package main

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


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

func file_index(mem []int, id int) []int {
    offset := -1
    for i, elem := range mem {
        if elem == id {
            offset = i
            break
        }
    }
    if offset == -1 { return nil }
    
    length := 1
    for i := offset + 1; i < len(mem); i++ {
        if mem[i] == id {
            length++
        } else {
            break
        }
    }

    return []int{offset, offset + length}
}

func cp(mem []int, id int) bool {
    file_idx := file_index(mem, id)
    file_len := file_idx[1] - file_idx[0]
    target_mem := mem[:file_idx[0]]
    
    i := 0
    for {
        if i > len(target_mem) { break }

        space_idx := file_index(target_mem[i:], -1)
        if space_idx == nil { break }

        space_len := space_idx[1] - space_idx[0]
        if space_len >= file_len {
            // copy 
            copy(mem[i + space_idx[0]: i + space_idx[1]], mem[file_idx[0]:file_idx[1]])
            return true
        }
        i += space_idx[1] + 1
    }

    return false
}

// 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
    for i := file_id - 1; i > 0; i-- {
        file_idx := file_index(mem, i)
        if cp(mem, i) {
            // clear old file
            for i := file_idx[0]; i < file_idx[1]; i++ { mem[i] = -1 }
        }
    }

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

    fmt.Println(checksum)
}