add issorted to groupby

Author: sl-solution

src/sort/groupby.jl

@@ -23,14 +23,19 @@ _columns(ds::GroupBy) = _columns(ds.parent)
 index(ds::GroupBy) = index(ds.parent)
 Base.parent(ds::GroupBy) = ds.parent
 
-function groupby(ds::Dataset, cols::MultiColumnIndex; alg = HeapSortAlg(), rev = false, mapformats::Bool = true, stable = true)
+function groupby(ds::Dataset, cols::MultiColumnIndex; alg = HeapSortAlg(), rev = false, mapformats::Bool = true, stable = true, issorted = false)
     # @assert !isgrouped(ds) "`groupby` is not yet implemented for already grouped data sets"
     colsidx = index(ds)[cols]
-    a = _sortperm(ds, cols, rev, a = alg, mapformats = mapformats, stable = stable)
-    GroupBy(ds,colsidx, a[2], a[1], a[3])
+    if issorted
+        selected_columns, ranges, last_valid_index = _find_starts_of_groups(ds::Dataset, colsidx, nrow(ds) < typemax(Int32) ? Val(Int32) : Val(Int64))
+        return GroupBy(ds, colsidx, 1:nrow(ds), ranges, last_valid_index)
+    else
+        a = _sortperm(ds, cols, rev, a = alg, mapformats = mapformats, stable = stable)
+        GroupBy(ds,colsidx, a[2], a[1], a[3])
+    end
 end
 
-groupby(ds::Dataset, col::ColumnIndex; alg = HeapSortAlg(), rev = false, mapformats::Bool = true, stable = true) = groupby(ds, [col], alg = alg, rev = rev, mapformats = mapformats, stable = stable)
+groupby(ds::Dataset, col::ColumnIndex; alg = HeapSortAlg(), rev = false, mapformats::Bool = true, stable = true, issorted = false) = groupby(ds, [col], alg = alg, rev = rev, mapformats = mapformats, stable = stable, issorted = issorted)
 
 function _threaded_permute_for_groupby(x, perm)
     if DataAPI.refpool(x) !== nothing