Increase SSO from 22 to 23 chars.

1 files changed, 36 insertions, 15 deletions

diff --git a/src/string.hh b/src/string.hh
index 004be04c..3bbb907e 100644
--- a/src/string.hh
+++ b/src/string.hh
@@ -1,6 +1,7 @@
 #ifndef string_hh_INCLUDED
 #define string_hh_INCLUDED
 
+#include <bit>
 #include "memory.hh"
 #include "hash.hh"
 #include "units.hh"
@@ -156,17 +157,17 @@ public:
 
     // String data storage using small string optimization.
     //
-    // the LSB of the last byte is used to flag if we are using the small buffer
-    // or an allocated one. On big endian systems that means the allocated
+    // the MSB of the last byte is used to flag if we are using the allocated buffer
+    // (1) or a small one (0). On big endian systems that means the allocated
     // capacity must be pair, on little endian systems that means the allocated
     // capacity cannot use its most significant byte, so we effectively limit
-    // capacity to 2^24 on 32bit arch, and 2^60 on 64.
+    // capacity to 2^24 on 32bit arch, and 2^56 on 64.
     struct Data
     {
         using Alloc = Allocator<char, MemoryDomain::String>;
 
         Data() { set_empty(); }
-        Data(NoCopy, const char* data, size_t size) : u{Long{const_cast<char*>(data), size, 0}} {}
+        Data(NoCopy, const char* data, size_t size) : u{Long{const_cast<char*>(data), size, {0}, Short::inactive_mask}} {}
 
         Data(const char* data, size_t size, size_t capacity);
         Data(const char* data, size_t size) : Data(data, size, size) {}
@@ -177,9 +178,9 @@ public:
         Data& operator=(const Data& other);
         Data& operator=(Data&& other) noexcept;
 
-        bool is_long() const { return (u.s.size & 1) == 0; }
-        size_t size() const { return is_long() ? u.l.size : (u.s.size >> 1); }
-        size_t capacity() const { return is_long() ? u.l.capacity : Short::capacity; }
+        bool is_long() const { return (u.s.remaining_size & Short::inactive_mask) > 0; }
+        size_t size() const { return is_long() ? u.l.size : (Short::capacity - u.s.remaining_size); }
+        size_t capacity() const { return is_long() ? u.l.capacity() : Short::capacity; }
 
         const char* data() const { return is_long() ? u.l.ptr : u.s.string; }
         char* data() { return is_long() ? u.l.ptr : u.s.string; }
@@ -195,18 +196,37 @@ public:
         struct Long
         {
             static constexpr size_t max_capacity =
-                (size_t)1 << 8 * (sizeof(size_t) - 1);
+                ((size_t)1 << (CHAR_BIT * (sizeof(size_t) - 1))) - 1;
 
             char* ptr;
             size_t size;
-            size_t capacity;
+            unsigned char m_capacity[sizeof(size_t) - 1];
+            unsigned char mode;
+            size_t capacity() const 
+            { 
+                size_t ret{};
+                auto* dest = ((unsigned char*)&ret) + 
+                             (std::endian::native == std::endian::big ? 1: 0);
+                memcpy(dest, m_capacity, sizeof m_capacity);
+                return ret;
+            }
+            void set_capacity(size_t cap) 
+            { 
+                auto* src = ((unsigned char*)&cap) + 
+                            (std::endian::native == std::endian::big ? 1: 0);
+                memcpy(m_capacity, src, sizeof m_capacity);
+                mode = Short::inactive_mask;
+            }
         };
 
         struct Short
         {
-            static constexpr size_t capacity = sizeof(Long) - 2;
-            char string[capacity+1];
-            unsigned char size;
+            static constexpr size_t capacity = sizeof(Long) - 1;
+            char string[capacity];
+            // When string is full remaining_size will be 0 and be the null terminator.
+            // When string is empty remaining size will be 0b00010111 and not collide with inactive_mask.
+            unsigned char remaining_size;
+            static constexpr unsigned char inactive_mask = 0b1000'0000;
         };
 
         union
@@ -217,11 +237,12 @@ public:
 
         void release()
         {
-            if (is_long() and u.l.capacity != 0)
-                Alloc{}.deallocate(u.l.ptr, u.l.capacity+1);
+            auto const cap = u.l.capacity();
+            if (is_long() and (cap != 0))
+                Alloc{}.deallocate(u.l.ptr, cap+1);
         }
 
-        void set_empty() { u.s.size = 1; u.s.string[0] = 0; }
+        void set_empty() { u.s.remaining_size = Short::capacity; u.s.string[0] = '\0'; }
         void set_short(const char* data, size_t size);
     };