package atomicheader

import (
	"encoding/binary"
	"hash/crc32"
	"os"
	"sync"

	"git.crumpington.com/public/jldb/lib/errs"
)

const (
	PageSize         = 512
	AvailabePageSize = 508

	ReservedBytes = PageSize * 4

	offsetSwitch = 1 * PageSize
	offset1      = 2 * PageSize
	offset2      = 3 * PageSize
)

type Handler struct {
	lock       sync.Mutex
	switchPage []byte // At offsetSwitch.
	page       []byte // Page buffer is re-used for reading and writing.

	currentPage int64 // Either 0 or 1.
	f           *os.File
}

func Init(f *os.File) error {
	if err := f.Truncate(ReservedBytes); err != nil {
		return errs.IO.WithErr(err)
	}

	switchPage := make([]byte, PageSize)
	switchPage[0] = 2
	if _, err := f.WriteAt(switchPage, offsetSwitch); err != nil {
		return errs.IO.WithErr(err)
	}
	return nil
}

func Open(f *os.File) (*Handler, error) {
	switchPage := make([]byte, PageSize)

	if _, err := f.ReadAt(switchPage, offsetSwitch); err != nil {
		return nil, errs.IO.WithErr(err)
	}

	h := &Handler{
		switchPage:  switchPage,
		page:        make([]byte, PageSize),
		currentPage: int64(switchPage[0]),
		f:           f,
	}

	if h.currentPage != 1 && h.currentPage != 2 {
		return nil, errs.Corrupt.WithMsg("invalid page id: %d", h.currentPage)
	}

	return h, nil
}

// Read reads the currently active header page.
func (h *Handler) Read(read func(page []byte) error) error {
	h.lock.Lock()
	defer h.lock.Unlock()

	if _, err := h.f.ReadAt(h.page, h.currentOffset()); err != nil {
		return errs.IO.WithErr(err)
	}

	computedCRC := crc32.ChecksumIEEE(h.page[:PageSize-4])
	storedCRC := binary.LittleEndian.Uint32(h.page[PageSize-4:])
	if computedCRC != storedCRC {
		return errs.Corrupt.WithMsg("checksum mismatch")
	}

	return read(h.page)
}

// Write writes the currently active header page. The page buffer given to the
// function may contain old data, so the caller may need to zero some bytes if
// necessary.
func (h *Handler) Write(update func(page []byte) error) error {
	h.lock.Lock()
	defer h.lock.Unlock()

	if err := update(h.page); err != nil {
		return err
	}

	crc := crc32.ChecksumIEEE(h.page[:PageSize-4])
	binary.LittleEndian.PutUint32(h.page[PageSize-4:], crc)

	newPageNum := 1 + h.currentPage%2
	newOffset := h.getOffset(newPageNum)

	if _, err := h.f.WriteAt(h.page, newOffset); err != nil {
		return errs.IO.WithErr(err)
	}

	if err := h.f.Sync(); err != nil {
		return errs.IO.WithErr(err)
	}

	h.switchPage[0] = byte(newPageNum)
	if _, err := h.f.WriteAt(h.switchPage, offsetSwitch); err != nil {
		return errs.IO.WithErr(err)
	}

	if err := h.f.Sync(); err != nil {
		return errs.IO.WithErr(err)
	}

	h.currentPage = newPageNum
	return nil
}

// ----------------------------------------------------------------------------

func (h *Handler) currentOffset() int64 {
	return h.getOffset(h.currentPage)
}

func (h *Handler) getOffset(pageNum int64) int64 {
	switch pageNum {
	case 1:
		return offset1
	case 2:
		return offset2
	default:
		panic("Invalid page number.")
	}
}