Match literals without a stack array and keep parser states in bytes (#5854)

* Match literals without a stack array and keep parser states in bytes

scan_after_whitespace() built the expected text of true, false, and null in
local std::array objects. With -fstack-protector-strong (the default of
Ubuntu's GCC, among others), a local array gives the function a stack canary
check, which then ran on every token. scan_literal() now takes the text
after the first character as a string literal, with its length as a template
parameter; for contiguous input it compares the rest at once and falls back
to the byte loop on a mismatch, so errors are reported at the same
character. It is out of line, which keeps the token dispatch small.

The parser kept the nesting of arrays and objects in a std::vector<bool>,
whose push_back() GCC calls out of line; it is a std::vector<std::uint8_t>
now.

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

* Skip the literal fast path test without exceptions

The test case compares the error messages of both lexer paths, so it does
not compile with JSON_NOEXCEPTION (the caught exception does not exist).

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

* Allow the string literal parameter of scan_literal() for clang-tidy

The parameter takes the literal by array reference so that its length is
a template parameter; this is the same exemption the C array overloads of
to_json() and from_json() have.

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

---------

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

tests/src/unit-class_lexer.cpp

@@ -13,6 +13,7 @@
 using nlohmann::json;
 
 #include <array> // array
+#include <cctype> // toupper
 #include <cstdint> // uint32_t, uint64_t
 #include <cstdio> // snprintf
 #include <cmath> // isfinite
@@ -599,6 +600,67 @@ TEST_CASE("lexer number fast path")
 #endif
 }
 
+#if !defined(JSON_NOEXCEPTION)
+// this test case compares the errors of both lexer paths, which abort when
+// exceptions are off
+TEST_CASE("lexer literal fast path")
+{
+    // For contiguous input, the rest of a literal is compared at once, and a
+    // mismatch is left to the byte path, which reports the error at the
+    // character that differs. Every truncation and every wrong character of
+    // each literal, at the end of the input or followed by more text, must
+    // give the byte path's result (a std::list is read one character at a
+    // time).
+    const auto outcome = [](const std::string & doc, bool byte_path) -> std::string
+    {
+        JSON_TRY
+        {
+            const std::list<char> chars(doc.begin(), doc.end());
+            return (byte_path ? json::parse(chars.begin(), chars.end()) : json::parse(doc)).dump();
+        }
+        JSON_CATCH (const json::exception & e)
+        {
+            return e.what();
+        }
+    };
+
+    std::vector<std::string> mismatches;
+    for (const std::string literal :
+            {"true", "false", "null"
+            })
+    {
+        std::vector<std::string> variants;
+        for (std::size_t length = 1; length <= literal.size(); ++length)
+        {
+            variants.push_back(literal.substr(0, length));
+        }
+        for (std::size_t i = 1; i < literal.size(); ++i)
+        {
+            std::string changed = literal;
+            changed[i] = 'X';
+            variants.push_back(changed);
+            changed[i] = static_cast<char>(std::toupper(static_cast<unsigned char>(literal[i])));
+            variants.push_back(changed);
+        }
+        for (const auto& variant : variants)
+        {
+            for (const std::string& doc :
+                    {
+                        variant, "[" + variant + "]", "[" + variant, variant + " ", variant + "1", "{\"k\":" + variant + "}"
+                    })
+            {
+                if (outcome(doc, false) != outcome(doc, true))
+                {
+                    mismatches.push_back(doc);
+                }
+            }
+        }
+    }
+    CAPTURE(mismatches)
+    CHECK(mismatches.empty());
+}
+#endif
+
 TEST_CASE("lexer string fast path")
 {
     // Build a byte string from explicit values: a hex escape in a string

tests/src/unit-stream-bulk-scan.cpp

@@ -171,7 +171,8 @@ std::vector<std::string> corpus()
         R"("unterminated)", "\"control\x01character\"", "\"ill-formed \xc3\x28 UTF-8\"", R"("bad \x escape")",
         R"("\ud800 lone surrogate")",
         // literals
-        "true", "false", "null", "tru", "nul", "falsey",
+        "true", "false", "null", "tru", "nul", "falsey", "[true,false,null]", "nulL", "truex", "[fals e]", "t",
+        "true1", "null null", "[false]",
         // structures
         "[]", "{}", "[1,2,3]", R"({"a":1,"b":[true,false,null],"c":{"d":"e"}})", "[1,]", R"({"a" 1})", "[[[[[]]]]]",
         // pretty-printed