[R] grouping and counting in dataframe

[zem]

sry, 
new try: 

tm<-c(12345,42352,12435,67546,24234,76543,31243,13334,64562,64123) 
gr<-c(1,3,1,2,2,4,2,3,3,3) 
d<-data.frame(cbind(time,gr))

where tm are unix times and gr the factor grouping by
i have a skalar for example k=500
now i need to calculate in for every row how much examples in the group are
in the interval [i-500;i+500] and i is the active tm-element, like this: 

>d
    time gr ct
1  12345  1  2
2  42352  3  0
3  12435  1  2
4  67546  2  0
5  24234  2  0
6  76543  4  0
7  31243  2  0
8  13334  3  0
9  64562  3  2
10 64123  3  2

i hope that was a better illustration of my problem

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