Original supplier in scd type 2

[#1]

I have source table with the following structure:

from	to	prodID	date
supplier1	warehouse1	1	10.10.2014
warehouse1	warehouse2	1	12.10.2014
supplier2	warehouse1	1	13.10.2014
warehouse2	shop1	1	14.10.2014
supplier3	shop4	1	15.10.2014
warehouse1	shop3	1	16.10.2014

and I want to find the last original supplier in the separate field:

from	to	prodID	date	original supplier
supplier1	warehouse1	1	10.10.2014	supplier1
warehouse1	warehouse2	1	12.10.2014	supplier1
supplier2	warehouse1	1	13.10.2014	supplier2
warehouse2	shop1	1	14.10.2014	supplier1
supplier3	shop4	1	15.10.2014	supplier3
warehouse1	shop3	1	16.10.2014	supplier2

I struggle with this task for months. The difficulty is in the  size of source table ( about 15M records for 15 month ) and that I can't build the tree using "hierarchy" cause I have to connect with the closest last date (not the same).

Any ideas would be appreciated.

[Gysbert_Wassenaar]

I think there's not enough information to solve this reliably. How can we tell if a value in the from column is a supplier? How can we tell what is the start and the end of a supply run?

[#3]

We can define suppliers, shops, and warehouses based on mask (warehouse id starts with '0', shop - '1', suppliers - 2..9).

We assume that supply starts from 'date' field and ends when we get the next supply.