Implement resize!(...; first=true)

base/array.jl

@@ -1443,12 +1443,16 @@ function _prepend!(a::Vector, ::IteratorSize, iter)
 end
 
 """
-    resize!(a::Vector, n::Integer) -> Vector
+    resize!(a::Vector, n::Integer; first::Bool=false) -> Vector
 
 Resize `a` to contain `n` elements. If `n` is smaller than the current collection
 length, the first `n` elements will be retained. If `n` is larger, the new elements are not
 guaranteed to be initialized.
 
+If `first` is true, then the new elements are inserted at the start of the collection. In
+this case, if `n` is smaller than the current collection length, the last `n` elements will
+be retained.
+
 # Examples
 ```jldoctest
 julia> resize!([6, 5, 4, 3, 2, 1], 3)
@@ -1472,15 +1476,15 @@ julia> a[1:6]
  1
 ```
 """
-function resize!(a::Vector, nl::Integer)
+function resize!(a::Vector, nl::Integer; first::Bool=false)
     l = length(a)
     if nl > l
-        _growend!(a, nl-l)
+        first ? _growbeg!(a, nl-l) : _growend!(a, nl-l)
     elseif nl != l
         if nl < 0
             _throw_argerror("new length must be ≥ 0")
         end
-        _deleteend!(a, l-nl)
+        first ? _deletebeg!(a, l-nl) : _deleteend!(a, l-nl)
     end
     return a
 end

base/bitarray.jl

@@ -811,13 +811,14 @@ function sizehint!(B::BitVector, sz::Integer)
     return B
 end
 
-resize!(B::BitVector, n::Integer) = _resize_int!(B, Int(n))
-function _resize_int!(B::BitVector, n::Int)
+resize!(B::BitVector, n::Integer; first::Bool=false) = _resize_int!(B, Int(n), first)
+function _resize_int!(B::BitVector, n::Int, first::Bool)
     n0 = length(B)
     n == n0 && return B
     n >= 0 || throw(BoundsError(B, n))
     if n < n0
-        deleteat!(B, n+1:n0)
+        r = first ? (1:n0-n) : (n+1:n0)
+        deleteat!(B, r)
         return B
     end
     Bc = B.chunks
@@ -828,6 +829,7 @@ function _resize_int!(B::BitVector, n::Int)
         Bc[end] = UInt64(0)
     end
     B.len = n
+    first && copy_chunks!(Bc, 1 + n - n0, Bc, 1, n0)
     return B
 end
 

test/bitarray.jl

@@ -204,6 +204,12 @@ timesofar("promotions")
         @check_bit_operation resize!(b1, v1 ÷ 2) BitVector
         gr(b) = (resize!(b, v1)[(v1÷2):end] .= 1; b)
         @check_bit_operation gr(b1) BitVector
+
+        b1 = bitrand(v1)
+        @test_throws BoundsError resize!(b1, -1; first=true)
+        b2 = copy(b1)
+        @test resize!(b1, v1+100; first=true)[end-v1+1:end] == b2
+        @test resize!(b1, v1+200)[end-100-v1+1:end-100] == b2
     end
 
     @testset "sizeof (issue #7515)" begin

test/core.jl

@@ -6465,6 +6465,15 @@ for U in unboxedunions
             @test A[1] === initvalue2(F2)
             @test typeof(A[end]) === F2
 
+            A = U[initvalue2(F2) for i = 1:len]
+            resize!(A, 1; first=true)
+            @test length(A) === 1
+            @test A[1] === initvalue2(F2)
+            resize!(A, len; first=true)
+            @test length(A) === len
+            @test A[end] === initvalue2(F2)
+            @test typeof(A[end]) === F2
+
             # deleteat!
             F = Base.uniontypes(U)[2]
             A = U[rand(F(1):F(len)) for i = 1:len]
@@ -6653,7 +6662,7 @@ setindex!(A, missing, 2)
 setindex!(A, 0x03, 3)
 setindex!(A, missing, 4)
 setindex!(A, 0x05, 5)
-Base._growat!(A, 1, 1)
+resize!(A, 6; first=true)
 
 @test getindex(A, 1) === missing
 @test getindex(A, 2) === 0x01
@@ -6663,7 +6672,7 @@ Base._growat!(A, 1, 1)
 @test getindex(A, 6) === 0x05
 
 # grow_at_beg 2
-Base._growat!(A, 1, 1)
+resize!(A, 7; first=true)
 @test getindex(A, 1) === missing
 @test getindex(A, 2) === missing
 @test getindex(A, 3) === 0x01
@@ -6684,7 +6693,7 @@ Base._growat!(A, 2, 1)
 @test getindex(A, 8) === 0x05
 
 # grow_at_beg 9
-Base._growat!(A, 1, 1)
+resize!(A, 9; first=true)
 @test getindex(A, 1) === missing
 @test getindex(A, 2) === missing
 @test getindex(A, 3) === missing
@@ -6744,6 +6753,11 @@ Base._growat!(A, 2, 3)
 @test getindex(A, 9) === missing
 @test getindex(A, 10) === 0x05
 
+resize!(A, 2; first=true)
+@test getindex(A, 1) === missing
+@test getindex(A, 2) === 0x05
+@test length(A) === 2
+
 # grow_at_beg 3
 A = Vector{Union{Missing, UInt8}}(undef, 1048577)
 setindex!(A, 0x01, 1)