package patcher

import (
	"context"
	"errors"
	"fmt"
	"io"

	objectSDK "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object"
	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/transformer"
)

var (
	ErrOffsetExceedsSize         = errors.New("patch offset exceeds object size")
	ErrEmptyPayloadPatch         = errors.New("patch must contain payload")
	ErrInvalidPatchObjectAddress = errors.New("invalid patch object address")
	ErrInvalidPatchOffsetOrder   = errors.New("invalid patch offset order")
)

// PatchRes is the result of patch application.
type PatchRes struct {
	AccessIdentifiers *transformer.AccessIdentifiers
}

// PatchApplier is the interface that provides method to apply header and payload patches.
type PatchApplier interface {
	// ApplyPatch applies the patch for an object. All patches must contain the same target object address.
	// Patch offsets must be passed with non-descending order.
	// Patching the payload is performed within all ApplyPatch ivocations.
	//
	// Unsuccessful call returns false. Error is checked with Close.
	ApplyPatch(ctx context.Context, patch *objectSDK.Patch) bool

	// Close closes PatchApplier when the patch stream is over.
	Close(context.Context) (PatchRes, error)
}

// HeaderProvider is the interface that provides a method to get an original's object header.
type HeaderProvider interface {
	// GetObjectHeader gets an original's object header.
	GetObjectHeader(ctx context.Context, addr oid.Address) (*objectSDK.Object, error)
}

// RangeProvider is the interface that provides a method to get original object payload
// by a given range.
type RangeProvider interface {
	// ReadRange reads an original object payload by the given range.
	// The method returns io.Reader over the data range only. This means if the data is read out,
	// then ReadRange has to be invoked to provide reader over the next range.
	ReadRange(ctx context.Context, addr oid.Address, rng *objectSDK.Range) io.Reader
}

type patcher struct {
	rangeProvider RangeProvider

	objectWriter transformer.ChunkedObjectWriter

	hdrProvider HeaderProvider

	currOffset uint64

	originalPayloadSize uint64

	firstApplyPatchCall bool

	addr oid.Address

	closeErr error
}

func New(hdrProvider HeaderProvider, objectRangeSplitter RangeProvider, objectWriter transformer.ChunkedObjectWriter) PatchApplier {
	return &patcher{
		rangeProvider: objectRangeSplitter,

		objectWriter: objectWriter,

		hdrProvider: hdrProvider,

		firstApplyPatchCall: true,
	}
}

func (p *patcher) ApplyPatch(ctx context.Context, currPatch *objectSDK.Patch) bool {
	if currPatch == nil {
		return true
	}

	if p.firstApplyPatchCall {
		p.firstApplyPatchCall = false

		p.addr = currPatch.Address

		hdr, err := p.hdrProvider.GetObjectHeader(ctx, currPatch.Address)
		if err != nil {
			p.closeErr = fmt.Errorf("get header error: %w", err)
			return false
		}

		p.originalPayloadSize = hdr.PayloadSize()

		if !currPatch.ReplaceAttributes {
			mergedAttrs := mergeAttributes(currPatch.NewAttributes, hdr.Attributes())
			hdr.SetAttributes(mergedAttrs...)
		} else {
			hdr.SetAttributes(currPatch.NewAttributes...)
		}

		if err = p.objectWriter.WriteHeader(ctx, hdr); err != nil {
			p.closeErr = fmt.Errorf("write header error: %w", err)
			return false
		}

		// only header patch
		if currPatch.PayloadPatch == nil {
			return true
		}
	} else {
		// All patches can be applied only for the same object.
		if !p.addr.Equals(currPatch.Address) {
			p.closeErr = fmt.Errorf("%w: expected = %s, got = %s",
				ErrInvalidPatchObjectAddress,
				p.addr.EncodeToString(),
				currPatch.Address.EncodeToString())
			return false
		}

		if currPatch.PayloadPatch == nil {
			p.closeErr = ErrEmptyPayloadPatch
			return false
		}
	}

	if currPatch.PayloadPatch.Range.GetOffset() < p.currOffset {
		p.closeErr = fmt.Errorf("%w: current = %d, previous = %d", ErrInvalidPatchOffsetOrder, currPatch.PayloadPatch.Range.GetOffset(), p.currOffset)
		return false
	}

	if currPatch.PayloadPatch.Range.GetOffset() > p.originalPayloadSize {
		p.closeErr = fmt.Errorf("%w: offset = %d, object size = %d", ErrOffsetExceedsSize, currPatch.PayloadPatch.Range.GetOffset(), p.originalPayloadSize)
		return false
	}

	var err error
	if p.currOffset, err = p.applyPatch(ctx, currPatch, p.currOffset); err != nil {
		p.closeErr = fmt.Errorf("apply patch error: %w", err)
		return false
	}

	return true
}

func (p *patcher) Close(ctx context.Context) (PatchRes, error) {
	if p.closeErr != nil {
		return PatchRes{}, p.closeErr
	}

	rng := new(objectSDK.Range)
	rng.SetOffset(p.currOffset)
	rng.SetLength(p.originalPayloadSize - p.currOffset)

	rdr := p.rangeProvider.ReadRange(ctx, p.addr, rng)
	for {
		remain := make([]byte, 1024)
		n, err := rdr.Read(remain)
		if err != nil {
			if err == io.EOF {
				break
			}
			return PatchRes{}, fmt.Errorf("read error: %w", err)
		}
		_, err = p.objectWriter.Write(ctx, remain[:n])
		if err != nil {
			return PatchRes{}, fmt.Errorf("write error: %w", err)
		}
	}

	aid, err := p.objectWriter.Close(ctx)
	if err != nil {
		return PatchRes{}, fmt.Errorf("close object writer error: %w", err)
	}

	return PatchRes{
		AccessIdentifiers: aid,
	}, nil
}

func (p *patcher) applyPatch(ctx context.Context, currPatch *objectSDK.Patch, offset uint64) (newOffset uint64, err error) {
	// write the original payload chunk before the start of the patch
	if currPatch.PayloadPatch.Range.GetOffset() > offset {
		rng := new(objectSDK.Range)
		rng.SetOffset(offset)
		rng.SetLength(currPatch.PayloadPatch.Range.GetOffset() - offset)

		rdr := p.rangeProvider.ReadRange(ctx, p.addr, rng)
		for {
			orig := make([]byte, 1024)
			var n int
			n, err = rdr.Read(orig)
			if err != nil {
				if err == io.EOF {
					break
				}
				err = fmt.Errorf("read error: %w", err)
				return
			}
			_, err = p.objectWriter.Write(ctx, orig[:n])
			if err != nil {
				err = fmt.Errorf("write error: %w", err)
				return
			}
		}

		newOffset = currPatch.PayloadPatch.Range.GetOffset()
	}

	// apply patch
	if _, err = p.objectWriter.Write(ctx, currPatch.PayloadPatch.Chunk); err != nil {
		return
	}

	if currPatch.PayloadPatch.Range.GetLength() > 0 {
		newOffset += currPatch.PayloadPatch.Range.GetLength()
	}

	return
}

func mergeAttributes(newAttrs, oldAttrs []objectSDK.Attribute) []objectSDK.Attribute {
	attrMap := make(map[string]string)

	for _, attr := range oldAttrs {
		attrMap[attr.Key()] = attr.Value()
	}

	for _, newAttr := range newAttrs {
		attrMap[newAttr.Key()] = newAttr.Value()
	}

	var mergedAttrs []objectSDK.Attribute
	for key, value := range attrMap {
		var attr objectSDK.Attribute
		attr.SetKey(key)
		attr.SetValue(value)
		mergedAttrs = append(mergedAttrs, attr)
	}

	return mergedAttrs
}