Random walk generate() method with bootstrapping uses unsafe recycling

[mitchelloharawild]

This was identified in the AFRICAST 2025 workshop by @bahmanrostamitabar

[mitchelloharawild]

MRE:

library(fable)
#> Loading required package: fabletools
#> Registered S3 method overwritten by 'tsibble':
#>   method               from 
#>   as_tibble.grouped_df dplyr
library(tsibble)
#> 
#> Attaching package: 'tsibble'
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, union
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
aus_beer <- tsibbledata::aus_production |> select(Beer) |> filter_index("2000 Q1" ~ "2009 Q1")
fit <- aus_beer |> filter_index(. ~ "2009 Q1") |> model(snaive = SNAIVE(Beer))
fit |> generate(h = 5, times = 5)
#> Warning: There were 5 warnings in `as_tibble()`.
#> The first warning was:
#> ℹ In argument: `.sim = sim_rw(.innov)`.
#> ℹ In group 1: `.rep = "1"`.
#> Caused by warning in `rep_len(future, length(dx)) + cumulative_e`:
#> ! longer object length is not a multiple of shorter object length
#> ℹ Run `dplyr::last_dplyr_warnings()` to see the 4 remaining warnings.
#> # A tsibble: 25 x 4 [1Q]
#> # Key:       .model, .rep [5]
#>    .model Quarter .rep   .sim
#>    <chr>    <qtr> <chr> <dbl>
#>  1 snaive 2009 Q2 1      390.
#>  2 snaive 2009 Q3 1      427.
#>  3 snaive 2009 Q4 1      515.
#>  4 snaive 2010 Q1 1      402.
#>  5 snaive 2010 Q2 1      418.
#>  6 snaive 2009 Q2 2      386.
#>  7 snaive 2009 Q3 2      433.
#>  8 snaive 2009 Q4 2      493.
#>  9 snaive 2010 Q1 2      433.
#> 10 snaive 2010 Q2 2      406.
#> # ℹ 15 more rows

^{Created on 2026-01-16 with reprex v2.1.1}