ARROW-14169: [R] altrep for factors

[romainfrancois]

draft pull request, on top of #11369

encouraging:

library(arrow, warn.conflicts = FALSE)
#> See arrow_info() for available features

f <- Array$create(iris$Species)$as_vector()
.Internal(inspect(f))
#> @7fe3c95c3860 13 INTSXP g0c0 [OBJ,REF(65535),ATT] arrow::ChunkedArray<0x7fe3c75fffc8, dictionary<values=string, indices=int8, ordered=0>, 1 chunks, 0 nulls> len=150
#> ATTRIB:
#>   @7fe3c95c3518 02 LISTSXP g0c0 [REF(1)] 
#>     TAG: @7fe3be80d5e0 01 SYMSXP g1c0 [MARK,REF(65535),LCK,gp=0x4000] "levels" (has value)
#>     @7fe3c95c3550 16 STRSXP g0c0 [REF(65535)] arrow::ChunkedArray<0x7fe3c75e92c8, string, 1 chunks, 0 nulls> len=3
#>     TAG: @7fe3be80d9d0 01 SYMSXP g1c0 [MARK,REF(46357),LCK,gp=0x6000] "class" (has value)
#>     @7fe3ca3eefc0 16 STRSXP g0c1 [REF(1)] (len=1, tl=0)
#>       @7fe3be88d3e8 09 CHARSXP g1c1 [MARK,REF(383),gp=0x61] [ASCII] [cached] "factor"
f
#>   [1] setosa     setosa     setosa     setosa     setosa     setosa    
#>   [7] setosa     setosa     setosa     setosa     setosa     setosa    
#>  [13] setosa     setosa     setosa     setosa     setosa     setosa    
#>  [19] setosa     setosa     setosa     setosa     setosa     setosa    
#>  [25] setosa     setosa     setosa     setosa     setosa     setosa    
#>  [31] setosa     setosa     setosa     setosa     setosa     setosa    
#>  [37] setosa     setosa     setosa     setosa     setosa     setosa    
#>  [43] setosa     setosa     setosa     setosa     setosa     setosa    
#>  [49] setosa     setosa     versicolor versicolor versicolor versicolor
#>  [55] versicolor versicolor versicolor versicolor versicolor versicolor
#>  [61] versicolor versicolor versicolor versicolor versicolor versicolor
#>  [67] versicolor versicolor versicolor versicolor versicolor versicolor
#>  [73] versicolor versicolor versicolor versicolor versicolor versicolor
#>  [79] versicolor versicolor versicolor versicolor versicolor versicolor
#>  [85] versicolor versicolor versicolor versicolor versicolor versicolor
#>  [91] versicolor versicolor versicolor versicolor versicolor versicolor
#>  [97] versicolor versicolor versicolor versicolor virginica  virginica 
#> [103] virginica  virginica  virginica  virginica  virginica  virginica 
#> [109] virginica  virginica  virginica  virginica  virginica  virginica 
#> [115] virginica  virginica  virginica  virginica  virginica  virginica 
#> [121] virginica  virginica  virginica  virginica  virginica  virginica 
#> [127] virginica  virginica  virginica  virginica  virginica  virginica 
#> [133] virginica  virginica  virginica  virginica  virginica  virginica 
#> [139] virginica  virginica  virginica  virginica  virginica  virginica 
#> [145] virginica  virginica  virginica  virginica  virginica  virginica 
#> Levels: setosa versicolor virginica
.Internal(inspect(f))
#> @7fe3c95c3860 13 INTSXP g0c0 [OBJ,REF(65535),ATT] arrow::ChunkedArray<0x7fe3c75fffc8, dictionary<values=string, indices=int8, ordered=0>, 1 chunks, 0 nulls> len=150
#> ATTRIB:
#>   @7fe3c95c3518 02 LISTSXP g0c0 [REF(1)] 
#>     TAG: @7fe3be80d5e0 01 SYMSXP g1c0 [MARK,REF(65535),LCK,gp=0x4000] "levels" (has value)
#>     @7fe3c95c3550 16 STRSXP g0c0 [REF(65535)] arrow::ChunkedArray<0x7fe3c75e92c8, string, 1 chunks, 0 nulls> len=3
#>     TAG: @7fe3be80d9d0 01 SYMSXP g1c0 [MARK,REF(46450),LCK,gp=0x6000] "class" (has value)
#>     @7fe3ca3eefc0 16 STRSXP g0c1 [REF(65535)] (len=1, tl=0)
#>       @7fe3be88d3e8 09 CHARSXP g1c1 [MARK,REF(385),gp=0x61] [ASCII] [cached] "factor"
identical(f, iris$Species)
#> [1] TRUE

^{Created on 2021-10-13 by the reprex package (v2.0.1.9000)}

[github-actions]

https://issues.apache.org/jira/browse/ARROW-14169

[romainfrancois]

This currently only deals with a single chunk so that we don't need to be concerned about unification as in Converter_Dictionary.

[pitrou]

This probably needs rebasing now?

[romainfrancois]

I cherry picked the relevant commit in #11738