package main

import (
    "os"
    "fmt"
    "strconv"
    "strings"
)


// general util
func check(e error) {
    if e != nil { panic(e) }
}

func pop(arr []int, index *int) int {
    /* get last element that is not a -1 and replacing it with -1 int the source slice */
    for i := len(arr) - 1; i >= 0; i-- {
        if arr[i] != -1 {
            (*index) = i
            elem := arr[i]
            arr[i] = -1
            return elem
        }
    }
    return -1
}

func insert(arr []int, n int) int {
    /* insert element at the first -1, return the index of this -1 */
    for i, elem := range arr {
        if elem == -1 {
            arr[i] = n
            return i
        }
    }
    return -1
}

// main loop
func main() {

    // read in file
    dat, err := os.ReadFile("data.txt")
    check(err)
    dat_str := strings.TrimSpace(string(dat))
    
    // contruct input as int arr 
    input := make([]int, 0, len(dat_str))
    mem_size := 0
    for _, char := range []rune(dat_str) {
        num, err := strconv.Atoi(string(char))
        check(err)
        mem_size += num
        input = append(input, num)
    }

    // convert to actual memory representation
    mem := make([]int, 0, mem_size)
    is_file := true
    file_id := 0
    for _, elem := range input {
        if is_file {
            for range elem {
                mem = append(mem, file_id)
            }
            file_id++
        } else {
            for range elem {
                mem = append(mem, -1)
            }
        }
        is_file = !is_file
    }

    // reorder
    idx_pop    := -1
    idx_insert := -2

    for idx_pop != idx_insert {
        elem := pop(mem, &idx_pop)
        idx_insert = insert(mem, elem)
    }

    // calc checksum
    checksum := 0
    for i, elem := range mem {
        if elem == -1 { break }
        checksum += i * elem
    }

    fmt.Println(checksum)
}