speed up filter of vector

base/array.jl

@@ -2364,7 +2364,28 @@ function filter!(f, a::AbstractVector)
     return a
 end
 
-filter(f, a::Vector) = mapfilter(f, push!, a, empty(a))
+function filter!(f, a::Vector)
+    j = 1
+    for ai in a
+        @inbounds a[j] = ai
+        j = ifelse(f(ai), j+1, j)
+    end
+    deleteat!(a, j:length(a))
+    sizehint!(a, length(a))
+    a
+end
+
+function filter(f, a::AbstractVector{T}) where T
+    j = 1
+    b = Vector{T}(undef, length(a))
+    for ai in a
+        @inbounds b[j] = ai
+        j = ifelse(f(ai), j+1, j)
+    end
+    deleteat!(b, j:length(b))
+    sizehint!(b, length(b))
+    b
+end
 
 # set-like operators for vectors
 # These are moderately efficient, preserve order, and remove dupes.