Speed up processing mostly visible strings in convertIntoString

This commit causes small (~5%) slowdown when processing strings
that consist mostly of escaped characters, but improves throughput
by about 100% for strings that consist mostly of characters that
do not need escaping.

src/catch2/catch_tostring.cpp

@@ -61,35 +61,35 @@ namespace Detail {
         std::string ret;
         // This is enough for the "don't escape invisibles" case, and a good
         // lower bound on the "escape invisibles" case.
-        ret.reserve(string.size() + 2);
+        ret.reserve( string.size() + 2 );
 
-        if (!escapeInvisibles) {
+        if ( !escapeInvisibles ) {
             ret += '"';
             ret += string;
             ret += '"';
             return ret;
         }
 
+        size_t last_start = 0;
+        auto write_to = [&]( size_t idx ) {
+            if ( last_start < idx ) {
+                ret += string.substr( last_start, idx - last_start );
+            }
+            last_start = idx + 1;
+        };
+
         ret += '"';
-        for (char c : string) {
+        for ( size_t i = 0; i < string.size(); ++i ) {
-            switch (c) {
+            const char c = string[i];
-            case '\r':
+            if ( c == '\r' || c == '\n' || c == '\t' || c == '\f' ) {
-                ret.append("\\r");
+                write_to( i );
-                break;
+                if ( c == '\r' ) { ret.append( "\\r" ); }
-            case '\n':
+                if ( c == '\n' ) { ret.append( "\\n" ); }
-                ret.append("\\n");
+                if ( c == '\t' ) { ret.append( "\\t" ); }
-                break;
+                if ( c == '\f' ) { ret.append( "\\f" ); }
-            case '\t':
-                ret.append("\\t");
-                break;
-            case '\f':
-                ret.append("\\f");
-                break;
-            default:
-                ret.push_back(c);
-                break;
             }
         }
+        write_to( string.size() );
         ret += '"';
 
         return ret;

tests/SelfTest/IntrospectiveTests/ToString.tests.cpp

@@ -6,11 +6,13 @@
 
 // SPDX-License-Identifier: BSL-1.0
 
+#include <catch2/benchmark/catch_benchmark.hpp>
+#include <catch2/catch_template_test_macros.hpp>
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/generators/catch_generators.hpp>
 #include <catch2/internal/catch_enum_values_registry.hpp>
 #include <catch2/matchers/catch_matchers_string.hpp>
 #include <catch2/matchers/catch_matchers_vector.hpp>
-#include <catch2/catch_test_macros.hpp>
-#include <catch2/catch_template_test_macros.hpp>
 
 #include <chrono>
 
@@ -139,3 +141,26 @@ TEST_CASE( "Exception thrown inside stringify does not fail the test", "[toStrin
     ThrowsOnStringification tos;
     CHECK( tos == tos );
 }
+
+TEST_CASE( "string escaping benchmark", "[toString][!benchmark]" ) {
+    const auto input_length = GENERATE( as<size_t>{}, 10, 100, 10'000, 100'000 );
+    std::string test_input( input_length, 'a' );
+    BENCHMARK( "no-escape string, no-escaping, len=" +
+               std::to_string( input_length ) ) {
+        return Catch::Detail::convertIntoString( test_input, false );
+    };
+    BENCHMARK( "no-escape string, escaping, len=" +
+               std::to_string( input_length ) ) {
+        return Catch::Detail::convertIntoString( test_input, true );
+    };
+
+    std::string escape_input( input_length, '\r' );
+    BENCHMARK( "full escape string, no-escaping, len=" +
+               std::to_string( input_length ) ) {
+        return Catch::Detail::convertIntoString( escape_input, false );
+    };
+    BENCHMARK( "full escape string, escaping, len=" +
+               std::to_string( input_length ) ) {
+        return Catch::Detail::convertIntoString( escape_input, true );
+    };
+}