amazon-bill-guesser
Answering the age-old question "did I actually buy that?" w.r.t. Amazon credit card charges.
Problem
Amazon's charges on my credit card do not always match my order totals, nor do they match my individual line items. Instead, the credit card charges may show up based on some partition of the order items, where each charge is the sum of one of the partition members. So for an order with three items costing 1,2, and 3, there are five possible ways charges might show up:
(->> (combo/partitions [1 2 3])
(map (fn [part]
(map #(apply + %) part))))
=> ((6) (3 3) (4 2) (1 5) (1 2 3))
If you happen to have many order splits, this becomes tedious...
Approach
... and by "tedious" I mean "intractable". Finding order line items that match a credit card charge is actually the subset sum problem, which is actually actually the knapsack problem.
The complexity of the algorithms I explored depends on N, the number of decision variables, and/or on P, the precision needed to cover the actual numbers. For the Amazon bill, N is the max number of items in a single order, and P is the number of bits needed to represent the total of the most expensive order.
In my own usage N is pretty small, so this library performs an exhaustive exponential time search. If you have larger N, you will need to try something else.
Usage
First install Clojure and clone this repo.
Download a report of type "Items" from https://amazon.com, navigating via:
Account & Lists -> Your Account -> Download order reports
Start a REPL from the repo directory:
clojure
Then start the guesser and follow the prompts:
(require '[com.stuarthalloway.amazon-bill-guesser :as abg])
(abg/guesser "your-csv-file-name")
Caveats
I did not investigate what happens when an order is partially fulfilled within the time range of a report, and partially outside it. I always pick a time range broad enough to include everything I care about.
Better Approach
It is possible that Amazon provides a report that contains enough information to answer this question directly, but I couldn't find it, and other people report similar problems.