// Package serial provides the 5-byte uint40 serial type and truncated hash
// types used by the store's index system.
package serial

import (
	"crypto/sha256"
	"encoding/binary"
)

const (
	// Len is the byte length of a serial (5 bytes = 40 bits).
	Len = 5
	// Max is the maximum serial value (2^40 - 1).
	Max uint64 = 1<<40 - 1
	// HashLen is the byte length of truncated hashes used in index keys.
	HashLen = 8
)

// Put writes a uint64 as a 5-byte big-endian serial into dst.
// dst must be at least 5 bytes.
func Put(dst []byte, v uint64) {
	dst[0] = byte(v >> 32)
	dst[1] = byte(v >> 24)
	dst[2] = byte(v >> 16)
	dst[3] = byte(v >> 8)
	dst[4] = byte(v)
}

// Get reads a 5-byte big-endian serial from src and returns it as uint64.
func Get(src []byte) uint64 {
	return uint64(src[0])<<32 |
		uint64(src[1])<<24 |
		uint64(src[2])<<16 |
		uint64(src[3])<<8 |
		uint64(src[4])
}

// Append appends a 5-byte serial to dst.
func Append(dst []byte, v uint64) []byte {
	return append(dst, byte(v>>32), byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
}

// IdHash returns an 8-byte truncated SHA-256 of a 32-byte event ID.
func IdHash(id []byte) []byte {
	h := sha256.Sum256(id)
	return h[:HashLen]
}

// PubHash returns an 8-byte truncated SHA-256 of a 32-byte pubkey.
func PubHash(pub []byte) []byte {
	h := sha256.Sum256(pub)
	return h[:HashLen]
}

// Ident returns an 8-byte truncated SHA-256 of an arbitrary value (tag value, word).
func Ident(val []byte) []byte {
	h := sha256.Sum256(val)
	return h[:HashLen]
}

// Timestamp writes a unix timestamp as 8-byte big-endian into dst.
func PutTimestamp(dst []byte, ts int64) {
	binary.BigEndian.PutUint64(dst, uint64(ts))
}

// GetTimestamp reads an 8-byte big-endian timestamp from src.
func GetTimestamp(src []byte) int64 {
	return int64(binary.BigEndian.Uint64(src))
}

// AppendTimestamp appends an 8-byte big-endian timestamp to dst.
func AppendTimestamp(dst []byte, ts int64) []byte {
	var buf [8]byte
	binary.BigEndian.PutUint64(buf[:], uint64(ts))
	return append(dst, buf[:]...)
}

// PutKind writes a kind as 2-byte big-endian into dst.
func PutKind(dst []byte, k uint16) {
	binary.BigEndian.PutUint16(dst, k)
}

// GetKind reads a 2-byte big-endian kind from src.
func GetKind(src []byte) uint16 {
	return binary.BigEndian.Uint16(src)
}

// AppendKind appends a 2-byte big-endian kind to dst.
func AppendKind(dst []byte, k uint16) []byte {
	return append(dst, byte(k>>8), byte(k))
}