Flagging Hospital Readmissions with PowerPivot

I'm trying to develop a means of calculating hospital readmissions. I'd like to keep as much of the calculation as possible in PowerPivot. I have a functional solution at the moment, but it uses a couple array functions and helper columns that become quite slow when large amounts of data are involved.

Objective: list all patients who are re-admitted to the hospital within 30 days of discharge.
Criteria: initial admission must be for heart failure (HF), subsequent admission can be for any reason.

Data:
PatientData (attachment) lists a unique patient identifier (MRN), a unique visit identifier (FIN), dates, and diagnosis codes.
CodeData (attchment) list all the diagnosis codes that indicate heart failure.

I've been working on this one for a couple days now. My current solution involves adding two helper columns to the PatientData table. One that flags heart failure admissions, and another that flags possible readmissions by the same patient. The array functions involved are really slowing things down, however. Any help would be much appreciated.

Thanks.