// Copyright 2023 The LevelDB-Go and Pebble Authors. All rights reserved. Use
// of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.

package objstorageprovider

import (
	"context"
	"fmt"
	"os"
	"sync"

	"github.com/cockroachdb/pebble/internal/invariants"
	"github.com/cockroachdb/pebble/objstorage"
	"github.com/cockroachdb/pebble/vfs"
)

const fileMaxReadaheadSize = 256 * 1024 /* 256KB */

// fileReadable implements objstorage.Readable on top of a vfs.File.
//
// The implementation might use Prealloc and might reopen the file with
// SequentialReadsOption.
type fileReadable struct {
	file vfs.File
	size int64

	// The following fields are used to possibly open the file again using the
	// sequential reads option (see vfsReadHandle).
	filename string
	fs       vfs.FS
}

var _ objstorage.Readable = (*fileReadable)(nil)

func newFileReadable(file vfs.File, fs vfs.FS, filename string) (*fileReadable, error) {
	info, err := file.Stat()
	if err != nil {
		return nil, err
	}
	r := &fileReadable{
		file:     file,
		size:     info.Size(),
		filename: filename,
		fs:       fs,
	}
	invariants.SetFinalizer(r, func(obj interface{}) {
		if obj.(*fileReadable).file != nil {
			fmt.Fprintf(os.Stderr, "Readable was not closed")
			os.Exit(1)
		}
	})
	return r, nil
}

// ReadAt is part of the objstorage.Readable interface.
func (r *fileReadable) ReadAt(_ context.Context, p []byte, off int64) error {
	n, err := r.file.ReadAt(p, off)
	if invariants.Enabled && err == nil && n != len(p) {
		panic("short read")
	}
	return err
}

// Close is part of the objstorage.Readable interface.
func (r *fileReadable) Close() error {
	defer func() { r.file = nil }()
	return r.file.Close()
}

// Size is part of the objstorage.Readable interface.
func (r *fileReadable) Size() int64 {
	return r.size
}

// NewReadHandle is part of the objstorage.Readable interface.
func (r *fileReadable) NewReadHandle(_ context.Context) objstorage.ReadHandle {
	rh := readHandlePool.Get().(*vfsReadHandle)
	rh.r = r
	rh.rs = makeReadaheadState(fileMaxReadaheadSize)
	return rh
}

type vfsReadHandle struct {
	r  *fileReadable
	rs readaheadState

	// sequentialFile holds a file descriptor to the same underlying File,
	// except with fadvise(FADV_SEQUENTIAL) called on it to take advantage of
	// OS-level readahead. Once this is non-nil, the other variables in
	// readaheadState don't matter much as we defer to OS-level readahead.
	sequentialFile vfs.File
}

var _ objstorage.ReadHandle = (*vfsReadHandle)(nil)

var readHandlePool = sync.Pool{
	New: func() interface{} {
		i := &vfsReadHandle{}
		// Note: this is a no-op if invariants are disabled or race is enabled.
		invariants.SetFinalizer(i, func(obj interface{}) {
			if obj.(*vfsReadHandle).r != nil {
				fmt.Fprintf(os.Stderr, "ReadHandle was not closed")
				os.Exit(1)
			}
		})
		return i
	},
}

// Close is part of the objstorage.ReadHandle interface.
func (rh *vfsReadHandle) Close() error {
	var err error
	if rh.sequentialFile != nil {
		err = rh.sequentialFile.Close()
	}
	*rh = vfsReadHandle{}
	readHandlePool.Put(rh)
	return err
}

// ReadAt is part of the objstorage.ReadHandle interface.
func (rh *vfsReadHandle) ReadAt(_ context.Context, p []byte, offset int64) error {
	var n int
	var err error
	if rh.sequentialFile != nil {
		// Use OS-level read-ahead.
		n, err = rh.sequentialFile.ReadAt(p, offset)
	} else {
		if readaheadSize := rh.rs.maybeReadahead(offset, int64(len(p))); readaheadSize > 0 {
			if readaheadSize >= fileMaxReadaheadSize {
				// We've reached the maximum readahead size. Beyond this point, rely on
				// OS-level readahead.
				rh.switchToOSReadahead()
			} else {
				_ = rh.r.file.Prefetch(offset, readaheadSize)
			}
		}
		n, err = rh.r.file.ReadAt(p, offset)
	}
	if invariants.Enabled && err == nil && n != len(p) {
		panic("short read")
	}
	return err
}

// SetupForCompaction is part of the objstorage.ReadHandle interface.
func (rh *vfsReadHandle) SetupForCompaction() {
	rh.switchToOSReadahead()
}

func (rh *vfsReadHandle) switchToOSReadahead() {
	if rh.sequentialFile != nil {
		return
	}

	// TODO(radu): we could share the reopened file descriptor across multiple
	// handles.
	f, err := rh.r.fs.Open(rh.r.filename, vfs.SequentialReadsOption)
	if err == nil {
		rh.sequentialFile = f
	}
}

// RecordCacheHit is part of the objstorage.ReadHandle interface.
func (rh *vfsReadHandle) RecordCacheHit(_ context.Context, offset, size int64) {
	if rh.sequentialFile != nil {
		// Using OS-level readahead, so do nothing.
		return
	}
	rh.rs.recordCacheHit(offset, size)
}

// TestingCheckMaxReadahead returns true if the ReadHandle has switched to
// OS-level read-ahead.
func TestingCheckMaxReadahead(rh objstorage.ReadHandle) bool {
	switch rh := rh.(type) {
	case *vfsReadHandle:
		return rh.sequentialFile != nil
	case *PreallocatedReadHandle:
		return rh.sequentialFile != nil
	default:
		panic("unknown ReadHandle type")
	}
}

// PreallocatedReadHandle is used to avoid an allocation in NewReadHandle; see
// UsePreallocatedReadHandle.
type PreallocatedReadHandle struct {
	vfsReadHandle
}

// Close is part of the objstorage.ReadHandle interface.
func (rh *PreallocatedReadHandle) Close() error {
	var err error
	if rh.sequentialFile != nil {
		err = rh.sequentialFile.Close()
	}
	rh.vfsReadHandle = vfsReadHandle{}
	return err
}

// UsePreallocatedReadHandle is equivalent to calling readable.NewReadHandle()
// but uses the existing storage of a PreallocatedReadHandle when possible
// (currently this happens if we are reading from a local file).
// The returned handle still needs to be closed.
func UsePreallocatedReadHandle(
	ctx context.Context, readable objstorage.Readable, rh *PreallocatedReadHandle,
) objstorage.ReadHandle {
	if r, ok := readable.(*fileReadable); ok {
		// See fileReadable.NewReadHandle.
		rh.vfsReadHandle = vfsReadHandle{r: r}
		return rh
	}
	return readable.NewReadHandle(ctx)
}
