refactor parse_unsigned

Author: grisumbras

include/boost/json/detail/sse2.hpp

@@ -315,53 +315,17 @@ inline int count_digits( char const* p ) noexcept
 
 inline uint64_t parse_unsigned( uint64_t r, char const * p, std::size_t n ) noexcept
 {
-    while( n >= 4 )
+    constexpr int N = 4;
+    auto const e = p + n;
+    for( ; (e - p) >= N; p += N )
     {
-        // faster on on clang for x86,
-        // slower on gcc
-#ifdef __clang__
-        r = r * 10 + p[0] - '0';
-        r = r * 10 + p[1] - '0';
-        r = r * 10 + p[2] - '0';
-        r = r * 10 + p[3] - '0';
-#else
-        uint32_t v;
-        std::memcpy( &v, p, 4 );
-
-        v -= 0x30303030;
-
-        unsigned w0 = v & 0xFF;
-        unsigned w1 = (v >> 8) & 0xFF;
-        unsigned w2 = (v >> 16) & 0xFF;
-        unsigned w3 = (v >> 24);
-
-#ifdef BOOST_JSON_BIG_ENDIAN
-        r = (((r * 10 + w3) * 10 + w2) * 10 + w1) * 10 + w0;
-#else
-        r = (((r * 10 + w0) * 10 + w1) * 10 + w2) * 10 + w3;
-#endif
-#endif
-        p += 4;
-        n -= 4;
+        for( auto i = 0; i < N; ++i )
+            r = r * 10 + p[i] - '0';
     }
 
-    switch( n )
-    {
-    case 0:
-        break;
-    case 1:
-        r = r * 10 + p[0] - '0';
-        break;
-    case 2:
-        r = r * 10 + p[0] - '0';
-        r = r * 10 + p[1] - '0';
-        break;
-    case 3:
-        r = r * 10 + p[0] - '0';
-        r = r * 10 + p[1] - '0';
-        r = r * 10 + p[2] - '0';
-        break;
-    }
+    for( ; p != e; ++p )
+        r = r * 10 + *p - '0';
+
     return r;
 }