Question

I have a vector which contains a list of Hash maps in Clojure and I have an add-watch on this vector to see any changes made. Is there an easy way to do a diff on the changes made to the hash map, so that maybe I could get a list of just the changed entries in the hash?

Note: This follows on from some earlier posts I have had where I have tried to persist changes to a database for a data structure stored in a ref. I have realised that the easiest way to save state is simple to watch the ref for changes and then store those changes. My ideal solution would be if the add-watch was passed a changelist as well 🙂

Answer 1

You probably need to define “diff” a little more precisely. For example, does an insertion in the middle of the vector count as a single change or as a change of that element and all subsequent ones? Also are your vectors guaranteed to be the same length?

Having said that, the simple approach would be something like:

• First check the length of the two vectors. If one is longer, then consider the extra elements as changes
• Then compare all the other elements with the corresponding element in the other vector using not= (this workes with hashes and will be very fast in the common case that the elements haven’t changed). Something to get you started: (map not= vector-1 vector-2)
• You can then use the answer from stackoverflow.com/questions/3387155/difference-between-two-maps that pmf mentioned if you want to find out exactly how the two maps differ.