accept() checks strings without copying them

json::accept() parses through json_sax_acceptor, which ignores the
values of strings and object keys, but the lexer still copied every
string token into its token buffer. That copy was 18 % of the
instructions of json::accept() on twitter.json (x86, GCC 16).

The lexer's flag for accept(), renamed from discard_number_values to
discard_values, now also selects a second instantiation of
scan_string() that performs the same checks (escapes, surrogates,
UTF-8, control characters, with the same error messages) without
writing to the token buffer. scan() chooses the instantiation once per
string: a run-time check inside the string loop made number-heavy
files up to 3 % slower on Apple M1.

json::accept() over 19 files, time after/before (geomean): 0.876 on x86
with GCC 16.2, 0.884 with Clang 23, 0.932 on Apple M1; up to 0.74 on
single files; files with few strings (canada, mesh, numbers) unchanged.

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

tests/src/unit-class_parser.cpp

@@ -2982,6 +2982,50 @@ TEST_CASE("accept() agrees with parse() on floats around the overflow limit")
     }
 }
 
+TEST_CASE("accept() agrees with parse() on strings, which it checks without copying them")
+{
+    // accept() scans strings and object keys with a separate instantiation of
+    // the string scanner that does not copy them into the token buffer; it
+    // must accept and reject exactly what parse() does
+    const std::vector<std::string> pieces =
+    {
+        "", "abc", "\\\"\\\\\\/\\b\\f\\n\\r\\t", "\\u0041\\u00e4\\u20AC", "\\ud83d\\ude00", "\\uD83D\\uDE00",
+        "\\ud800", "\\udc00", "\\ud800\\u0041", "\\ud800x", "\\udbff\\udfff", "\\u12", "\\u12g4", "\\x", "\\",
+        "\xc3\xa4", "\xe2\x82\xac", "\xf0\x9f\x98\x80", "\xf4\x8f\xbf\xbf", "\xc0\xaf", "\xed\xa0\x80", "\xf4\x90\x80\x80",
+        "\xff", "\xe2\x82", "\xc3", "\x01", "\x1f", "\x7f", "\n", std::string(1, '\0')
+    };
+
+    for (std::size_t run = 0; run <= 70; ++run)
+    {
+        for (const std::string& piece : pieces)
+        {
+            // a run of ordinary characters puts the piece at every offset of the bulk scanners
+            const std::string str = std::string(run, 'a') + piece;
+            for (const std::string& text :
+                    {
+                        "\"" + str + "\"", "[\"" + str + "\"]", "{\"" + str + "\":1}", "{\"k\":\"" + str + "\",\"" + str + "\":[\"" + str + "\"]}",
+                        "[\"" + str
+                    })
+            {
+                CAPTURE(text)
+                for (int options = 0; options < 4; ++options)
+                {
+                    const bool ignore_comments = (options & 1) != 0;
+                    const bool ignore_trailing_commas = (options & 2) != 0;
+                    const bool parses = !json::parse(text, nullptr, false, ignore_comments, ignore_trailing_commas).is_discarded();
+                    CHECK(json::accept(text, ignore_comments, ignore_trailing_commas) == parses);
+
+                    std::istringstream stream(text);
+                    CHECK(json::accept(stream, ignore_comments, ignore_trailing_commas) == parses);
+
+                    const std::list<char> chars(text.begin(), text.end());
+                    CHECK(json::accept(chars.begin(), chars.end(), ignore_comments, ignore_trailing_commas) == parses);
+                }
+            }
+        }
+    }
+}
+
 namespace
 {
 // a DOM parser that reports parse errors without throwing, as in the sax_no_exception example