Everything You Wanted to Know about Scrypt-Jane

I've talked a bit about a few scrypt-jane based coins previously -- specifically I've mentioned Microcoin, but there are several others. I've also talked about the alternative Proof of Work (PoW) algorithm used in Vertcoin, "Scrypt Adaptive N-Factor". Put simply, I like the VTC algorithm a lot more, but I've never really discussed why. So, this is as good a time as any to broach that subject.

Fundamentally, the difference between scrypt-jane and VTC's Adaptive-N-Factor PoW algorithms is pretty simple. VTC basically went with hand-picked dates for when to move on to various higher N-Factors, while most coins using scrypt-jane have a formula that decides when it's time to move up the difficulty ladder. In fact, VTC actually adjusted it's rollout schedule in a hard-fork of the blockchain after the first couple of weeks, at the same time they introduced KGW.

But I'm getting ahead of myself. First, I suppose I ought to quickly cover what N-Factor is used for. N-Factor represents the amount of data that a thread works on, with the N in N-Factor being the power of two  (minus 1) that's used. Standard scrypt uses and N-Factor of 9 (I think? Correct me if I'm wrong and I'll fix the table!). Here's the quick list of RAM requirements (poached from the VTC announcement thread):

N-Factor	N (2N+1)	Memory
9	1024	128KB
10	2048	256KB
11	4096	512KB
12	8192	1MB
13	16384	2MB
14	32768	4MB
15	65536	8MB
16	131072	16MB
17	262144	32MB
18	524288	64MB
19	1048576	128MB
20	2097152	256MB
21	4194304	512MB
22	8388608	1GB
23	16777216	2GB
24	33554432	4GB

As you can imagine, running the scrypt algorithm with N-Factor of 20 would require a huge chunk of RAM even on a CPU with eight threads -- 1GB just for the basic memory footprint. On a GPU like the R9 290X that has 2816 shader cores and 4GB RAM, any N-Factor above 13 is going to present some real problems. Now, let's get back to VTC's Adaptive-N vs. scrypt-jane.

With Vertcoin, the rollout of the above N-Factors has been set such that we start at N-Factor 11, we'll move to N=12 February 25, 2016; N=13 comes in September 30, 2017, and so on. The reasoning is that right now, scrypt ASICs are just starting to show up, and they're all built for N=10; to make a new ASIC for N=11 will require a solid 6-12 months at least -- assuming you can find a company that thinks it's even worth doing, considering N=12 will supplant N=11 -- and it would require a lot of RAM. And in the meantime, those with GPUs can still do a decent job of mining.

Truth be told, scrypt-jane doesn't actually define any schedule -- it's just a flexible implementation of scrypt that can be tweaked as desired. It supports several mixing and hashing functions, with scrypt being used with a varying N-Factor. In general, scrypt-jane implementations have set a minimum N-Factor of 4 and a maximum N-Factor of 30. That's fine, but the problem is how coins are scaling the N-Factor. Here's what happens with YACoin, YBCoin, Ultracoin, and Microcoin to name several that I've checked:

1. Calculate the number of seconds since the launch of the coin.
2. Look at that number in binary, and basically find how many significant binary digits are used, but don't count the three most significant digits.
3. With those three most significant binary digits, do a logical AND operation with 0b11 (or in other words, look at the two least significant bits).
4. Using the numbers from steps two and three: NFactor = ([BinSigDigits - 3] * 170 + [{ThreeMostSigBinDigits & 0b11} * 25] - 2320) / 100
5. The result is converted to an integer and limited to values between 4 and 30, inclusive.

That's one of those formulas that just doesn't seem too sensible to me, and the code unfortunately doesn't shed any light on why these numbers were selected. Why 170, 25, 2320, and 100? As my computer science instructor used to call such numbers: GOK (God Only Knows). This is why we have constants defined in code, so that you can give a short comment on what the number is used for. Bad coding practices unfortunately are par for the course in the cryptocurrency world. So what does this formula actually mean? Well, it means the N-Factors will be changed on the following time schedule:

N-Factor	Seconds Since Launch	Days at N-Factor
4	0-524287	6.07
5	524288-786431	3.04
6	786432-1048575	3.0
7	1048576-1835007	9.1
8	1835008-2097151	3.1
9	2097152-4194303	24.2
10	4194304-5242879	12.2
11	5242880-8388607	36.4
12	8388608-12582911	49
13	12582912-16777215	48
14	16777216-33554431	194
15	33554432-41943039	97
16	41943040-67108863	292
17	67108864-100663295	388
18	100663296-134217727	388
19	134217728-234881023	1166
20	234881024-268435455	388
21	268435456-536870911	3107
22	536870912-805306367	3107
23	805306368-1073741823	3107
24	1073741824-1879048191	9320
25	1879048192-2147483647	3107
26	2147483648-4294967295	24855
27	4294967296-5368709119	12428
28	5368709120-8589934591	37283
29	8589934592-12884901887	49710
30	12884901888+	(Indefinitely)

Personally... I think that's a stupid set of N-Factor progressions. There, I said it: the standard scrypt-jane implementation has non-sensible spacing between some of the N-Factors. Take the first 48.5 days when we're below N-Factor 10 as a prime example; what purpose does it serve to use such low N-Factors? Well, early on the GPU mining speed in scrypt-jane will be much higher compared to CPU mining, whereas once you hit N-Factors of 9 and above it starts to slow down. At that point, we're still only looking at 15-50 days between some pretty severe adjustments in hash rates from 10-13, which just creates a bit of chaos. Then round about N-Factor 14, we reach the point where CPUs become roughly as fast as GPUs, give or take -- we've actually got a real example of this, as YACoin is already at N-Factor 14 -- and then we start to slow down on the N-Factor changes.

So here's the question: is a coin that's predominantly limited to CPU mining a good thing? While I'm happy to mine such coins when it makes sense from a profitability standpoint, I generally assume that any good coin that sticks around long enough will end up with a GPU miner port, unless it's simply impractical to make one. But if the algorithm is simply not portable to GPUs for whatever reason, I don't inherently think that it's a bad thing. My problem with scrypt-jane is that it sort of goes the opposite route: it starts out by opening the floodgates for GPUs, but then after six months or so (194 days), we reach N-Factors where GPU mining becomes quite slow. That will result in most if not all GPUs leaving to go to "greener pastures", which is sort of the opposite of what you want to happen.

The typical progression is usually the opposite. We start with a CPU-only coin (BTC started this way, and so did LTC, QRK, and now DRK). After the coin/algorithm gains some momentum, smart people figure out a way to use GPUs to speed up the work. More time passes and if the coin(s) using the PoW algorithm remains viable, maybe people even try to make an FPGA or ASIC for the core algorithms. If you want to stop this sort of progression, well, honestly you can't truly stop it -- all you can do is make it more difficult. Remember that scrypt was originally created to be a CPU-only PoW, but that failed. Quark and X11 have now been ported to GPUs with varying degrees of success. Considering GPUs are becoming more like CPUs, at some point in the future it's not a stretch to assume that if something can't be mined (effectively) with a GPU, it probably can't be mined with a CPU either.

Wrapping up, I wanted to quickly look at the most common scrypt-jane coins and check on hash rates profitability -- for CPUs as well as GPUs. This is obviously just a snapshot in time, but going with the four coins mentioned earlier (which are similar in most respects in terms of PoW implementation, though the block reward schedules vary), here's what we have (as best as I can tell -- again, correct me if I'm wrong as it's a bit tricky to find the current N-Factor for most of these coins):

Coin	N-Factor	CPU Perf. (i7-4770K)	GPU Perf. (HD 7970)	Difficulty	BTC Rate	BTC/Day (CPU / GPU)
Microcoin	10	?	450	2.319	0.00000003	? / 0.0134
Ultracoin	11	?	235	6.828	0.00034	? / 0.0118
YACoin	14	0.97K	1.4K	0.0167	0.000031	0.00184 / 0.00258
YBCoin	14	0.97K	1.4K	0.119	0.0037	0.00191 / 0.002^8

So it's worth pointing out that the start times for the various coins determine when the N-Factor changes; Microcoin has a start time of 1389028879 (06 Jan 2014 17:21:19 GMT) while Ultracoin has a start time of 1388361600 (30 Dec 2013 00:00:00 GMT), so UTC is currently one N-Factor ahead of MRC. As for YAC and YBC, other than their reward structures they're both close enough in start times that they're on N-Factor 14. Also, I couldn't find a CPU miner that didn't hard-code the start time so I couldn't test CPU rates on MRC and YAC (yet -- if you have a binary for Windows, let me know).

The short summary: YAC and YBC are mostly not worth your CPU/GPU time, while for the time being MRC and UTC are doing okay. MRC will have a lot of coins, and the current exchange rate fluctuates around 2-4 satoshis, so you can see pretty large swings in value over the course of a single day. If you want to dump a bunch of MRC, you'll probably get 2 satoshi whereas being patient can get you 3 or 4 satoshi.

The big problem with mining any of these is that the scaling N-Factor happens so early that if you're not in before N-Factor 12 or so, you've missed the boat for GPUs. Which means if you are in early enough, holding the coins for the first 100-150 days or so (until we reach N-Factors of 12) isn't a bad idea. Check back in a couple months and I'll let you know how that strategy worked out for me!

Wrapping this up, sorry if I'm pissing any anybody's favorite pool, but I call it as I see it. There have been plenty of questionable coins released in the past year, and while early adopters have often made out with big winnings, johnny-come-lately is often left wasting their time and energy on a dead-end coin. Plus, I had some people ask about Ultracoin in particular (since it's relatively new), and this post is a result of my looking into that topic. What's really sad is the state of many of the coin releases, with poorly documented code that was clearly just cloned from another coin and tweaked as little as possible. Anyway, we'll see which coins are still around in another year and being actively mined; until then, happy (scrypt-jane?) mining!

Donations welcome
MRC: 1Ctnz6cHcMYiF9fz2pyd6orFuo1mDhKdWj
BTC: 153qS9Ze32hnV3fwirZLWNka4wBAowc21E
LTC: LfCLyykrNFftzpdWejR73hf478ZtBzQ9jE
VTC: VaYNYUUi3amUTKQAQSs9EM4xJyNRyqnoUG
UTC: Uj1rnV4fZBPAngTj7xcYHrRcxKkadRiqGo
YAC: Y1xTeeJFnwawaCPVHcvhH23w5oDGy1VRgC
YBC: Yg6wWEd2fcMyMcxbQ3eGg3dXo5aYs878pt