calculating the number of "open hours" per day between two dates - r

Question

I have a data frame with start dates and end dates, along with the number of people registered for an event. I would like to calculate the number of hours each party is present for within a specific timeframe (e.g., 07:00 - 17:00)
If I use the following example data.frame...
d <- data.frame(startDate = c(as.POSIXct("2011-06-04 08:00:00"), as.POSIXct("2011-06-03 08:00:00"),
as.POSIXct("2011-09-12 10:00:00")),
endDate = c(as.POSIXct("2011-06-06 11:00:00"), as.POSIXct("2011-06-04 11:00:00"),
as.POSIXct("2011-09-12 18:00:00")),
partysize = c(124,442,323))
open <- "07:00"
close <- "17:00"
I would like my result set to look something like this:
day numhours partysize
2011-06-04 9 124
2011-06-05 10 124
2011-06-06 4 124
2011-06-03 9 442
2011-06-04 4 442
2011-09-12 7 323
note: numhours is the number of hours the date was included between the open and close times
Thanks in advance,
--JT

Answer 1

Sorry its very messy and I used 7 and 17 instead of your open and close
app.days<-mapply(function(x,y){x+y*60*60*24},as.POSIXct(format(d$startDate,"%Y-%m-%d")),lapply(floor(-(d$startDate-d$endDate)/24),seq,from=0))
start.date<-mapply(function(x,y){pmax(x+7*60*60,y)},app.days,d$startDate)
end.date<-mapply(function(x,y){pmin(x+17*60*60,y)},app.days,d$endDate)
app.hours<-mapply(function(x,y){as.numeric(x-y)},end.date,start.date)
res<-mapply(function(x,y,z){data.frame(day=as.Date(x),numhours=y,partysize=z)},app.days,app.hours,as.list(d$partysize))
res1<-data.frame(day=as.Date(unlist(res[1,]),origin="1970-01-01"),numhours=unlist(res[2,]),partysize=unlist(res[3,]))
> res1
day numhours partysize
1 2011-06-04 9 124
2 2011-06-05 10 124
3 2011-06-06 4 124
4 2011-06-03 9 442
5 2011-06-04 4 442
6 2011-09-12 7 323
Basically we identify how many days each party size stays for. For a given day we find the applicable open and close. Then we subtract open from close. The dataframe is eventually formed but it could probably have been created in the res<- step.....