// Performs sbox permutations for DES encryption
// (rodric rabbah, <rabbah@mit.edu>)

bit->bit splitjoin Sboxes()
{
    split roundrobin(6);
    
    for (int i = 1; i <= 8; i++) {
        if (i == 1) add Sbox(S8); // lower 8 bits
        if (i == 2) add Sbox(S7); // next  8 bits
        if (i == 3) add Sbox(S6); // ...
        if (i == 4) add Sbox(S5);
        if (i == 5) add Sbox(S4);
        if (i == 6) add Sbox(S3);
        if (i == 7) add Sbox(S2);
        if (i == 8) add Sbox(S1); // last (upper) 8 bits
    }

    join roundrobin(4);
}

bit->bit filter Sbox(int[4][16] table)
{
    work push 4 pop 6 {
        int r = pop(); // r = first and last bit
        int c = pop(); // c = middle four bits
        c = (pop() << 1) | c;
        c = (pop() << 2) | c;
        c = (pop() << 3) | c;
        r = (pop() << 1) | r;

        int out = table[r][c];
        push((bit)((out & 0x1) >> 0));
        push((bit)((out & 0x2) >> 1));
        push((bit)((out & 0x4) >> 2));
        push((bit)((out & 0x8) >> 3));
    }
}