/*
   Copyright 2022 Erigon contributors

   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 bptree

import (
	"bufio"
	"crypto/rand"
	"fmt"
	"io"
	"math/big"
	"os"
	"strconv"
)

// Size in bytes of data blocks read/written from/to the file system.
const BLOCKSIZE int64 = 4096

// BinaryFile type represents an open binary file.
type BinaryFile struct {
	file      *os.File
	path      string
	blockSize int64
	size      int64
	opened    bool
}

// RandomBinaryReader reads data chuncks randomly from a binary file.
type RandomBinaryReader struct {
	sourceFile *BinaryFile
	chunckSize int
}

func (r RandomBinaryReader) Read(b []byte) (n int, err error) {
	numKeys := len(b) / r.chunckSize
	for i := 0; i < numKeys; i++ {
		bytesRead, err := r.readAtRandomOffset(b[i*r.chunckSize : i*r.chunckSize+r.chunckSize])
		if err != nil {
			return i*r.chunckSize + bytesRead, fmt.Errorf("cannot random read at iteration %d: %w", i, err)
		}
		n += bytesRead
	}
	remainderSize := len(b) % r.chunckSize
	bytesRead, err := r.readAtRandomOffset(b[numKeys*r.chunckSize : numKeys*r.chunckSize+remainderSize])
	if err != nil {
		return numKeys*r.chunckSize + bytesRead, fmt.Errorf("cannot random read remainder %d: %w", remainderSize, err)
	}
	n += bytesRead
	return n, nil
}

func (r RandomBinaryReader) readAtRandomOffset(b []byte) (n int, err error) {
	randomValue, err := rand.Int(rand.Reader, big.NewInt(r.sourceFile.size-int64(len(b))))
	if err != nil {
		return 0, fmt.Errorf("cannot generate random offset: %w", err)
	}
	randomOffset := randomValue.Int64()
	_, err = r.sourceFile.file.Seek(randomOffset, io.SeekStart)
	if err != nil {
		return 0, fmt.Errorf("cannot seek to offset %d: %w", randomOffset, err)
	}
	bytesRead, err := r.sourceFile.file.Read(b)
	if err != nil {
		return 0, fmt.Errorf("cannot read from source file: %w", err)
	}
	return bytesRead, nil
}

func CreateBinaryFileByRandomSampling(path string, size int64, sourceFile *BinaryFile, keySize int) *BinaryFile {
	return CreateBinaryFileFromReader(path, "_onlyexisting", size, RandomBinaryReader{sourceFile, keySize})
}

func CreateBinaryFileByPRNG(path string, size int64) *BinaryFile {
	return CreateBinaryFileFromReader(path, "", size, rand.Reader)
}

func CreateBinaryFileFromReader(path, suffix string, size int64, reader io.Reader) *BinaryFile {
	file, err := os.OpenFile(path+strconv.FormatInt(size, 10)+suffix, os.O_RDWR|os.O_CREATE, 0644)
	ensure(err == nil, fmt.Sprintf("CreateBinaryFileFromReader: cannot create file %s, error %s\n", file.Name(), err))

	err = file.Truncate(size)
	ensure(err == nil, fmt.Sprintf("CreateBinaryFileFromReader: cannot truncate file %s to %d, error %s\n", file.Name(), size, err))

	bufferedFile := bufio.NewWriter(file)
	numBlocks := size / BLOCKSIZE
	remainderSize := size % BLOCKSIZE
	buffer := make([]byte, BLOCKSIZE)
	for i := int64(0); i <= numBlocks; i++ {
		if i == numBlocks {
			buffer = make([]byte, remainderSize)
		}
		bytesRead, err := io.ReadFull(reader, buffer)
		ensure(bytesRead == len(buffer), fmt.Sprintf("CreateBinaryFileFromReader: insufficient bytes read %d, error %s\n", bytesRead, err))
		bytesWritten, err := bufferedFile.Write(buffer)
		ensure(bytesWritten == len(buffer), fmt.Sprintf("CreateBinaryFileFromReader: insufficient bytes written %d, error %s\n", bytesWritten, err))
	}

	err = bufferedFile.Flush()
	ensure(err == nil, fmt.Sprintf("CreateBinaryFileFromReader: error during flushing %s\n", err))

	binaryFile := &BinaryFile{path: file.Name(), blockSize: BLOCKSIZE, size: size, file: file, opened: true}
	binaryFile.rewind()
	return binaryFile
}

func OpenBinaryFile(path string) *BinaryFile {
	file, err := os.Open(path)
	ensure(err == nil, fmt.Sprintf("OpenBinaryFile: cannot open file %s, error %s\n", path, err))

	info, err := file.Stat()
	ensure(err == nil, fmt.Sprintf("OpenBinaryFile: cannot stat file %s error %s\n", path, err))
	ensure(info.Size() >= 0, fmt.Sprintf("OpenBinaryFile: negative size %d file %s\n", info.Size(), path))

	binaryFile := &BinaryFile{path: path, blockSize: BLOCKSIZE, size: info.Size(), file: file, opened: true}
	return binaryFile
}

func (f *BinaryFile) rewind() {
	offset, err := f.file.Seek(0, io.SeekStart)
	ensure(err == nil, fmt.Sprintf("rewind: error during seeking %s\n", err))
	ensure(offset == 0, fmt.Sprintf("rewind: unexpected offset after seeking: %d\n", offset))
}

func (f *BinaryFile) Name() string {
	return f.path
}

func (f *BinaryFile) Size() int64 {
	return f.size
}

func (f *BinaryFile) NewReader() *bufio.Reader {
	ensure(f.opened, fmt.Sprintf("NewReader: file %s is not opened\n", f.path))
	f.rewind()
	return bufio.NewReader(f.file)
}

func (f *BinaryFile) Close() {
	ensure(f.opened, fmt.Sprintf("Close: file %s is not opened\n", f.path))
	err := f.file.Close()
	ensure(err == nil, fmt.Sprintf("Close: cannot close file %s, error %s\n", f.path, err))
	f.opened = false
}