Interested in collaborating

[jonathanstrong]

Hello,

Earlier this evening I randomly came across your post on users.rust-lang.org from several months ago looking for feedback on this library. I wasn't previously aware of the project and was impressed by its breadth and the amount of work that's already gone into it. I run a trading operation on a rust codebase so this is definitely of major interest to me! I'd be interested in collaborating and wanted to send some initial thoughts from a couple hours with the code.

You mentioned in the post you were new to rust, so I included various recommendations from my own experience. Perhaps some of it will be obvious or things you already know - but I thought it couldn't hurt. Like you, I see rust as an incredible tool for this application, owing to the great performance, powerful type system, modern packaging system, etc. But there are definitely still bumps along the way during the learning curve.

In any event, here are various thoughts I jotted down while diving into the project for the first time:

• The number one thing I wanted/expected to see are some examples. Examples are integrated into cargo (cargo run --example pricing) and I often start there when I'm looking at a project for the first time.

• On the other hand, there's plenty of tests. (Second step is often git clone then cargo test for me). Perhaps some of the tests include code that would be suitable to use in examples. Do you have any recommendations on which ones to look at?

• One of the biggest issues I face is needing to use different number types in different situations and performance and ergonomics pitfalls associated with juggling all of them, converting from one to the other, etc. In many cases, correct precision is required and a decimal type is needed. (I have been using the rust wrapper to the decNumber lib for over a year, it seems by my lights to be the best choice at the moment). In many other cases, the performance hit from using decimal is very, very steep compared to the floating point primitives (f64/f32). Rust tends very much towards prudence when it comes to floating point, with a carefully designed api, but it still takes a lot of care to avoid blowups. A third option that sometimes presents itself is to use integer types as fixed-point, for instance when the precision of prices is static. This can be very fast and precise, but it's not always available. I saw that the library generally uses f64, I was wondering if you had further thoughts on how that has worked for you in this application.

• I'm intrigued by the date functionality here; it might be worth pursuing it as a stand-alone crate. Currently the chrono crate has a pretty good handle on power/completeness/correctness and it works very well with serde for deserialization. However, the main chrono DateTime<Utc> type is somewhat heavyweight: it's an i32 for the date, two u32s for the time, plus an offset enum (which I believe is zero-sized, however). I frequently use nanosecond timestamps stored in an i64/u64 for performance, and there's no good library that I've seen that focuses less on the kitchen sink, and more on the most performant possible way to store/operate on times.

• Regarding the Error struct defined in core::qm:

  pub struct Error {
      message: String 
  }

  (Haven't been able to tell if this is widely used throughout the library or not). Error-handling has been a moving target in rust, with the std Error trait falling out of favor and several different crates trying to make error-handling more ergonomic have risen and fallen. Personally, I have come to prefer using Error enums defined on a per-crate or per-mod basis. In this case, it might look something like this:

  pub enum Error {
      Io(std::io::Error),
      ShapeError(ndarray::ShapeError)
      // ...
  }

  A lot of the time I keep it even simpler with "zero-size" enums with no data associated with each variant, but the variant name alone is enough to act on the error.

  The advantages of localized, lightweight error types include:

  • In rust, matching on or otherwise manipulating/operating on types is very ergonomic and powerful, while matching/operating on strings is relatively painful, owing to the strict ownership semantics and String/str divergence.

  • If latency is a priority, as it often is in trading, incurring a heap allocation (for the message String) just to handle an error is somewhat wasteful.

  • This is especially true because in idiomatic rust, program crashes are very rare and there's little need to gather as much information as possible for piecing together what went wrong after the fact. Your debugging time is more likely to be spent on the front-end getting the program to compile in the first place.

• Regarding this comment/question:

  impl Black76 {
      pub fn new() -> Result<Black76, qm::Error> {
  
          // For some reason we cannot use the ? operator for this sort of error.
          // as a workaround, do it manually for now.
          match Normal::new(0.0, 1.0) {
              Ok(normal) => Ok(Black76 { normal: normal }),
              Err(e) => Err(qm::Error::new(&format!("RSStat error: {}", e)))
          }
      }
      // ...
  }

  The Normal::new function from the statrs crate returns Result<T, StatsError>. Since Black76::new returns Result<Self, qm::Error>, you would only be able to use the question mark operator if a From<_> impl exists to convert a StatsError into a qm::Error. You already have a long list of these From<_> impls in core::qm, so perhaps you discovered this along the way.

  Incidentally, the statrs::StatsError is a great example of a robust, but lightweight error enum that covers the various types of errors that can come from the library without any allocations. &'static str (string literals) used in many of the variants are very ergonomic for that purpose.

  In this case, however, handling the error isn't even necessary. The error can only arise if either argument is NaN or a negative value is passed as the std_dev argument, which is clearly is not the case since it's being passed float literals. In my judgement, this is a perfect use of .unwrap() or .expect("statrs::Normal::new(0.0, 1.0)"). I typically note why an unwrap is safe with a quick comment.

• One test failed on my machine (using nightly-2018-12-02), which seems to be a floating point issue:

  thread 'math::brent::tests::problem_of_the_day' panicked at 'result=192.80753261197134 expected=192.80752497643797', src/math/brent.rs:146:9

Hope these initial comments are helpful to you. I'm looking forward to delving further into this. I'd be interested to hear what the current status on the project is, and if you have any ideas for areas where collaboration would be especially useful.

Jonathan

[MarcusRainbow]

Hi Jonathan,
Wow! This depth of feedback is what I was hoping for when I put the project on GitHub. Thanks very much. Here are a few responses to your issues:

Examples:
Ideally what I wanted for examples was real world examples. Unfortunately, I do not have easy access to real market data. If you do, that would really help the project. Until I have some decent examples, the place I'd recommend looking at first for most users is the very top level tests -- facade/c_interface.rs. At present, the facade is all defined in terms of JSON inputs. Moving forward, I think there should be lower-level ways of getting data into QuantMath, but this does at least work and allow you to do pricing. As you are interested in interfacing from Rust, I'd start at the level below the facade -- pricers/selfpricer.rs and pricers/montecarlo.rs.

Numeric types:
I have always used f64 or its equivalent in other languages for financial maths. I can see that this gives inaccurate results, but in practice the input data is not that great anyway. For some applications such as accrual-valued equity swaps and settlement, I can see that you may need greater accuracy. My gut feel would be to write templated code for functions I knew might need decimal accuracy, but to avoid it for most of the library. I don't think anybody is going to want risk calculated more accurately than f64.

Date/time:
Please feel free to disagree, but my experience is that finance is either interested in nanosecond accuracy for high-frequency trading, or else it is good enough to hit the right date. Date-time brings a host of issues, like what is the settlement period for different times of day, do you need to handle intraday discounting, or how you handle timezones and changes to daylight-saving time. For every purpose I've experienced in finance, it's good enough to have a date plus some indication of how far you are through the trading day, such as an enum (open/close) or a day fraction -- volatility time, which may not be a linear function of real clock time.

Errors as strings:
I've always liked strings as errors in finance for the reason that you seldom want to recover from an error -- you just want to report as much detail as possible to the user and then abort that calculation. (I once worked on a library that tried all sorts of ways of getting a vol, and if all else failed, it used 30%! -- I don't want that sort of philosophy in QuantMath.) However, I take what you are saying about the cost and awkwardness of strings. Maybe a way forward would be to use lightweight error types for the low-level modules, and keep string errors for the higher levels, where we need to be able to report a lot of detail.

statrs error:
Your analysis is exactly correct here! Feel free to make the change you suggest.

The failing test:
I only have a couple of machines here that I can test the code on -- it does not surprise me that different architectures would give slightly different results. The difference we are seeing here is a relative error of about 1e-8, even though I was asking Brent for an accuracy of 1e-10. Maybe it is just differences between the trig functions on my and your machines (see http://notabs.org/fpuaccuracy/)

Contributions:
Pretty much anything would be welcome, from code tidyup through major chunks of new functionality. What sort of trading are you doing, and what are your requirements from a pricing library?

Marcus

[jonathanstrong]

I'm glad you found it helpful! To give you a bit more background about myself, I trade on the crypto markets, at relatively "high frequency" (the infrastructure is very young, it's milliseconds that matter still, not nanoseconds). Although I studied programming, I worked as a political reporter after school; my interest in trading came from an experience of using machine learning to forecast the outcomes of congressional votes, which we hoped to sell to hedge funds. (The forecasts were perfect, the sales less so -- it's still hard to trade an informational edge like that, we found).

Anyway, that's all to say it's less that I know exactly what I want to use here than I am very excited by the wealth of knowledge the code contains and all that I can learn from it. For instance, I had never encountered the term "bump" before yesterday, which I gather means to update calculations incrementally on receiving market new data. It's also to point out that my reference point of latency-sensitive code that runs 24/7 (no "trading day") and must never crash is pretty different from running a daily analysis - so my performance concerns may be overzealous.

I collect lots and lots of market data but I doubt it's the kind needed here. However, I found the functions in risk::marketdata::tests that generate dummy data so that should be enough to craft some examples out of.

One major thing I noticed from my further review of the code was the extensive use of single-use reference count types (RcRateCurve, RcDividendStream, etc). Also from the use of several serde alternatives I gather there was some friction with std::rc::Rc and serializing/deserializing the data.

On one previous occasion, I remember facing a similar problem that I solved by enabling the "rc" feature in serde (it's listed in the Cargo.toml. I believe it allows Rc<T> to be serialized as T, although it's been a while.

In general, it's far preferable to use a generic Rc type that wraps the underlying types, e.g. Rc<RateCurve> instead of RcRateCurve, as a RateCurve type really has no business mucking about with reference counting, and the one-off types create a lot of noise in trying to read and understand the code.

Can you explain what problems you faced that led you to these design choices? I have in mind several occasions where I ended up doing similar things simply because it seemed to be the only way to accomplish what I wanted the code to do. I gather that the intended use is to be able to load the underlying data from file, so serialization is important. Was the use of reference counting types primarily for convenience, or necessity (cyclic data structures, etc.)?

On a related front, many of the reference counted types use Arc, the thread-safe reference counted type that uses atomics under the hood. In my experience, Arc is surprisingly expensive - the type has to be quite large for simply cloning it to be slower. Also, I didn't find any multi-threaded code in the library itself. But the scope of the data here also matters. Broadly speaking, in a serious use of this library, how many "things" are we talking about? Thousands? Single-digit millions? At hundreds of millions the tab really starts to rack up from Arc, String, etc., but maybe that's way beyond what's plausible (in my high freq world the data piles up at a staggering pace).

I've forked the library and begun some work around the edges, I plan on submitting various pull requests here and there coming forward. First significant one will be to construct an example based on the monte carlo tests. Thanks for making this code public! The scope of the project is quite amazing and I'm very pleased I found it.