Merge pull request #380 from elbeno/advance-to-boundary

✨ Add `byterator::advance_to_alignment`

Author: bdeane-intel

docs/byterator.adoc

@@ -102,3 +102,27 @@ The last case above is particularly useful for dealing with enumerations:
 enum struct E : std::uint32_t { A, B, C };
 auto v3 = i.read<std::uint8_t, E>();
 ----
+
+`byterator` can also advance to alignment boundaries:
+
+[source,cpp]
+----
+i.advance_to_alignment<std::uint32_t>();   // advance to next uint32_t alignment
+i.advance_to_alignment<std::uint32_t>(0);  // the same thing - now a no-op
+i.advance_to_alignment<std::uint32_t>(-1); // advance to previous uint32_t alignment
+
+i.advance_to_alignment<std::uint32_t>(n);  // advance to nth next uint32_t alignment
+i.advance_to_alignment<std::uint32_t>(-n); // advance to nth previous uint32_t alignment
+----
+
+If the `byterator` is already located at the requested alignment boundary (and
+the argument is zero or omitted), it doesn't change.
+
+NOTE: `advance_to_alignment` advances to the absolute alignment, _not_ the
+alignment relative to the underlying storage. For example, if the start of the
+underlying storage is _not_ 4-byte aligned, calling
+`advance_to_alignment<std::uint32_t>()` will result in an offset from the start
+that is _not_ a multiple of 4.
+
+TIP: `advance_to_alignment<T>(n)` is equivalent to `advance_to_alignment<T>()`
+followed by `advance<T>()` `n` times.

include/stdx/byterator.hpp

@@ -150,6 +150,15 @@ template <typename T> class byterator {
         return advance(sizeof(V));
     }
 
+    template <typename V = std::uint8_t>
+    auto advance_to_alignment(difference_type n = 0) -> decltype(auto) {
+        auto p = bit_cast<std::uintptr_t>(ptr);
+        constexpr auto offset = sizeof(V) - 1u;
+        constexpr auto mask = ~offset;
+        ptr = bit_cast<byte_t *>((p + offset) & mask);
+        return advance(n * static_cast<difference_type>(sizeof(V)));
+    }
+
     template <typename V = std::uint8_t, typename R = V,
               std::enable_if_t<std::is_trivially_copyable_v<V>, int> = 0>
     [[nodiscard]] auto read() -> R {

test/byterator.cpp

@@ -389,3 +389,50 @@ TEST_CASE("read enum (constrained size alias)", "[byterator]") {
     CHECK(i.readu8<E2>() == E2::A);
     CHECK((i == j));
 }
+
+TEST_CASE("advance to next alignment", "[byterator]") {
+    auto const a =
+        std::array{stdx::to_be<std::uint64_t>(0x0102'0304'0506'0708),
+                   stdx::to_be<std::uint64_t>(0x0a0b'0c0d'0e0f'1011)};
+    auto base = stdx::byterator{std::begin(a)};
+    auto i = stdx::byterator{std::begin(a)};
+    i.advance_to_alignment();
+    CHECK(i == base);
+
+    ++i;
+    i.advance_to_alignment();
+    CHECK(i == std::next(base));
+
+    i.advance_to_alignment<std::uint16_t>();
+    CHECK(i == std::next(base, 2));
+
+    i.advance_to_alignment<std::uint32_t>();
+    CHECK(i == std::next(base, 4));
+
+    i.advance_to_alignment<std::uint64_t>();
+    CHECK(i == std::next(base, 8));
+
+    i.advance_to_alignment<std::uint64_t>(); // already there
+    CHECK(i == std::next(base, 8));
+}
+
+TEST_CASE("advance to previous alignment", "[byterator]") {
+    auto const a =
+        std::array{stdx::to_be<std::uint64_t>(0x0102'0304'0506'0708),
+                   stdx::to_be<std::uint64_t>(0x0a0b'0c0d'0e0f'1011)};
+    auto base = stdx::byterator{std::begin(a)};
+    auto i = stdx::byterator{std::begin(a)};
+
+    i.advance(8);
+    i.advance_to_alignment(-1);
+    CHECK(i == std::next(base, 7));
+
+    i.advance_to_alignment<std::uint16_t>(-1);
+    CHECK(i == std::next(base, 6));
+
+    i.advance_to_alignment<std::uint32_t>(-1);
+    CHECK(i == std::next(base, 4));
+
+    i.advance_to_alignment<std::uint64_t>(-1);
+    CHECK(i == base);
+}