## Billions of Code Name Permutations in 32 bits

My friend over at Possibly Wrong created a code name generator. By coincidence I happened to be thinking about code names myself while recently replaying XCOM: Enemy Within (2012/2013). The game generates a random code name for each mission, and I wondered how often it repeats. The UFOpaedia page on the topic gives the word lists: 53 adjectives and 76 nouns, for a total of 4028 possible code names. A typical game has around 60 missions, and if code names are generated naively on the fly, then per the birthday paradox around half of all games will see a repeated mission code name! Fortunately this is easy to avoid, and the particular configuration here lends itself to an interesting implementation.

Mission code names are built using “adjective noun”. Some examples from the game’s word list:

• Fallen Jester
• Hidden Crown

To generate a code name, we could select a random adjective and a random noun, but as discussed it wouldn’t take long for a collision. The naive approach is to keep a database of previously-generated names, and to consult this database when generating new names. That works, but there’s an even better solution: use a random permutation. Done well, we don’t need to keep track of previous names, and the generator won’t repeat until it’s exhausted all possibilities.

Further, the total number of possible code names, 4028, is suspiciously shy of 4,096, a power of two (`2**12`). That makes designing and implementing an efficient permutation that much easier.

### A linear congruential generator

A classic, obvious solution is a linear congruential generator (LCG). A full-period, 12-bit LCG is nothing more than a permutation of the numbers 0 to 4,095. When generating names, we can skip over the extra 68 values and pretend it’s a permutation of 4,028 elements. An LCG is constructed like so:

``````f(n) = (f(n-1)*A + C) % M
``````

Typically the seed is used for `f(0)`. M is selected based on the problem space or implementation efficiency, and usually a power of two. In this case it will be 4,096. Then there are some rules for choosing A and C.

Simply choosing a random `f(0)` per game isn’t great. The code name order will always be the same, and we’re only choosing where in the cycle to start. It would be better to vary the permutation itself, which we can do by also choosing unique A and C constants per game.

Choosing C is easy: It must be relatively prime with M, i.e. it must be odd. Since it’s addition modulo M, there’s no reason to choose `C >= M` since the results are identical to a smaller C. If we think of C as a 12-bit integer, 1 bit is locked in, and the other 11 bits are free to vary:

``````xxxxxxxxxxx1
``````

Choosing A is more complicated: must be odd, `A-1` must be divisible by 4, and `A-1` should be divisible by 8 (better results). Again, thinking of this in terms of a 12-bit number, this locks in 3 bits and leaves 9 bits free:

``````xxxxxxxxx101
``````

This ensures all the must and should properties of A.

Finally `0 <= f(0) < M`. Because of modular arithmetic larger, values are redundant, and all possible values are valid since the LCG, being full-period, will cycle through all of them. This is just choosing the starting point in a particular permutation cycle. As a 12-bit number, all 12 bits are free:

``````xxxxxxxxxxxx
``````

That’s `9 + 11 + 12 = 32` free bits to fill randomly: again, how incredibly convenient! Every 32-bit integer defines some unique code name permutation… almost. Any 32-bit descriptor where `f(0) >= 4028` will collide with at least one other due to skipping, and so around 1.7% of the state space is redundant. A small loss that should shrink with slightly better word list planning. I don’t think anyone will notice.

### Slice and dice

I love compact state machines, and this is an opportunity to put one to good use. My code name generator will be just one function:

``````uint32_t codename(uint32_t state, char *buf);
``````

This takes one of those 32-bit permutation descriptors, writes the first code name to `buf`, and returns a descriptor for another permutation that starts with the next name. All we have to do is keep track of that 32-bit number and we’ll never need to worry about repeating code names until all have been exhausted.

First, lets extract A, C, and `f(0)`, which I’m calling S. The low bits are A, middle bits are C, and high bits are S. Note the OR with 1 and 5 to lock in the hard-set bits.

``````long a = (state <<  3 | 5) & 0xfff;  //  9 bits
long c = (state >>  8 | 1) & 0xfff;  // 11 bits
long s =  state >> 20;               // 12 bits
``````

Next iterate the LCG until we have a number in range:

``````do {
s = (s*a + c) & 0xfff;
} while (s >= 4028);
``````

Once we have an appropriate LCG state, compute the adjective/noun indexes and build a code name:

``````int i = s % 53;
int j = s / 53;
``````

Finally assemble the next 32-bit state. Since A and C don’t change, these are passed through while the old S is masked out and replaced with the new S.

``````return (state & 0xfffff) | (uint32_t)s<<20;
``````

Putting it all together:

``````static const char *adjvs[] = { /* ... */ };
static const char *nouns[] = { /* ... */ };

uint32_t codename(uint32_t state, char *buf)
{
long a = (state <<  3 | 5) & 0xfff;  //  9 bits
long c = (state >>  8 | 1) & 0xfff;  // 11 bits
long s =  state >> 20;               // 12 bits

do {
s = (s*a + c) & 0xfff;

int i = s % COUNTOF(adjvs);
int j = s / COUNTOF(adjvs);
return (state & 0xfffff) | (uint32_t)s<<20;
}
``````

The caller just needs to generate an initial 32-bit integer. Any 32-bit integer is valid — even zero — so this could just be, say, the unix epoch (`time(2)`), but adjacent values will have similar-ish permutations. I intentionally placed S in the high bits, which are least likely to vary, since it only affects where the cycle begins, while A and C have a much more dramatic impact and so are placed at more variable locations.

Regardless, it would be better to hash such an input so that adjacent time values map to distant states. It also helps hide poorer (less random) choices for A multipliers. I happen to have designed some great functions for exactly this purpose. Here’s one of my best:

``````static uint32_t
hash32(uint32_t x)
{
x += 0x3243f6a8U; x ^= x >> 15;
x *= 0xd168aaadU; x ^= x >> 15;
x *= 0xaf723597U; x ^= x >> 15;
return x;
}
``````

This would be perfectly reasonable for generating all possible names in a random order:

``````uint32_t state = hash32(time(0));
for (int i = 0; i < 4028; i++) {
char buf;
state = codename(state, buf);
puts(buf);
}
``````

To further help cover up poorer A multipliers, it’s better for the word list to be pre-shuffled in its static storage. If that underlying order happens to show through, at least it will be less obvious (i.e. not in alphabetical order). Shuffling the string list in my source is just a few keystrokes in Vim, so this is easy enough.

### Robustness

If you’re set on making the `codename` function easier to use such that consumers don’t need to think about hashes, you could “encode” and “decode” the descriptor going in an out of the function:

``````uint32_t codename(uint32_t state, char *buf)
{
state += 0x3243f6a8U; state ^= state >> 17;
state *= 0x9e485565U; state ^= state >> 16;
state *= 0xef1d6b47U; state ^= state >> 16;

// ...

state = (state & 0xfffff) | (uint32_t)s<<20;
state ^= state >> 16; state *= 0xeb00ce77U;
state ^= state >> 16; state *= 0x88ccd46dU;
state ^= state >> 17; state -= 0x3243f6a8U;
return state;
}
``````

This permutes the state coming in, and reverses that permutation on the way out (read: inverse hash). This breaks up similar starting points.

### A random-access code name permutation

Of course this isn’t the only way to build a permutation. I recently picked up another trick: Kensler permutation. The key insight is cycle-walking, allowing for random-access to a permutation of a smaller domain (e.g. 4,028 elements) through permutation of a larger domain (e.g. 4096 elements).

Here’s such a code name generator built around a bespoke 12-bit xorshift-multiply permutation. I used 4 “rounds” since xorshift-multiply is less effective the smaller the permutation.

``````// Generate the nth code name for this seed.
void codename_n(char *buf, uint32_t seed, int n)
{
uint32_t i = n;
do {
i ^= i >> 6; i ^= seed >>  0; i *= 0x325; i &= 0xfff;
i ^= i >> 6; i ^= seed >>  8; i *= 0x3f5; i &= 0xfff;
i ^= i >> 6; i ^= seed >> 16; i *= 0xa89; i &= 0xfff;
i ^= i >> 6; i ^= seed >> 24; i *= 0x85b; i &= 0xfff;
i ^= i >> 6;

int a = i % COUNTOF(adjvs);
int b = i / COUNTOF(adjvs);
snprintf(buf, 22, "%s %s", adjvs[a], nouns[b]);
}
``````

While this is more flexible, avoids poorer permutations, and doesn’t have state space collisions, I still have a soft spot for my LCG-based state machine generator.

### Source code

You can find the complete, working source code with both generators here: `codename.c`. I used real US Secret Service code names for my word list. Some sample outputs:

• PLASTIC HUMMINGBIRD
• BLACK VENUS
• SILENT SUNBURN
• BRONZE AUTHOR