/*
   Copyright The containerd Authors.

   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

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

package erofs

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

	"github.com/containerd/containerd/v2/core/content"
	"github.com/containerd/containerd/v2/core/diff"
	"github.com/containerd/containerd/v2/core/images"
	"github.com/containerd/containerd/v2/core/mount"
	"github.com/containerd/containerd/v2/plugins/snapshots/erofs/erofsutils"
	"github.com/containerd/errdefs"
	"github.com/containerd/log"
	digest "github.com/opencontainers/go-digest"
	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
)

var emptyDesc = ocispec.Descriptor{}

type differ interface {
	diff.Applier
	diff.Comparer
}

// erofsDiff does erofs comparison and application
type erofsDiff struct {
	store         content.Store
	mkfsExtraOpts []string
}

func NewErofsDiffer(store content.Store, mkfsExtraOpts []string) differ {
	return &erofsDiff{
		store:         store,
		mkfsExtraOpts: mkfsExtraOpts,
	}
}

// Compare creates a diff between the given mounts and uploads the result
// to the content store.
func (s erofsDiff) Compare(ctx context.Context, lower, upper []mount.Mount, opts ...diff.Opt) (d ocispec.Descriptor, err error) {
	return emptyDesc, fmt.Errorf("erofsDiff does not implement Compare method: %w", errdefs.ErrNotImplemented)
}

func (s erofsDiff) Apply(ctx context.Context, desc ocispec.Descriptor, mounts []mount.Mount, opts ...diff.ApplyOpt) (d ocispec.Descriptor, err error) {
	t1 := time.Now()
	defer func() {
		if err == nil {
			log.G(ctx).WithFields(log.Fields{
				"d":      time.Since(t1),
				"digest": desc.Digest,
				"size":   desc.Size,
				"media":  desc.MediaType,
			}).Debugf("diff applied")
		}
	}()

	if _, err := images.DiffCompression(ctx, desc.MediaType); err != nil {
		return emptyDesc, fmt.Errorf("currently unsupported media type: %s", desc.MediaType)
	}

	var config diff.ApplyConfig
	for _, o := range opts {
		if err := o(ctx, desc, &config); err != nil {
			return emptyDesc, fmt.Errorf("failed to apply config opt: %w", err)
		}
	}

	layer, err := erofsutils.MountsToLayer(mounts)
	if err != nil {
		return emptyDesc, err
	}

	ra, err := s.store.ReaderAt(ctx, desc)
	if err != nil {
		return emptyDesc, fmt.Errorf("failed to get reader from content store: %w", err)
	}
	defer ra.Close()

	processor := diff.NewProcessorChain(desc.MediaType, content.NewReader(ra))
	for {
		if processor, err = diff.GetProcessor(ctx, processor, config.ProcessorPayloads); err != nil {
			return emptyDesc, fmt.Errorf("failed to get stream processor for %s: %w", desc.MediaType, err)
		}
		if processor.MediaType() == ocispec.MediaTypeImageLayer {
			break
		}
	}
	defer processor.Close()

	digester := digest.Canonical.Digester()
	rc := &readCounter{
		r: io.TeeReader(processor, digester.Hash()),
	}

	layerBlobPath := path.Join(layer, "layer.erofs")
	err = erofsutils.ConvertTarErofs(ctx, rc, layerBlobPath, s.mkfsExtraOpts)
	if err != nil {
		return emptyDesc, fmt.Errorf("failed to convert erofs: %w", err)
	}

	// Read any trailing data
	if _, err := io.Copy(io.Discard, rc); err != nil {
		return emptyDesc, err
	}

	return ocispec.Descriptor{
		MediaType: ocispec.MediaTypeImageLayer,
		Size:      rc.c,
		Digest:    digester.Digest(),
	}, nil
}

type readCounter struct {
	r io.Reader
	c int64
}

func (rc *readCounter) Read(p []byte) (n int, err error) {
	n, err = rc.r.Read(p)
	rc.c += int64(n)
	return
}