R Under development (unstable) (2024-01-07 r85787 ucrt) -- "Unsuffered Consequences"
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> library(FeatureHashing)
> df <- data.frame(a = c("a,b,c", "a,c", "b,c", "a"))
> m <- hashed.model.matrix(~ split(a, delim = ",", type = "existence") - 1, df, 
+                          hash.size = 4, signed.hash = FALSE, create.mapping = TRUE)
> hash.mapping(m)
aa ab ac 
 4  4  1 
> stopifnot(m@x == 1)
> 
> proc.time()
   user  system elapsed 
   1.48    0.21    1.67