package main

import (
	"fmt"
	"database/sql"
	_ "github.com/lib/pq"
	"log"
	"math/rand"
	"time"
	"strconv"
	"runtime"
)

var (
	txn *sql.Tx
	err error
	db *sql.DB
	stmt *sql.Stmt
	conString = "user= host= password= dbname= sslmode=disable"
	customerList []int
)

// populates a database with test data
func main() {
	rand.Seed(time.Now().UnixNano())
	startTime := time.Now().UnixNano()
	var memStats runtime.MemStats

	if db, err = sql.Open("postgres", conString); err != nil {
		log.Fatalln(err)
	}
	defer db.Close()

	runtime.ReadMemStats(&memStats)
	fmt.Printf("Memory in use: %v Mb\n", memStats.Alloc / 1024 / 1024)
	fmt.Printf("Obtained from system: %d Mb\n", memStats.Sys / 1024 / 1024)

	customerList = loadCustomerID()

	runtime.ReadMemStats(&memStats)
	fmt.Printf("Memory in use: %v Mb\n", memStats.Alloc / 1024 / 1024)
	fmt.Printf("Obtained from system: %v Mb\n", memStats.Sys / 1024 / 1024)

	endTime := time.Now().UnixNano()
	fmt.Printf("completed in: %d ms\n", (endTime - startTime) / int64(time.Millisecond))
}

// get the maximum customer_id in the customer table
func getMaxCustomerID() int {
	var maxCustomerID string
	row := db.QueryRow("SELECT max(customer_id) as max_customer from customer")
	err := row.Scan(&maxCustomerID)
	if err != nil {
		log.Fatal(err)
	}
	intMaxCustomerID, err := strconv.Atoi(maxCustomerID)
	if err != nil {
		log.Fatal(err)
	}

	return intMaxCustomerID
}

// load list of available customerID
func loadCustomerID() []int {
	var list []int

	const limit = 1000000

	maxCustomerID := getMaxCustomerID()

	for offset := 0; offset < maxCustomerID; offset += limit {
		fmt.Printf("limit: %d offset: %d\n", limit, offset)
		rows, err := db.Query("SELECT customer_id FROM customer LIMIT $1 OFFSET $2", limit, offset)
		if err != nil && err == sql.ErrNoRows {
			break // no more records
		}

		if (err != nil) {
			log.Fatalln(err)
		}

		for rows.Next() {
			var record string
			err = rows.Scan(&record)
			if (err != nil) {
				log.Fatalln(err)
			}
			customerID, _ := strconv.Atoi(record)
			list = append(list, customerID)
		}
	}
	return list
}