r - All possible combinations of a set that sum to a target value

Question

I have an input vector such as:

weights <- seq(0, 1, by = 0.2)

I would like to generate all the combinations of weights (repeats allowed) such that the sum is equal to 1. I came up with

l <- rep(list(weights), 10)
combinations <- expand.grid(l)
combinations[which(apply(combinations, 1, sum) == 1),]

The problem is of course I generate far more combinations that I need. Is there a way to get it done more efficiently?

EDIT: Thanks for the answers. That's the first part of the problem. As @Frank pointed out, now that I have all the "solutions" that add up to 1, the problem is to get all the permutations (not sure if it is the right word) from the solutions in a vector of length 10. For instance:

s1 <- c(0, 0, 0.2, 0, 0, 0, 0.8, 0, 0, 0)
s2 <- c(0.8, 0, 0, 0, 0, 0, 0, 0, 0.2, 0)
etc...

Answer 1

Take a look at partitions library,

library(partitions)
ps <- parts(10)
res <- ps[,apply(ps, 2, function(x) all(x[x>0] %% 2 == 0))] / 10