// Package store is torly's storage engine.
//
// It follows the design used by morly: an append-only segmented write-ahead
// log holds event bytes, and fixed-width sorted flat files hold the indexes.
// Nothing is memory-mapped and no record is updated in place, which is what
// makes the design a good fit for TinyGo and for immutable signed messages.
//
// A serial is a physical address in the log: segment<<32 | offset. Index
// records carry the serial of the event they point at, so a lookup is
// index search followed by one log read.
package store

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

const (
	// SerialLen is the byte length of a serial (uint40).
	SerialLen = 5
	// MaxSerial is the largest uint40 value.
	MaxSerial uint64 = 1<<40 - 1
	// HashLen is the byte length of truncated hashes in index keys.
	HashLen = 8
)

// Hash8 returns the first eight bytes of SHA-256(data).
func Hash8(data []byte) (out [HashLen]byte) {
	sum := sha256.Sum256(data)
	copy(out[:], sum[:HashLen])
	return out
}

// PutSerial writes v as a 5-byte big-endian serial.
func PutSerial(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)
}

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

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

// PutTimestamp writes ts as an 8-byte big-endian value, preserving ordering.
func PutTimestamp(dst []byte, ts int64) { binary.BigEndian.PutUint64(dst, uint64(ts)) }

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

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

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