// Package headers provides headers for the db binary files
package headers

import (
	"encoding/binary"
	"errors"
)

type Headers struct {
	HeaderString []byte // Header string
	PageSize     uint64 // Page size for chunks
	DBVersion    uint16 // Version of the db

	// Reserved for future use.
	WAL    bool   // Enable write-ahead-logging
	Commit []byte // SHA256 sum of the current DB - Could be switched to ULID/UUID in future.
}

// constant HeaderString
const HeaderString = "zutto/db/"

//Commit data length - mapped to be 32 bytes for now, to match SHA256SUM
const CommitDataLength = 32

// Static lengths of the headers HeaderString length(uint16, 2 bytes)- Pagesize (uint64, 8 bytes) + DBVersion (int16, 2 bytes) + WAL (int8//bool, 1 byte) + Commit (SHA256 32 bytes)
const StaticHeadersLength = 13 + CommitDataLength

// Generate generates []byte output from the headers
func (h *Headers) Generate() *[]byte {
	var HeaderStringLength int = len(h.HeaderString)

	//Output array
	var data []byte = make([]byte, HeaderStringLength+StaticHeadersLength)

	//Headerstring
	binary.LittleEndian.PutUint16(data[0:2], uint16(HeaderStringLength))
	copy(data[2:2+HeaderStringLength], h.HeaderString[:])

	//add the simpler metadata
	binary.LittleEndian.PutUint64(data[2+HeaderStringLength:10+HeaderStringLength], h.PageSize)
	binary.LittleEndian.PutUint16(data[10+HeaderStringLength:12+HeaderStringLength], h.DBVersion)

	//Convert bool-value to byte with a helper function true = byte(1), false = byte(0)
	data[12+HeaderStringLength] = ToSingleByte(h.WAL)

	//commit data, extr
	copy(data[13+HeaderStringLength:13+CommitDataLength+HeaderStringLength], h.Commit[0:CommitDataLength])

	return &data
}

// Parse parses the headers
// Accepts 2 parameters, Input *[]byte's and Padding
// Returns error in case of errors.
func (h *Headers) Parse(Input *[]byte, Padding int) error {
	if Padding+len((*Input)) < StaticHeadersLength {
		return errors.New("Input was not correct length")
	}

	var HeaderStringLength int = int(binary.LittleEndian.Uint16((*Input)[Padding : Padding+2]))
	h.HeaderString = (*Input)[2+Padding : Padding+2+HeaderStringLength]
	h.PageSize = binary.LittleEndian.Uint64((*Input)[2+HeaderStringLength+Padding : Padding+10+HeaderStringLength])
	h.DBVersion = binary.LittleEndian.Uint16((*Input)[10+HeaderStringLength+Padding : Padding+12+HeaderStringLength])

	h.WAL = ByteToBool((*Input)[12+HeaderStringLength])
	h.Commit = ((*Input)[13+HeaderStringLength+Padding : Padding+13+CommitDataLength+HeaderStringLength])

	return nil
}