInput{
		Bucket:      &c.bucket,
		Key:         &key,
		ContentType: &contentType,
	})
	if err != nil {
		return fmt.Errorf("create multipart: %w", err)
	}
	uploadID := create.UploadId

	var parts []types.CompletedPart
	buf := make([]byte, partSize)
	partNum := int32(1)

	for {
		n, readErr := io.ReadFull(body, buf)
		if n == 0 && readErr != nil {
			break
		}

		upload, err := c.s3.UploadPart(ctx, &s3.UploadPartInput{
			Bucket:     &c.bucket,
			Key:        &key,
			UploadId:   uploadID,
			PartNumber: &partNum,
			Body:       io.NopCloser(io.LimitReader(bytesReader(buf[:n]), int64(n))),
		})
		if err != nil {
			c.s3.AbortMultipartUpload(ctx, &s3.AbortMultipartUploadInput{
				Bucket:   &c.bucket,
				Key:      &key,
				UploadId: uploadID,
			})
			return fmt.Errorf("upload part %d: %w", partNum, err)
		}

		parts = append(parts, types.CompletedPart{
			PartNumber: &partNum,
			ETag:       upload.ETag,
		})
		partNum++

		if readErr != nil {
			break
		}
	}

	_, err = c.s3.CompleteMultipartUpload(ctx, &s3.CompleteMultipartUploadInput{
		Bucket:   &c.bucket,
		Key:      &key,
		UploadId: uploadID,
		MultipartUpload: &types.CompletedMultipartUpload{
			Parts: parts,
		},
	})
	if err != nil {
		return fmt.Errorf("complete multipart: %w", err)
	}

	return nil
}

// PresignGet generates a presigned download URL valid for 5 minutes.
func (c *Client) PresignGet(ctx context.Context, key, filename string) (string, error) {
	resp, err := c.presign.PresignGetObject(ctx, &s3.GetObjectInput{
		Bucket:                     &c.bucket,
		Key:                        &key,
		ResponseContentDisposition: aws.String(fmt.Sprintf(`attachment; filename="%s"`, filename)),
	}, s3.WithPresignExpires(presignExpiry))
	if err != nil {
		return "", err
	}
	return resp.URL, nil
}

// Delete removes an object from R2.
func (c *Client) Delete(ctx context.Context, key string) error {
	_, err := c.s3.DeleteObject(ctx, &s3.DeleteObjectInput{
		Bucket: &c.bucket,
		Key:    &key,
	})
	return err
}

// bytesReader wraps a byte slice as an io.Reader.
type bytesReaderImpl struct {
	data []byte
	pos  int
}

func bytesReader(b []byte) io.Reader {
	// Make a copy so the buffer can be reused
	cp := make([]byte, len(b))
	copy(cp, b)
	return &bytesReaderImpl{data: cp}
}

func (r *bytesReaderImpl) Read(p []byte) (int, error) {
	if r.pos >= len(r.data) {
		return 0, io.EOF
	}
	n := copy(p, r.data[r.pos:])
	r.pos += n
	return n, nil
}