Big speed up: Early return for `vec_rep()` and `vec_rep_each()`

[mgirlich]

The edge case times = 1 could be made much more efficient by returning early. Maybe also times = 0 could benefit a little from an early return. This creates nice speed ups e.g. in tidyr::expand_grid().

Benchmarks

library(vctrs)

x <- seq(10e6)
x[[1]] <- 1
vec_rep2 <- function(x, times) {
  if (times == 1L) {
    return(x)
  } else if (times == 0L) {
    vec_ptype(x)
  }
  
  vec_rep(x, times)
}

bench::mark(
  vec_rep = vec_rep(x, 1),
  vec_rep2 = vec_rep2(x, 1),
)
#> # A tibble: 2 x 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 vec_rep      17.5ms     18ms      44.0     114MB     106.
#> 2 vec_rep2      382ns    451ns 1189079.         0B       0

bench::mark(
  vec_rep = vec_rep(x, 0),
  vec_rep2 = vec_rep2(x, 0),
)
#> # A tibble: 2 x 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 vec_rep       819ns    907ns   795080.        0B      0  
#> 2 vec_rep2     1.93µs   2.12µs   396239.        0B     39.6

vec_rep_each2 <- function(x, times) {
  if (times == 1L) {
    return(x)
  } else if (times == 0L) {
    vec_ptype(x)
  }
  
  vec_rep_each(x, times)
}

bench::mark(
  vec_rep_each = vec_rep_each(x, 1),
  vec_rep_each2 = vec_rep_each2(x, 1),
)
#> # A tibble: 2 x 6
#>   expression         min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>    <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 vec_rep_each    25.1ms   25.5ms      39.2     114MB     108.
#> 2 vec_rep_each2    413ns    445ns 1361489.         0B       0

bench::mark(
  vec_rep_each = vec_rep_each(x, 0),
  vec_rep_each2 = vec_rep_each2(x, 0),
)
#> # A tibble: 2 x 6
#>   expression         min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>    <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 vec_rep_each    6.21ms    6.6ms      152.        0B        0
#> 2 vec_rep_each2   6.17ms   6.71ms      149.        0B        0

^{Created on 2021-06-16 by the reprex package (v2.0.0)}