// Copyright (c) 2018 David Crawshaw <david@zentus.com>
// Copyright (c) 2021 Roxy Light <roxy@zombiezen.com>
//
// Permission to use, copy, modify, and distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
//
// SPDX-License-Identifier: ISC

// Package sqlitefile provides bytes buffers backed by a temporary SQLite table.
package sqlitefile

import (
	"fmt"
	"io"

	"zombiezen.com/go/sqlite"
)

// File is a readable, writable, and seekable series of temporary SQLite blobs.
type File struct {
	err    error
	conn   *sqlite.Conn
	blobs  []*sqlite.Blob
	rowids []int64
	off    bbpos
	len    bbpos
}

// NewFile creates a new [File] with a reasonable initial capacity.
func NewFile(conn *sqlite.Conn) (*File, error) {
	return NewFileSize(conn, 16*1024)
}

// NewFileSize creates a new [File] with the given number of bytes of capacity.
func NewFileSize(conn *sqlite.Conn, initSize int) (*File, error) {
	bb := &File{conn: conn}
	stmt := conn.Prep("CREATE TEMP TABLE IF NOT EXISTS BlobBuffer (blob BLOB);")
	if _, err := stmt.Step(); err != nil {
		return nil, err
	}
	if err := bb.addblob(int64(initSize)); err != nil {
		return nil, err
	}
	return bb, nil
}

func (bb *File) addblob(size int64) error {
	stmt := bb.conn.Prep("INSERT INTO BlobBuffer (blob) VALUES ($blob);")
	stmt.SetZeroBlob("$blob", size)
	if _, err := stmt.Step(); err != nil {
		return err
	}
	rowid := bb.conn.LastInsertRowID()
	blob, err := bb.conn.OpenBlob("temp", "BlobBuffer", "blob", rowid, true)
	if err != nil {
		return err
	}
	bb.blobs = append(bb.blobs, blob)
	bb.rowids = append(bb.rowids, rowid)
	return nil
}

// grow adds an sqlite blob if the buffer is out of space.
func (bb *File) grow() error {
	lastSize := bb.blobs[len(bb.blobs)-1].Size()
	size := lastSize * 2
	if err := bb.addblob(size); err != nil {
		return err
	}
	return nil
}

// rem reports the remaining available bytes in the pointed-to blob
func (bb *File) rem(pos bbpos) int64 {
	return bb.blobs[pos.index].Size() - pos.pos
}

func (bb *File) eq(p1, p2 bbpos) bool {
	if p1 == p2 {
		return true
	}
	if p1.index == p2.index+1 && bb.rem(p1) == 0 && p2.pos == 0 {
		return true
	}
	if p2.index == p1.index+1 && bb.rem(p2) == 0 && p1.pos == 0 {
		return true
	}
	return false
}

func (bb *File) gt(p1, p2 bbpos) bool {
	if bb.eq(p1, p2) {
		return false
	}
	if p1.index != p2.index {
		return p1.index > p2.index
	}
	return p1.pos > p2.pos
}

func (bb *File) zero(p1, p2 bbpos) error {
	var zeros [4096]byte
	for bb.gt(p2, p1) {
		w := bb.rem(p1)
		if w == 0 {
			p1.index++
			p1.pos = 0
			w = bb.rem(p1)
		}
		if p1.index == p2.index {
			w = p2.pos
		}
		if w > int64(len(zeros)) {
			w = int64(len(zeros))
		}
		if _, err := bb.blobs[p1.index].Seek(p1.pos, io.SeekStart); err != nil {
			return err
		}
		nn, err := bb.blobs[p1.index].Write(zeros[:w])
		if err != nil {
			return err
		}
		p1.pos += int64(nn)
	}
	return nil
}

// Write writes p to the file.
// See [io.Writer] for details.
func (bb *File) Write(p []byte) (n int, err error) {
	if bb.err != nil {
		return 0, bb.err
	}

	if bb.gt(bb.off, bb.len) {
		if err := bb.zero(bb.len, bb.off); err != nil {
			bb.err = err
			return 0, err
		}
	}

	for len(p) > 0 {
		w := bb.rem(bb.off)
		if w == 0 {
			if bb.off.index == len(bb.blobs)-1 {
				if bb.err = bb.grow(); bb.err != nil {
					return n, bb.err
				}
			}
			bb.off.index++
			bb.off.pos = 0
			w = bb.rem(bb.off)
		}
		if int64(len(p)) < w {
			w = int64(len(p))
		}
		if _, err := bb.blobs[bb.off.index].Seek(bb.off.pos, io.SeekStart); err != nil {
			bb.err = err
			break
		}
		nn, err := bb.blobs[bb.off.index].Write(p[:w])
		n += nn
		p = p[nn:]
		bb.off.pos += int64(nn)
		if bb.gt(bb.off, bb.len) {
			bb.len = bb.off
		}
		if err != nil {
			bb.err = err
			break
		}
	}

	return n, bb.err
}

// Read reads data into p.
// See [io.Reader] for details.
func (bb *File) Read(p []byte) (n int, err error) {
	if bb.err != nil {
		return 0, bb.err
	}

	for len(p) > 0 && bb.gt(bb.len, bb.off) {
		if bb.rem(bb.off) == 0 {
			bb.off.index++
			bb.off.pos = 0
		}

		var bsize int64
		if bb.len.index == bb.off.index {
			bsize = bb.len.pos
		} else {
			bsize = bb.blobs[bb.off.index].Size()
		}
		w := bsize - bb.off.pos
		if int64(len(p)) < w {
			w = int64(len(p))
		}
		if _, err := bb.blobs[bb.off.index].Seek(bb.off.pos, io.SeekStart); err != nil {
			bb.err = err
			return n, err
		}
		nn, err := io.ReadFull(bb.blobs[bb.off.index], p[:w])
		n += nn
		p = p[nn:]
		bb.off.pos += int64(nn)
		if err != nil {
			bb.err = err
			return n, err
		}
	}

	if n == 0 && (bb.eq(bb.off, bb.len) || bb.gt(bb.off, bb.len)) {
		return 0, io.EOF
	}

	return n, nil
}

// Seek changes the read/write position in the file.
// See [io.Seeker] for details.
func (bb *File) Seek(offset int64, whence int) (int64, error) {
	if bb.err != nil {
		return 0, bb.err
	}

	const (
		SeekStart   = 0
		SeekCurrent = 1
		SeekEnd     = 2
	)
	switch whence {
	case SeekStart:
		// use offset directly
	case SeekCurrent:
		for i := 0; i < bb.off.index; i++ {
			offset += bb.blobs[i].Size()
		}
		offset += bb.off.pos
	case SeekEnd:
		offset += bb.Len()
	}
	if offset < 0 {
		return -1, fmt.Errorf("sqlitex.File: attempting to seek before beginning of blob (%d)", offset)
	}

	rem := offset
	bb.off.index = 0
	for i := 0; rem > bb.blobs[i].Size(); i++ {
		bb.off.index = i + 1
		rem -= bb.blobs[i].Size()

		if i == len(bb.blobs)-1 {
			if err := bb.grow(); err != nil {
				return offset - rem, err
			}
		}
	}
	bb.off.pos = rem

	return offset, nil
}

// Truncate changes the size of the file.
func (bb *File) Truncate(size int64) error {
	for {
		for i := 0; i < len(bb.blobs); i++ {
			bsize := bb.blobs[i].Size()
			if bsize > size {
				newlen := bbpos{index: i, pos: size}
				if err := bb.zero(bb.len, newlen); err != nil {
					return err
				}
				bb.len = newlen
				return nil
			}
			size -= bsize
		}
		if err := bb.grow(); err != nil {
			return err
		}
	}
}

// Len returns the size of the file in bytes.
func (bb *File) Len() (n int64) {
	for i := 0; i < bb.len.index; i++ {
		n += bb.blobs[i].Size()
	}
	n += bb.len.pos
	return n
}

// Cap returns the allocated capacity of the file in bytes.
func (bb *File) Cap() (n int64) {
	for i := 0; i < len(bb.blobs); i++ {
		n += bb.blobs[i].Size()
	}
	return n
}

// Close releases all resources associated with the file,
// removing the storage from the database.
func (bb *File) Close() error {
	if bb.err != nil {
		return bb.err
	}
	for _, blob := range bb.blobs {
		err := blob.Close()
		if bb.err == nil {
			bb.err = err
		}
	}
	stmt := bb.conn.Prep("DELETE FROM BlobBuffer WHERE rowid = $rowid;")
	for _, rowid := range bb.rowids {
		stmt.Reset()
		stmt.SetInt64("$rowid", rowid)
		if _, err := stmt.Step(); err != nil && bb.err == nil {
			bb.err = err
		}
	}
	bb.blobs = nil
	bb.rowids = nil
	return bb.err
}

type bbpos struct {
	index int   // bb.blobs[index]
	pos   int64 // point inside that blob
}