godb/internals/vm/vm.go

package vm

import (
	"encoding/binary"
	"fmt"
	"os"
	"strings"

	. "godb/internals/buffer"
)

type MetaCommandResults int

const (
	MetaCommandSuccess MetaCommandResults = iota
	MetaCommandUnrecognised
)

type PrepareResults int

const (
	PrepareSuccess PrepareResults = iota
	PrepareUnrecognised
	PrepareSyntaxError
)

type statementType int

const (
	StatementSelect statementType = iota
	StatementInsert
	StatementUnrecognized
)

const (
	idSize       = 8
	usernameSize = 32
	emailSize    = 255

	idOffset       = 0
	usernameOffset = idOffset + idSize
	emailOffset    = usernameOffset + usernameSize

	pageSize    = 4096
	maxPages    = 100
	rowSize     = idSize + usernameSize + emailSize
	rowsPerPage = pageSize / rowSize
	maxRows     = rowsPerPage * maxPages
)

type ExecuteResult int

const (
	ExecuteSuccess ExecuteResult = iota
	ExecuteTableFull
	ExecuteFail
)

type row struct {
	id       uint
	username string
	email    string
}

type Statement struct {
	stype statementType
	row   row
}

type Table struct {
	numOfRows uint
	pages     [maxPages]any
}

func NewStatement() *Statement {
	statement := &Statement{}
	return statement
}

func NewTable() *Table {
	newTable := Table{numOfRows: 0}
	for i := range maxPages {
		newTable.pages[i] = nil
	}
	return &newTable
}

func rowToUse(table *Table, rowNo uint) []byte {
	pageNo := rowNo / rowsPerPage
	var page []byte
	if table.pages[pageNo] == nil {
		page = make([]byte, pageSize)
		table.pages[pageNo] = page
	} else {
		i := table.pages[pageNo]
		if t, ok := i.([]byte); !ok {
			page = []byte(t)
		} else {
			page = t
		}
	}
	rowOffset := rowNo % rowsPerPage
	byteOffset := rowOffset * rowSize
	return page[byteOffset : byteOffset+rowSize]
}

func (row *row) seralizeRow(page []byte) {
	binary.NativeEndian.PutUint64(page[idOffset:], uint64(row.id))

	usernameBuff := page[usernameOffset : usernameOffset+usernameSize]
	clear(usernameBuff)
	copy(usernameBuff, row.username)

	emailBuff := page[emailOffset : emailOffset+emailSize]
	clear(emailBuff)
	copy(emailBuff, row.email)
}

func (row *row) deSeralizeRow(page []byte) {
	row.id = uint(binary.NativeEndian.Uint64(page[idOffset:idSize]))
	row.username = string(page[usernameOffset : usernameOffset+usernameSize])
	row.email = string(page[emailOffset : emailOffset+emailSize])
}

func (statement *Statement) execInsert(table *Table) ExecuteResult {
	if table.numOfRows >= maxRows {
		return ExecuteTableFull
	}
	statement.row.seralizeRow(rowToUse(table, table.numOfRows))
	table.numOfRows += 1
	return ExecuteSuccess
}

func (statement *Statement) execSelect(table *Table) ExecuteResult {
	var row row
	for i := range table.numOfRows {
		row.deSeralizeRow(rowToUse(table, i))
		fmt.Printf("%v\n", row)
	}
	return ExecuteSuccess
}

func DoMetaCommands(inputBuffer *InputBuffer) MetaCommandResults {
	metacommand := MetaCommandUnrecognised
	if inputBuffer.Buffer == ".exit" {
		fmt.Println("bye!")
		os.Exit(0)
	} else {
		metacommand = MetaCommandUnrecognised
	}
	return metacommand
}

func (statement *Statement) PrepareStatements(inputBuffer *InputBuffer) PrepareResults {
	statement.stype = StatementUnrecognized
	if strings.HasPrefix(inputBuffer.Buffer, "insert") {
		argsAssigned, err := fmt.Sscanf(inputBuffer.Buffer, "insert %d %s %s", &statement.row.id, &statement.row.username, &statement.row.email)
		if err != nil {
			fmt.Println("error parsing insert statement")
			fmt.Println(err.Error())
		}
		if err != nil || argsAssigned > 3 {
			return PrepareSyntaxError
		}
		statement.stype = StatementInsert
		return PrepareSuccess
	}
	if strings.HasPrefix(inputBuffer.Buffer, "select") {
		statement.stype = StatementSelect
		return PrepareSuccess
	}
	return PrepareUnrecognised
}

func (statement *Statement) ExecuteStatement(table *Table) ExecuteResult {
	var result ExecuteResult = ExecuteFail
	switch statement.stype {
	case StatementInsert:
		result = statement.execInsert(table)
	case StatementSelect:
		result = statement.execSelect(table)
	}
	return result
}