efehaskell / hiring-test

A take-home haskell programming test for potential new hires

NStack Coding Test

The system is intended to simulate receiving and tracking trades in real-time financial markets. A selection of markets execute trades for simple financial instruments (e.g. bonds) in three different currencies: USD, EUR, and GBP. A market will notify us whenever we make a trade, indicating which currency we traded, the size of the trade (quantity), and whether we bought or sold.

We aim to build a simulator that can replay previously saved trade data, in real-time, for each market. These simulators can then be used to test an automated aggregation system. The aggregator will merge invidual market output into a single multi-market output (i.e. show all trades from all markets), and then aggregate the history of individual trades to produce a running summary of total quantities traded.

We store previous market data in 3 CSV files, market_a.csv, market_b.csv and market_c.csv respectively. The CSV files store a list of trades, in the format:

TIMESTAMP,CURRENCY,QTY,DIRECTION

where

• TIMESTAMP is elapsed milli-seconds since Market open (i.e. when the program begins)
• CURRENCY is one of USD | EUR | GBP, the three currencies we are trading
• QTY is an integer number of instruments, indicating how many we bought or sold
• DIRECTION is one of BUY | SELL

Each row corresponds to a single trade that occurred at the given timestamp. A trade shows that we have either bought that much qty of the instrument, or sold that much.

e.g.

1003,USD,2000,BUY
2422,EUR,1000,SELL
etc...

As we buy or sell, our own stock of each instrument goes up or down. For example, if we buy 1000 USD and then buy 2000 more USD, we currently own 3000 USD instruments (we would say we are long 3000). If we then sell 2000 USD and then sell 2000 USD more, we are now at -1000 USD. A negative position is entirely valid (we would say we are short 1000 - effectively we owe 1000).

The task

1. Using a Haskell iteratee library of your choice (e.g. Pipes, Conduit, Machines, etc.), write a function that given a CSV filename, returns an iteratee producer that replays data from that stored CSV in real-time and emits it downstream.

e.g. using Pipes, you might end up with something like:

-- emits in real-time
replayCsv :: FilePath -> Producer Order IO ()

2. Using the producers from (1), write a program that runs all the market simulators concurrently and combines the inputs from them into a combined 'multi-market' view, emitting all trades from the input markets in real-time. You should then transform the stream of combined orders into the aggregated state of the market given all orders up to that point, e.g. total cumulative positions for all three currency symbols. Every 1 seconds the program should write the current aggregated state to stdout. A negative number indicates a 'short' position, e.g. we have sold more than we have bought. The output format should be CURRENCY QTY for each currency, comma separated, e.g.

USD 2000, EUR -1000, GBP 3000
USD 1000, EUR -1000, GBP 3000
USD 1000, EUR 0, GBP 3000

e.g. again using Pipes, you might end up with:

-- Prints to std out every 1s
aggregateAndLog :: [Producer Order IO ()] -> IO ()

3. Write an application that takes a list of csv files as command line arguments, and replays those as markets using (1) and aggregates the output through (2)

Submission

Your solution should take the form of a Haskell program with a stack build file, such that it can be built using stack build. To initialise the stack project, you can use the command:

stack new nhire

Your application should be called nhire-exe. It should produce the correct output when run with:

nhire-exe market_a.csv market_b.csv market_c.csv