package contentutil

import (
	"context"
	"io"

	"github.com/containerd/containerd/v2/core/content"
	"github.com/containerd/containerd/v2/core/remotes"
	digest "github.com/opencontainers/go-digest"
	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
	"github.com/pkg/errors"
)

type ReferrersProvider interface {
	content.Provider
	remotes.ReferrersFetcher
}

func FromFetcher(f remotes.Fetcher) ReferrersProvider {
	return &fetchedProvider{
		f: f,
	}
}

type fetchedProvider struct {
	f remotes.Fetcher
}

func (p *fetchedProvider) ReaderAt(ctx context.Context, desc ocispecs.Descriptor) (content.ReaderAt, error) {
	rc, err := p.f.Fetch(ctx, desc)
	if err != nil {
		return nil, err
	}

	return &readerAt{Reader: rc, Closer: rc, size: desc.Size}, nil
}

func (p *fetchedProvider) FetchReferrers(ctx context.Context, dgst digest.Digest, opts ...remotes.FetchReferrersOpt) ([]ocispecs.Descriptor, error) {
	refs, ok := p.f.(remotes.ReferrersFetcher)
	if !ok {
		return nil, errors.New("fetcher does not support referrers")
	}
	return refs.FetchReferrers(ctx, dgst, opts...)
}

type readerAt struct {
	io.Reader
	io.Closer
	size   int64
	offset int64
}

func (r *readerAt) ReadAt(b []byte, off int64) (int, error) {
	if r.offset != off {
		if seeker, ok := r.Reader.(io.Seeker); ok {
			if _, err := seeker.Seek(off, io.SeekStart); err != nil {
				return 0, err
			}
			r.offset = off
		} else {
			if ra, ok := r.Reader.(io.ReaderAt); ok {
				return ra.ReadAt(b, off)
			}
			return 0, errors.New("unsupported offset")
		}
	}

	var totalN int
	for len(b) > 0 {
		n, err := r.Read(b)
		if errors.Is(err, io.EOF) && n == len(b) {
			err = nil
		}
		r.offset += int64(n)
		totalN += n
		b = b[n:]
		if err != nil {
			return totalN, err
		}
	}
	return totalN, nil
}

func (r *readerAt) Size() int64 {
	return r.size
}