Store node integers by halves on big-endian targets

The non-little-endian path of the node writer wrote the integer's native
word over len and next, so len got the high half there. Compose and split
the value explicitly (len is the low half, next the high half); the
little-endian path stays a plain memcpy.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_view_builder.cpp

@@ -401,3 +401,61 @@ TEST_CASE("json_view builder")
         }
     }
 }
+
+TEST_CASE("json_view node integer bits")
+{
+    using nlohmann::detail::view::integer_bits;
+    using nlohmann::detail::view::set_integer_bits;
+
+    // an integer lives in len (low half) and next (high half), on any byte
+    // order; a big-endian target must not store the native word over both
+    SECTION("set_integer_bits and integer_bits")
+    {
+        node n = {};
+        for (const std::uint64_t v :
+                {
+                    std::uint64_t{0}, std::uint64_t{1}, std::uint64_t{0xFFFFFFFFu}, std::uint64_t{0x100000000u},
+                    std::uint64_t{0x0000000200000003u}, std::uint64_t{0x0123456789ABCDEFu}, std::uint64_t{0xFFFFFFFFFFFFFFFEu}
+                })
+        {
+            CAPTURE(v)
+            n.kind = 0x5A;
+            n.flags = 0xA5;
+            n.extra = 0x1234;
+            n.off = 0x89ABCDEFu;
+            set_integer_bits(n, v);
+            CHECK(integer_bits(n) == v);
+            CHECK(n.len == static_cast<std::uint32_t>(v));
+            CHECK(n.next == static_cast<std::uint32_t>(v >> 32));
+            // the other fields are untouched
+            CHECK(n.kind == 0x5A);
+            CHECK(n.flags == 0xA5);
+            CHECK(n.extra == 0x1234);
+            CHECK(n.off == 0x89ABCDEFu);
+        }
+    }
+
+    SECTION("parsed integers")
+    {
+        struct integer_case
+        {
+            const char* text;
+            std::uint64_t bits;
+        };
+        for (const integer_case c :
+                {
+                    integer_case{"[8589934595]", 0x0000000200000003u}, integer_case{"[4294967296]", 0x100000000u}, integer_case{"[4294967295]", 0xFFFFFFFFu},
+                    integer_case{"[-2]", 0xFFFFFFFFFFFFFFFEu}, integer_case{"[-4294967297]", 0xFFFFFFFEFFFFFFFFu}, integer_case{"[18446744073709551615]", 0xFFFFFFFFFFFFFFFFu},
+                    integer_case{"[7]", 7u}
+                })
+        {
+            CAPTURE(c.text)
+            const built b = build(c.text, false, false, true);
+            REQUIRE(b.ok);
+            const node& n = b.data->tape[1];
+            CHECK(integer_bits(n) == c.bits);
+            CHECK(n.len == static_cast<std::uint32_t>(c.bits));
+            CHECK(n.next == static_cast<std::uint32_t>(c.bits >> 32));
+        }
+    }
+}