const fs = require('fs');

const input = fs.readFileSync(__dirname + '/input.txt', 'utf8');

interface MyFile { id: number, addr: number, size: number };

const files: Array<MyFile> = [];

let pointer = 0;
let id = 0;

for (let i = 0; i < input.length; i++) {
    const size = parseInt(input[i]);
    if (i % 2 === 0) {
        files.push({ id, size, addr: pointer });
        id++;
    }
    pointer += size;
}

function printDisk() {
    const disk: Array<string> = [];
    files.forEach((file) => {
        for (let i = file.addr; i < file.addr + file.size; i++) {
            disk[i] = ''+file.id;
        }
    });

    let diskStr = '';
    for (let i = 0; i < disk.length; i++) {
        diskStr += disk[i] || '.';
    }
    console.log(diskStr);
}

function getFirstSpace(): null|{ addr: number, size: number, index: number } {
    for (let i = 0; i < files.length; i++) {
        const file = files[i];
        const nextFile = files[i+1];
        if (file && nextFile) {
            const fileEnd = file.addr + file.size;
            if (nextFile.addr > fileEnd) {
                return {
                    addr: fileEnd,
                    size: nextFile.addr - fileEnd,
                    index: i + 1,
                };
            }
        }
    }
    return null;
}


top:
while (true) {
    const lastFile = files.pop()!;
    let remainingSize = lastFile.size;
    while (remainingSize > 0) {
        const firstFreeSpace = getFirstSpace();
        if (!firstFreeSpace || firstFreeSpace.addr >= lastFile.addr) {
            const secondLastFile = files[files.length - 1];
            files.push({
                addr: secondLastFile.addr + secondLastFile.size,
                size: remainingSize,
                id: lastFile.id,
            });
            break top;
        }
        let filePartSize;
        let remainingFreeSpace;
        if (remainingSize < firstFreeSpace.size) {
            filePartSize = remainingSize;
            remainingFreeSpace = firstFreeSpace.size - remainingSize;
        } else {
            filePartSize = firstFreeSpace.size;
            remainingFreeSpace = 0;
        }
        files.splice(firstFreeSpace.index, 0, { addr: firstFreeSpace.addr, id: lastFile.id, size: filePartSize });
        remainingSize -= filePartSize;
    }
}

let checksum = 0;

files.forEach((file) => {
    for (let i = file.addr; i < file.addr + file.size; i++) {
        checksum += i * file.id;
    }
});

console.log(checksum);