1 files changed, 49 insertions, 6 deletions
diff --git a/asl/span.hpp b/asl/span.hpp
index efc9eec..5ffcf68 100644
--- a/asl/span.hpp
+++ b/asl/span.hpp
@@ -13,7 +13,12 @@ static constexpr int64_t dynamic_size = -1;
 template<is_object T, int64_t kSize = dynamic_size>
 class span
 {
-    static constexpr bool kIsDynamic = kSize < 0;
+    static constexpr bool is_dynamic(int64_t size)
+    {
+        return size < 0;
+    }
+    
+    static constexpr bool kIsDynamic = is_dynamic(kSize);
 
     using SizeType = select_t<kIsDynamic, int64_t, empty>;
 
@@ -21,7 +26,7 @@ class span
     ASL_NO_UNIQUE_ADDRESS SizeType m_size{};
 
 public:
-    constexpr span() = default;
+    constexpr span() requires (kIsDynamic || kSize == 0) = default;
 
     constexpr span(T* data, int64_t size)
         requires kIsDynamic
@@ -55,7 +60,7 @@ public:
         requires (
             (
                 kIsDynamic ||
-                span<U, kOtherSize>::kIsDynamic ||
+                is_dynamic(kOtherSize) ||
                 kOtherSize == kSize
             ) && convertible_from<T(&)[], U(&)[]>
         )
@@ -92,11 +97,49 @@ public:
         return m_data[i]; // NOLINT(*-pointer-arithmetic)
     }
 
-    // @Todo subspan, first, last
+    // @Todo first, last
     // @Todo as_(mutable_)bytes
 
-    template<is_object U, int64_t kOtherSize>
-    friend class span;
+    template<int64_t kOffset, int64_t kSubSize = dynamic_size>
+    constexpr auto subspan() const
+        requires (
+            kOffset >= 0 &&
+            (kIsDynamic || kOffset <= kSize) &&
+            (kIsDynamic || is_dynamic(kSubSize) || kSubSize <= kSize - kOffset)
+        )
+    {
+        ASL_ASSERT(kOffset <= size());
+
+        if constexpr (is_dynamic(kSubSize))
+        {
+            if constexpr (kIsDynamic)
+            {
+                return span<T>(data() + kOffset, size() - kOffset);
+            }
+            else
+            {
+                return span<T, kSize - kOffset>(data() + kOffset, size() - kOffset);
+            }
+        }
+        else
+        {
+            ASL_ASSERT(kSubSize <= size() - kOffset);
+            return span<T, kSubSize>(data() + kOffset, kSubSize);
+        }
+    }
+
+    constexpr span<T> subspan(int64_t offset, int64_t sub_size = dynamic_size) const
+    {
+        ASL_ASSERT(offset <= size());
+        
+        if (is_dynamic(sub_size))
+        {
+            return span<T>{ data() + offset, size() - offset };
+        }
+        
+        ASL_ASSERT(sub_size <= size() - offset);
+        return span<T>{ data() + offset, sub_size };
+    }
 };
 
 } // namespace asl