Computing the difference between two counters on successive events in Pig

For each user, we need to compute the difference beween the counters of two consecutive events:

Input:

user_id,  counter,  time_stamp

USER_1, 30000, 2015_03_01

USER_2, 20000, 2015_03_01

USER_1, 40000, 2015_03_02

USER_2, 30000, 2015_03_02

USER_1, 50000,  2015_03_03

USER_2, 40000,  2015_03_03

Output:

user_id,  Delta in counter,  time_stamp

USER_1, NULL, 2015_03_01

USER_1, 10000, 2015_03_02

USER_1, 10000,  2015_03_03

USER_2, NULL, 2015_03_01

USER_2, 10000, 2015_03_02

USER_2, 10000,  2015_03_03

Any hints on how to do this (or something close) in Pig?

Is Pig the right tool for this? or Hive?