Include cumulative balance fields in cashflow records

[yellowbean]

Background

Current cashflow projection will yield a record as snapshot on a date like

Date Balance Principal Default Loss Interest

But it pose drawbacks:

• To get cumulative balance of Default ,engine has to loop over all records each time. if it is not cached ( it is not cached ) ,that's duplication of summing
• The record only will lose information the balance on each single period sum up doesn't tie out with previous one.

Balance	Principal	Interest	Default
100	20	5	10
75	20	3	4	<--

as point of time of the Arrow, there's no way to infer the total balance of original pool is 100+20+10 = 130

But given the cumulative fields included, the integrity of pool performance record is preserved.

Balance	Principal	Interest	Default	CumuDefault	CumuPrincipal
100	20	5	10	10	20
75	20	3	5	15	40	<--

This will yield extra benefits like , delinquency pool which has extra balance field .

Propose solution

• include Cumulative Balance

[yellowbean]

It will break the syntax on engine level but won't be in Python wrapper level

[yellowbean]

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