Add isdisjoint (#34427)

This was added in #13192, but the PR became stale. We've also changed
a bit how we write these kinds of algorithms, so make use of the tools
we have (e.g. `fastin`).

Author: Keno

base/abstractset.jl

@@ -260,6 +260,8 @@ true
 """
 issubset, ⊆, ⊇
 
+const FASTIN_SET_THRESHOLD = 70
+
 function issubset(l, r)
     if haslength(r) && (isa(l, AbstractSet) || !hasfastin(r))
         rlen = length(r) # conditions above make this length computed only when needed
@@ -269,7 +271,7 @@ function issubset(l, r)
         end
         # when `in` would be too slow and r is big enough, convert it to a Set
         # this threshold was empirically determined (cf. #26198)
-        if !hasfastin(r) && rlen > 70
+        if !hasfastin(r) && rlen > FASTIN_SET_THRESHOLD
             return issubset(l, Set(r))
         end
     end
@@ -375,6 +377,30 @@ function issetequal(l, r)
     return issetequal(Set(l), Set(r))
 end
 
+## set disjoint comparison
+"""
+    isdisjoint(v1, v2) -> Bool
+
+Returns whether the collections `v1` and `v2` are disjoint, i.e. whether
+their intersection is empty.
+
+!!! compat "Julia 1.5"
+    This function requires at least Julia 1.5.
+"""
+function isdisjoint(l, r)
+    function _isdisjoint(l, r)
+        hasfastin(r) && return !any(in(r), l)
+        hasfastin(l) && return !any(in(l), r)
+        haslength(r) && length(r) < FASTIN_SET_THRESHOLD &&
+            return !any(in(r), l)
+        return !any(in(Set(r)), l)
+    end
+    if haslength(l) && haslength(r) && length(r) < length(l)
+        return _isdisjoint(r, l)
+    end
+    _isdisjoint(l, r)
+end
+
 ## partial ordering of sets by containment
 
 ==(l::AbstractSet, r::AbstractSet) = length(l) == length(r) && l ⊆ r

base/exports.jl

@@ -502,6 +502,7 @@ export
     in,
     intersect!,
     intersect,
+    isdisjoint,
     isempty,
     issubset,
     issetequal,

test/sets.jl

@@ -301,13 +301,15 @@ end
     @test !(Set([1,2,3]) <= Set([1,2,4]))
 end
 
-@testset "issubset, symdiff" begin
+@testset "issubset, symdiff, isdisjoint" begin
     for S in (Set, BitSet, Vector)
         for (l,r) in ((S([1,2]),     S([3,4])),
                       (S([5,6,7,8]), S([7,8,9])),
                       (S([1,2]),     S([3,4])),
                       (S([5,6,7,8]), S([7,8,9])),
                       (S([1,2,3]),   S()),
+                      (S(),          S()),
+                      (S(),          S([1,2,3])),
                       (S([1,2,3]),   S([1])),
                       (S([1,2,3]),   S([1,2])),
                       (S([1,2,3]),   S([1,2,3])),
@@ -317,6 +319,8 @@ end
             @test issubset(intersect(l,r), r)
             @test issubset(l, union(l,r))
             @test issubset(r, union(l,r))
+            @test isdisjoint(l,l) == isempty(l)
+            @test isdisjoint(l,r) == isempty(intersect(l,r))
             if S === Vector
                 @test sort(union(intersect(l,r),symdiff(l,r))) == sort(union(l,r))
             else