# Encryption

```
// Source -> Encrypted
// 10 -> 11 (1 + 0) (1 + 0)
// 1010 -> 1211 (1 + 0) (1 + 0 + 1) (0 + 1 + 0) (1 + 0)
//
// 1010
// |/
// 1... (1+0)

// 1010
// \|/
// .2.. (1+0+1)

// 1010
//  \|/
// ..1. (0+1+0)

// 1010
//   \|
// ...1 (1+0)

// Result: 1211

// Write function to go from encrypted to source
// Source numbers are always 1 or 0
```

## Using Just Strings

```cpp
// assuming string sequence - iterative solution
string next_n(const string &str, const unsigned pos, const unsigned n)
{
    unsigned short sum = 0, go_to = pos + n;
    for (int i = pos; i < go_to; i++)
        sum += (str[i] - '0');
    return to_string(sum);
}

string num_src_to_encryptd(const string &bits)
{
    if (bits.length() <= 1) return bits;
    string encrypted = "";
    encrypted.reserve(bits.length());

    encrypted += next_n(bits, 0, 2);
    for (int i = 0; i < bits.length() - 2; i++)
        encrypted += next_n(bits, i, 3);

    encrypted += next_n(bits, bits.length() - 2, 2);

    return encrypted;
}

int main()
{
    // string test
    string test0 = "1";
    cout << num_src_to_encryptd(test0) << endl;

    string test1 = "10";
    cout << num_src_to_encryptd(test1) << endl;

    string test2 = "1010";
    cout << num_src_to_encryptd(test2) << endl;

    string test3 = "10101100";
    cout << num_src_to_encryptd(test3) << endl;
}
```

## Strings plus some Math

```cpp
// assuming bit sequence = iterative solution
unsigned short num_at_MSD(const int num_digits, const int num, const int i) {
    return int(num / pow(10, num_digits - 1 - i)) % 10;
}

unsigned short num_digits(int num)
{
    int div = 1;
    int count = 0;
    while (div <= num) {
        div *= 10;
        count++;
    }
    return count;
}

string num_src_to_encryptd(const int num)
{
    string bit_seq = "";
    int size = num_digits(num), sum = 0;
    if (size <= 1) return to_string(num);


    bit_seq += to_string(num_at_MSD(size, num, 0) +
        num_at_MSD(size, num, 1));
    for (int i = 0; i < size - 2; i++) {
        for (int j = 0; j < 3; j++) {
            sum += num_at_MSD(size, num, i + j);
        }
        bit_seq += to_string(sum);
        sum = 0;
    }
    bit_seq += to_string(num_at_MSD(size, num, size - 2) +
        num_at_MSD(size, num, size - 1));

    return bit_seq;
}


int main()
{
    int test0 = 1; 
    cout << num_src_to_encryptd(test0) << endl;

    int test1 = 10; 
    cout << num_src_to_encryptd(test1) << endl;

    int test2 = 1010; 
    cout << num_src_to_encryptd(test2) << endl;

    int test3 = 10101100; 
    cout << num_src_to_encryptd(test3) << endl;
}
```

## Bitwise

```cpp
unsigned msb(int num)
{
    unsigned count = 0;
    while (num >>= 1)
        count++;
    return count;
}

bool getBit(int num, int n) {
    return (num & (1 << n)) > 1;
}


int num_src_to_encryptd(const int num)
{
    int bit_seq = 0;
    int multiplier = 1;
    int size = msb(num) + 1, sum = 0;

    if (size <= 1) return num;
    bit_seq += (getBit(num, 0) + getBit(num, 1)) * multiplier;
    multiplier *= 10;

    for (int i = 0; i < size - 2; i++) {
        for (int j = 0; j < 3; j++) {
            sum += getBit(num, i + j) * multiplier;
        }
        bit_seq += sum;

        multiplier *= 10;
        sum = 0;
    }
    bit_seq += (getBit(num, size - 1) + getBit(num, size - 2)) * multiplier;

    return bit_seq;
}


int main()
{
    int test0 = 1; // 1
    cout << num_src_to_encryptd(test0) << endl;

    int test1 = 2; // 10
    cout << num_src_to_encryptd(test1) << endl;

    int test2 = 10; // 1010
    cout << num_src_to_encryptd(test2) << endl;

    int test3 = 172; // 10101100
    cout << num_src_to_encryptd(test3) << endl;
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://maksimdan.gitbook.io/interview-practice-problems/leetcode_sessions/cryptography/encryption.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.