// Copyright 2016 Google LLC
//
// 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 storage

import (
	"context"
	"fmt"
	"hash/crc32"
	"io"
	"io/ioutil"
	"net/http"
	"strings"
	"sync"
	"time"

	"cloud.google.com/go/internal/trace"
)

var crc32cTable = crc32.MakeTable(crc32.Castagnoli)

// ReaderObjectAttrs are attributes about the object being read. These are populated
// during the New call. This struct only holds a subset of object attributes: to
// get the full set of attributes, use ObjectHandle.Attrs.
//
// Each field is read-only.
type ReaderObjectAttrs struct {
	// Size is the length of the object's content.
	Size int64

	// StartOffset is the byte offset within the object
	// from which reading begins.
	// This value is only non-zero for range requests.
	StartOffset int64

	// ContentType is the MIME type of the object's content.
	ContentType string

	// ContentEncoding is the encoding of the object's content.
	ContentEncoding string

	// CacheControl specifies whether and for how long browser and Internet
	// caches are allowed to cache your objects.
	CacheControl string

	// LastModified is the time that the object was last modified.
	LastModified time.Time

	// Generation is the generation number of the object's content.
	Generation int64

	// Metageneration is the version of the metadata for this object at
	// this generation. This field is used for preconditions and for
	// detecting changes in metadata. A metageneration number is only
	// meaningful in the context of a particular generation of a
	// particular object.
	Metageneration int64

	// CRC32C is the CRC32 checksum of the entire object's content using the
	// Castagnoli93 polynomial, if available.
	CRC32C uint32

	// Decompressed is true if the object is stored as a gzip file and was
	// decompressed when read.
	// Objects are automatically decompressed if the object's metadata property
	// "Content-Encoding" is set to "gzip" or satisfies decompressive
	// transcoding as per https://cloud.google.com/storage/docs/transcoding.
	//
	// To prevent decompression on reads, use [ObjectHandle.ReadCompressed].
	Decompressed bool
}

// NewReader creates a new Reader to read the contents of the
// object.
// ErrObjectNotExist will be returned if the object is not found.
//
// The caller must call Close on the returned Reader when done reading.
//
// By default, reads are made using the Cloud Storage XML API. We recommend
// using the JSON API instead, which can be done by setting [WithJSONReads]
// when calling [NewClient]. This ensures consistency with other client
// operations, which all use JSON. JSON will become the default in a future
// release.
func (o *ObjectHandle) NewReader(ctx context.Context) (*Reader, error) {
	return o.NewRangeReader(ctx, 0, -1)
}

// NewRangeReader reads part of an object, reading at most length bytes
// starting at the given offset. If length is negative, the object is read
// until the end. If offset is negative, the object is read abs(offset) bytes
// from the end, and length must also be negative to indicate all remaining
// bytes will be read.
//
// If the object's metadata property "Content-Encoding" is set to "gzip" or satisfies
// decompressive transcoding per https://cloud.google.com/storage/docs/transcoding
// that file will be served back whole, regardless of the requested range as
// Google Cloud Storage dictates. If decompressive transcoding occurs,
// [Reader.Attrs.Decompressed] will be true.
//
// By default, reads are made using the Cloud Storage XML API. We recommend
// using the JSON API instead, which can be done by setting [WithJSONReads]
// when calling [NewClient]. This ensures consistency with other client
// operations, which all use JSON. JSON will become the default in a future
// release.
func (o *ObjectHandle) NewRangeReader(ctx context.Context, offset, length int64) (r *Reader, err error) {
	// This span covers the life of the reader. It is closed via the context
	// in Reader.Close.
	ctx = trace.StartSpan(ctx, "cloud.google.com/go/storage.Object.Reader")

	if err := o.validate(); err != nil {
		return nil, err
	}
	if offset < 0 && length >= 0 {
		return nil, fmt.Errorf("storage: invalid offset %d < 0 requires negative length", offset)
	}
	if o.conds != nil {
		if err := o.conds.validate("NewRangeReader"); err != nil {
			return nil, err
		}
	}

	opts := makeStorageOpts(true, o.retry, o.userProject)

	params := &newRangeReaderParams{
		bucket:         o.bucket,
		object:         o.object,
		gen:            o.gen,
		offset:         offset,
		length:         length,
		encryptionKey:  o.encryptionKey,
		conds:          o.conds,
		readCompressed: o.readCompressed,
		handle:         &o.readHandle,
	}

	r, err = o.c.tc.NewRangeReader(ctx, params, opts...)

	// Pass the context so that the span can be closed in Reader.Close, or close the
	// span now if there is an error.
	if err == nil {
		r.ctx = ctx
	} else {
		trace.EndSpan(ctx, err)
	}

	return r, err
}

// NewMultiRangeDownloader creates a multi-range reader for an object.
// Must be called on a gRPC client created using [NewGRPCClient].
//
// This uses the gRPC-specific bi-directional read API, which is in private
// preview; please contact your account manager if interested.
func (o *ObjectHandle) NewMultiRangeDownloader(ctx context.Context) (mrd *MultiRangeDownloader, err error) {
	// This span covers the life of the reader. It is closed via the context
	// in Reader.Close.
	ctx = trace.StartSpan(ctx, "cloud.google.com/go/storage.Object.MultiRangeDownloader")

	if err := o.validate(); err != nil {
		return nil, err
	}
	if o.conds != nil {
		if err := o.conds.validate("NewMultiRangeDownloader"); err != nil {
			return nil, err
		}
	}

	opts := makeStorageOpts(true, o.retry, o.userProject)

	params := &newMultiRangeDownloaderParams{
		bucket:        o.bucket,
		conds:         o.conds,
		encryptionKey: o.encryptionKey,
		gen:           o.gen,
		object:        o.object,
		handle:        &o.readHandle,
	}

	r, err := o.c.tc.NewMultiRangeDownloader(ctx, params, opts...)

	// Pass the context so that the span can be closed in MultiRangeDownloader.Close(), or close the
	// span now if there is an error.
	if err == nil {
		r.ctx = ctx
	} else {
		trace.EndSpan(ctx, err)
	}

	return r, err
}

// decompressiveTranscoding returns true if the request was served decompressed
// and different than its original storage form. This happens when the "Content-Encoding"
// header is "gzip".
// See:
//   - https://cloud.google.com/storage/docs/transcoding#transcoding_and_gzip
//   - https://github.com/googleapis/google-cloud-go/issues/1800
func decompressiveTranscoding(res *http.Response) bool {
	// Decompressive Transcoding.
	return res.Header.Get("Content-Encoding") == "gzip" ||
		res.Header.Get("X-Goog-Stored-Content-Encoding") == "gzip"
}

func uncompressedByServer(res *http.Response) bool {
	// If the data is stored as gzip but is not encoded as gzip, then it
	// was uncompressed by the server.
	return res.Header.Get("X-Goog-Stored-Content-Encoding") == "gzip" &&
		res.Header.Get("Content-Encoding") != "gzip"
}

// parseCRC32c parses the crc32c hash from the X-Goog-Hash header.
// It can parse headers in the form [crc32c=xxx md5=xxx] (XML responses) or the
// form [crc32c=xxx,md5=xxx] (JSON responses). The md5 hash is ignored.
func parseCRC32c(res *http.Response) (uint32, bool) {
	const prefix = "crc32c="
	for _, spec := range res.Header["X-Goog-Hash"] {
		values := strings.Split(spec, ",")

		for _, v := range values {
			if strings.HasPrefix(v, prefix) {
				c, err := decodeUint32(v[len(prefix):])
				if err == nil {
					return c, true
				}
			}
		}

	}
	return 0, false
}

// setConditionsHeaders sets precondition request headers for downloads
// using the XML API. It assumes that the conditions have been validated.
func setConditionsHeaders(headers http.Header, conds *Conditions) error {
	if conds == nil {
		return nil
	}
	if conds.MetagenerationMatch != 0 {
		headers.Set("x-goog-if-metageneration-match", fmt.Sprint(conds.MetagenerationMatch))
	}
	switch {
	case conds.GenerationMatch != 0:
		headers.Set("x-goog-if-generation-match", fmt.Sprint(conds.GenerationMatch))
	case conds.DoesNotExist:
		headers.Set("x-goog-if-generation-match", "0")
	}
	return nil
}

var emptyBody = ioutil.NopCloser(strings.NewReader(""))

// Reader reads a Cloud Storage object.
// It implements io.Reader.
//
// Typically, a Reader computes the CRC of the downloaded content and compares it to
// the stored CRC, returning an error from Read if there is a mismatch. This integrity check
// is skipped if transcoding occurs. See https://cloud.google.com/storage/docs/transcoding.
type Reader struct {
	Attrs          ReaderObjectAttrs
	objectMetadata *map[string]string

	seen, remain, size int64
	checkCRC           bool // Did we check the CRC? This is now only used by tests.

	reader io.ReadCloser
	ctx    context.Context
	mu     sync.Mutex
	handle *ReadHandle
}

// Close closes the Reader. It must be called when done reading.
func (r *Reader) Close() error {
	err := r.reader.Close()
	trace.EndSpan(r.ctx, err)
	return err
}

func (r *Reader) Read(p []byte) (int, error) {
	n, err := r.reader.Read(p)
	if r.remain != -1 {
		r.remain -= int64(n)
	}
	return n, err
}

// WriteTo writes all the data from the Reader to w. Fulfills the io.WriterTo interface.
// This is called implicitly when calling io.Copy on a Reader.
func (r *Reader) WriteTo(w io.Writer) (int64, error) {
	// This implicitly calls r.reader.WriteTo for gRPC only. JSON and XML don't have an
	// implementation of WriteTo.
	n, err := io.Copy(w, r.reader)
	if r.remain != -1 {
		r.remain -= int64(n)
	}
	return n, err
}

// Size returns the size of the object in bytes.
// The returned value is always the same and is not affected by
// calls to Read or Close.
//
// Deprecated: use Reader.Attrs.Size.
func (r *Reader) Size() int64 {
	return r.Attrs.Size
}

// Remain returns the number of bytes left to read, or -1 if unknown.
func (r *Reader) Remain() int64 {
	return r.remain
}

// ContentType returns the content type of the object.
//
// Deprecated: use Reader.Attrs.ContentType.
func (r *Reader) ContentType() string {
	return r.Attrs.ContentType
}

// ContentEncoding returns the content encoding of the object.
//
// Deprecated: use Reader.Attrs.ContentEncoding.
func (r *Reader) ContentEncoding() string {
	return r.Attrs.ContentEncoding
}

// CacheControl returns the cache control of the object.
//
// Deprecated: use Reader.Attrs.CacheControl.
func (r *Reader) CacheControl() string {
	return r.Attrs.CacheControl
}

// LastModified returns the value of the Last-Modified header.
//
// Deprecated: use Reader.Attrs.LastModified.
func (r *Reader) LastModified() (time.Time, error) {
	return r.Attrs.LastModified, nil
}

// Metadata returns user-provided metadata, in key/value pairs.
//
// It can be nil if no metadata is present, or if the client uses the JSON
// API for downloads. Only the XML and gRPC APIs support getting
// custom metadata via the Reader; for JSON make a separate call to
// ObjectHandle.Attrs.
func (r *Reader) Metadata() map[string]string {
	if r.objectMetadata != nil {
		return *r.objectMetadata
	}
	return nil
}

// ReadHandle returns the read handle associated with an object.
// ReadHandle will be periodically refreshed.
//
// ReadHandle requires the gRPC-specific bi-directional read API, which is in
// private preview; please contact your account manager if interested.
// Note that this only valid for gRPC and only with zonal buckets.
func (r *Reader) ReadHandle() ReadHandle {
	if r.handle == nil {
		r.handle = &ReadHandle{}
	}
	r.mu.Lock()
	defer r.mu.Unlock()
	return (*r.handle)
}

// MultiRangeDownloader reads a Cloud Storage object.
//
// Typically, a MultiRangeDownloader opens a stream to which we can add
// different ranges to read from the object.
//
// This API is currently in preview and is not yet available for general use.
type MultiRangeDownloader struct {
	Attrs  ReaderObjectAttrs
	reader multiRangeDownloader
	ctx    context.Context
}

type multiRangeDownloader interface {
	add(output io.Writer, offset, limit int64, callback func(int64, int64, error))
	wait()
	close() error
	getHandle() []byte
}

// Add adds a new range to MultiRangeDownloader.
//
// The offset for the first byte to return in the read, relative to the start
// of the object.
//
// A negative offset value will be interpreted as the number of bytes from the
// end of the object to be returned. Requesting a negative offset with magnitude
// larger than the size of the object will return the entire object. An offset
// larger than the size of the object will result in an OutOfRange error.
//
// A limit of zero indicates that there is no limit, and a negative limit will
// cause an error.
//
// This will initiate the read range but is non-blocking; call callback to
// process the result. Add is thread-safe and can be called simultaneously
// from different goroutines.
func (mrd *MultiRangeDownloader) Add(output io.Writer, offset, length int64, callback func(int64, int64, error)) {
	mrd.reader.add(output, offset, length, callback)
}

// Close the MultiRangeDownloader. It must be called when done reading.
// Adding new ranges after this has been called will cause an error.
//
// This will immediately close the stream and can result in a
// "stream closed early" error if a response for a range is still not processed.
// Call [MultiRangeDownloader.Wait] to avoid this error.
func (mrd *MultiRangeDownloader) Close() error {
	err := mrd.reader.close()
	trace.EndSpan(mrd.ctx, err)
	return err
}

// Wait for all the responses to process on the stream.
// Adding new ranges after this has been called will cause an error.
// Wait will wait for all callbacks to finish.
func (mrd *MultiRangeDownloader) Wait() {
	mrd.reader.wait()
}

// GetHandle returns the read handle. This can be used to further speed up the
// follow up read if the same object is read through a different stream.
func (mrd *MultiRangeDownloader) GetHandle() []byte {
	return mrd.reader.getHandle()
}
