added ghead/gtail support for n>1

[ben-schwen]

Closes #5060

Adds Gforce support for ghead/gtail and n > 1.

Note: This leads to the change of test 3 test cases which explicitly checked the non support of ghead/gtail for n != 0.

[jangorecki]

Nice, this change should nicely speed up db-benchmark group q8. https://h2oai.github.io/db-benchmark/

[mattdowle]

Looks great!
I'm just looking at the lines of code, and the common lines of code in this file in general. I'm wondering why don't these functions gather the row numbers first, and then call subsetVectorRaw to get the result. That would collapse the lines of code in the file dramatically (all the type cases would go away) and if .SD has a lot of columns, then the processing of grpsize, ff etc to gather the row numbers need only happen once rather than for each column.

[ben-schwen]

@mattdowle that sounds pretty reasonable as long as we do not introduce something like na.rm or na.omit for ghead/gtail, since then we would still need the cases?

A proof of concept (if I understood it right) can be found in

data.table/src/gsumm.c

Lines 1107 to 1123 in 27a829c

	SEXP idx = allocVector(INTSXP, anslen);
	int *idxp = INTEGER(idx);
	int offset = 0;
	for (int i=0; i<ngrp; ++i) {
	const int thisgrpsize = grpsize[i];
	for (int j=0; j<val; ++j) {
	if (j > thisgrpsize-1) break;
	int k = ff[i]+j-1;
	if (isunsorted) k = oo[k]-1;
	k = (irowslen == -1) ? k : irows[k]-1;
	idxp[offset+j] = k+1;
	}
	if (val > thisgrpsize) offset += thisgrpsize; else offset += val;
	}
	SEXP ans = PROTECT(allocVector(TYPEOF(x), anslen));
	copyMostAttrib(x, ans);
	subsetVectorRaw(ans, x, idx, false);

cutting down the lines of code from >100 to <40. Same approach could be used for gfirst/gtail and gnthvalue.

[codecov]

Codecov Report

Merging #5089 (fde19a9) into master (b33dee6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5089      +/-   ##
==========================================
- Coverage   99.53%   99.53%   -0.01%     
==========================================
  Files          76       76              
  Lines       14468    14448      -20     
==========================================
- Hits        14401    14381      -20     
  Misses         67       67              

Impacted Files	Coverage Δ
R/data.table.R	99.94% <100.00%> (+<0.01%)	⬆️
src/gsumm.c	100.00% <100.00%> (ø)

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[jangorecki]

Looks great, do we have tests for n=0, n=NA and negative n?

[mattdowle]

Hm. Good point. Looks like we don't, and they don't work.