Reimplement random_bytes to be consistent with endianness

spec/std/random_spec.cr

@@ -190,6 +190,18 @@ describe "Random" do
   end
 
   describe "random_bytes" do
+    it "generates random bytes" do
+      rng = TestRNG.new([0xfa19443eu32, 1u32, 0x12345678u32])
+      rng.random_bytes(9).should eq Bytes[0x3e, 0x44, 0x19, 0xfa, 1, 0, 0, 0, 0x78]
+      rng.random_bytes(1).should eq Bytes[0x3e]
+      rng.random_bytes(4).should eq Bytes[1, 0, 0, 0]
+      rng.random_bytes(3).should eq Bytes[0x78, 0x56, 0x34]
+      rng.random_bytes(0).should eq Bytes.new(0)
+
+      rng = TestRNG.new([12u8, 255u8, 11u8, 5u8, 122u8, 200u8, 192u8])
+      rng.random_bytes(7).should eq Bytes[12, 255, 11, 5, 122, 200, 192]
+    end
+
     it "gets random bytes with default number of digits" do
       bytes = TestRNG.new(RNG_DATA_32).random_bytes
       bytes.size.should eq(16)

src/random.cr

@@ -302,19 +302,17 @@ module Random
   # slice # => [217, 118, 38, 196]
   # ```
   def random_bytes(buf : Bytes) : Nil
-    n = buf.size / sizeof(typeof(next_u))
-    remaining = buf.size - n * sizeof(typeof(next_u))
-
-    slice = buf.to_unsafe.as(typeof(next_u)*).to_slice(n)
-    slice.each_index { |i| slice[i] = next_u }
-
-    if remaining > 0
-      bytes = next_u
-      remaining.times do |i|
-        bits = i * 8
-        mask = typeof(next_u).new(0xff << bits)
-        buf[-i - 1] = UInt8.new((bytes & mask) >> bits)
+    ptr = buf.to_unsafe
+    finish = buf.to_unsafe + buf.size
+
+    while ptr < finish
+      random = next_u
+      rand_ptr = pointerof(random).as(UInt8*)
+      if IO::ByteFormat::SystemEndian != IO::ByteFormat::LittleEndian
+        rand_ptr.to_slice(sizeof(typeof(next_u))).reverse!
       end
+      rand_ptr.copy_to(ptr, {finish - ptr, sizeof(typeof(next_u))}.min)
+      ptr += sizeof(typeof(next_u))
     end
   end