accept(): skip digits without computing their value

accept() needs the value of a number only for a float whose exponent may
reach the largest value. skip_digits() finds the end of the digits without
the multiply-adds of accumulate_digits(), and once an 8-byte word fails the
digit test, the remaining loop needs no bounds check: a non-digit is in
that word. The value is computed afterwards for the rare floats that need
it.

accept() of canada.json: -16 % on Apple M1, -22 % with GCC 15 and -21 %
with Clang 21 on x86-64; files with many numbers -9 to -15 %.

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

tests/src/unit-contiguous_parser.cpp

@@ -518,6 +518,33 @@ TEST_CASE("contiguous parser")
         }
     }
 
+    SECTION("accept() of floats near the largest value")
+    {
+        // accept() skips digits without their value and computes it only for
+        // a float whose exponent may reach the largest value
+        for (const char* number :
+                {"17976931348623158e292", "17976931348623159e292", "1797693134862315.8e293", "1797693134862315.9e293",
+                 "0.17976931348623158e309", "0.17976931348623159e309", "-17976931348623159e292", "9999999999999999999e289"
+                })
+        {
+            check_both(number);
+            check_both(std::string("[") + number + "]");
+        }
+        CHECK(json::accept("17976931348623158e292"));
+        CHECK_FALSE(json::accept("17976931348623159e292"));
+        CHECK(json::accept("[1797693134862315.8e293]"));
+        CHECK_FALSE(json::accept("[1797693134862315.9e293]"));
+        CHECK(json::accept("0.17976931348623158e309"));
+        CHECK_FALSE(json::accept("0.17976931348623159e309"));
+        CHECK_FALSE(json::accept("-17976931348623159e292"));
+
+        using narrow_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int32_t, std::uint32_t, float>;
+        check_same<narrow_json>("34028235e31");
+        check_same<narrow_json>("34028236e31");
+        CHECK(narrow_json::accept("34028235e31"));
+        CHECK_FALSE(narrow_json::accept("34028236e31"));
+    }
+
     SECTION("callbacks, comments and trailing commas take the library parser")
     {
         // (the results are those of the options; the contiguous parser does not support them)