为ActivityType执行21天滚动总和的最快方法

setDT(dt)
dt[, ActivityDate := as.Date(ActivityDate, '%m/%d/%Y')]

# add index to keep track of rows
dt[, idx := .I]

# match the dates we're looking for using a rolling join and extract the row numbers
rr = dt[.(Name = Name, ActivityDate = ActivityDate - 21, refIdx = idx),
       .(idx, refIdx), on = c('Name', 'ActivityDate'), roll = -Inf]
#    idx refIdx
# 1:   1      1
# 2:   1      2
# 3:   1      3
# 4:   1      4
# 5:   5      5
# 6:   5      6
# 7:   6      7
# 8:   8      8
# 9:   8      9
#10:   8     10
#11:  11     11
#12:  11     12
#13:  12     13

# extract the above rows and count occurrences using dcast
dcast(rr[, {seq = idx:refIdx; dt[seq]}, by = 1:nrow(rr)], nrow ~ ActivityType)
#   nrow Email Webinar
#1     1     1       0
#2     2     2       0
#3     3     2       1
#4     4     2       2
#5     5     0       1
#6     6     1       1
#7     7     2       0
#8     8     1       0
#9     9     1       1
#10   10     1       2
#11   11     0       1
#12   12     1       1
#13   13     2       0