package pfile

import (
	"hash/crc32"
	"unsafe"

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

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

const (
	pageSize       = 512
	pageHeaderSize = 40
	pageDataSize   = pageSize - pageHeaderSize

	pageTypeFree = 0
	pageTypeHead = 1
	pageTypeData = 2
)

var emptyPage = func() dataPage {
	p := newDataPage()
	h := p.Header()
	h.CRC = p.ComputeCRC()
	return p
}()

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

type pageHeader struct {
	CRC          uint32 // IEEE CRC-32 checksum.
	PageType     uint32 // One of the PageType* constants.
	CollectionID uint64 //
	ItemID       uint64
	DataSize     uint64
	NextPage     uint64
}

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

type dataPage []byte

func newDataPage() dataPage {
	p := dataPage(make([]byte, pageSize))
	return p
}

func (p dataPage) Header() *pageHeader {
	return (*pageHeader)(unsafe.Pointer(&p[0]))
}

func (p dataPage) ComputeCRC() uint32 {
	return crc32.ChecksumIEEE(p[4:])
}

func (p dataPage) Data() []byte {
	return p[pageHeaderSize:]
}

func (p dataPage) Write(data []byte) int {
	return copy(p[pageHeaderSize:], data)
}

func (p dataPage) Validate() error {
	header := p.Header()
	if header.CRC != p.ComputeCRC() {
		return errs.Corrupt.WithMsg("CRC mismatch on data page.")
	}
	return nil
}