From fb301f424d2b15818557e5634879b874d7e0558e Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Thu, 20 Feb 2020 11:33:08 -0800
Subject: [PATCH] add `bitreverse` function (#34791)

---
 NEWS.md          |  1 +
 base/bitarray.jl | 16 +++-------------
 base/exports.jl  |  1 +
 base/int.jl      | 34 ++++++++++++++++++++++++++++++++++
 test/int.jl      | 11 +++++++++++
 5 files changed, 50 insertions(+), 13 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index d097474a91629..097320a65bfc9 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -53,6 +53,7 @@ New library functions
 * Add function `ismutable` and deprecate `isimmutable` to check whether something is mutable.([#34652])
 * `include` now accepts an optional `mapexpr` first argument to transform the parsed
   expressions before they are evaluated ([#34595]).
+* New function `bitreverse` for reversing the order of bits in a fixed-width integer ([#34791]).
 
 New library features
 --------------------
diff --git a/base/bitarray.jl b/base/bitarray.jl
index 7c98256bc2017..2bbb138c1d7e5 100644
--- a/base/bitarray.jl
+++ b/base/bitarray.jl
@@ -1187,16 +1187,6 @@ function reverse(A::BitArray; dims::Integer)
     return B
 end
 
-function reverse_bits(src::UInt64)
-    z    = src
-    z    = ((z >>>  1) & 0x5555555555555555) | ((z <<  1) & 0xaaaaaaaaaaaaaaaa)
-    z    = ((z >>>  2) & 0x3333333333333333) | ((z <<  2) & 0xcccccccccccccccc)
-    z    = ((z >>>  4) & 0x0f0f0f0f0f0f0f0f) | ((z <<  4) & 0xf0f0f0f0f0f0f0f0)
-    z    = ((z >>>  8) & 0x00ff00ff00ff00ff) | ((z <<  8) & 0xff00ff00ff00ff00)
-    z    = ((z >>> 16) & 0x0000ffff0000ffff) | ((z << 16) & 0xffff0000ffff0000)
-    return ((z >>> 32) & 0x00000000ffffffff) | ((z << 32) & 0xffffffff00000000)
-end
-
 function reverse!(B::BitVector)
     # Basic idea: each chunk is divided into two blocks of size k = n % 64, and
     # h = 64 - k. Walk from either end (with indices i and j) reversing chunks
@@ -1220,14 +1210,14 @@ function reverse!(B::BitVector)
 
     i, j = 0, length(B.chunks)
     u = UInt64(0)
-    v = reverse_bits(B.chunks[j])
+    v = bitreverse(B.chunks[j])
     B.chunks[j] = 0
     @inbounds while true
         i += 1
         if i == j
             break
         end
-        u = reverse_bits(B.chunks[i])
+        u = bitreverse(B.chunks[i])
         B.chunks[i] = 0
         B.chunks[j] |= u >>> h
         B.chunks[i] |= v >>> h
@@ -1236,7 +1226,7 @@ function reverse!(B::BitVector)
         if i == j
             break
         end
-        v = reverse_bits(B.chunks[j])
+        v = bitreverse(B.chunks[j])
         B.chunks[j] = 0
         B.chunks[i] |= v << k
         B.chunks[j] |= u << k
diff --git a/base/exports.jl b/base/exports.jl
index 6362bc2316598..e5346c56701a9 100644
--- a/base/exports.jl
+++ b/base/exports.jl
@@ -220,6 +220,7 @@ export
     atanh,
     big,
     binomial,
+    bitreverse,
     bswap,
     cbrt,
     ceil,
diff --git a/base/int.jl b/base/int.jl
index 668879656ab09..ea6fbe58563a9 100644
--- a/base/int.jl
+++ b/base/int.jl
@@ -860,3 +860,37 @@ for op in (:+, :-, :*, :&, :|, :xor)
         return $op(aT, bT)
     end
 end
+
+const _mask1_uint128 = (UInt128(0x5555555555555555) << 64) | UInt128(0x5555555555555555)
+const _mask2_uint128 = (UInt128(0x3333333333333333) << 64) | UInt128(0x3333333333333333)
+const _mask4_uint128 = (UInt128(0x0f0f0f0f0f0f0f0f) << 64) | UInt128(0x0f0f0f0f0f0f0f0f)
+
+"""
+    bitreverse(x)
+
+Reverse the order of bits in integer `x`. `x` must have a fixed bit width,
+e.g. be an `Int16` or `Int32`.
+
+!!! compat "Julia 1.5"
+    This function requires Julia 1.5 or later.
+
+# Examples
+```jldoctest
+julia> bitreverse(0x8080808080808080)
+0x0101010101010101
+
+julia> reverse(bitstring(0xa06e)) == bitstring(bitreverse(0xa06e))
+true
+```
+"""
+function bitreverse(x::BitInteger)
+    # TODO: consider using llvm.bitreverse intrinsic
+    z = unsigned(x)
+    mask1 = _mask1_uint128 % typeof(z)
+    mask2 = _mask2_uint128 % typeof(z)
+    mask4 = _mask4_uint128 % typeof(z)
+    z = ((z & mask1) << 1) | ((z >> 1) & mask1)
+    z = ((z & mask2) << 2) | ((z >> 2) & mask2)
+    z = ((z & mask4) << 4) | ((z >> 4) & mask4)
+    return bswap(z) % typeof(x)
+end
diff --git a/test/int.jl b/test/int.jl
index 9f8dbefd9984a..383849b221d2d 100644
--- a/test/int.jl
+++ b/test/int.jl
@@ -352,3 +352,14 @@ end
         end
     end
 end
+
+@testset "bitreverse" begin
+    for T in Base.BitInteger_types
+        x = rand(T)::T
+        @test bitreverse(x) isa T
+        @test reverse(bitstring(x)) == bitstring(bitreverse(x))
+    end
+    @test bitreverse(0x80) === 0x01
+    @test bitreverse(Int64(456618293)) === Int64(-6012608040035942400)
+    @test bitreverse(Int32(456618293)) === Int32(-1399919400)
+end