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.

1 files changed, 19 insertions, 19 deletions

diff --git a/src/catch2/catch_tostring.cpp b/src/catch2/catch_tostring.cpp
index 4d80b173..cc686567 100644
--- a/src/catch2/catch_tostring.cpp
+++ b/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) {
-            switch (c) {
-            case '\r':
-                ret.append("\\r");
-                break;
-            case '\n':
-                ret.append("\\n");
-                break;
-            case '\t':
-                ret.append("\\t");
-                break;
-            case '\f':
-                ret.append("\\f");
-                break;
-            default:
-                ret.push_back(c);
-                break;
+        for ( size_t i = 0; i < string.size(); ++i ) {
+            const char c = string[i];
+            if ( c == '\r' || c == '\n' || c == '\t' || c == '\f' ) {
+                write_to( i );
+                if ( c == '\r' ) { ret.append( "\\r" ); }
+                if ( c == '\n' ) { ret.append( "\\n" ); }
+                if ( c == '\t' ) { ret.append( "\\t" ); }
+                if ( c == '\f' ) { ret.append( "\\f" ); }
             }
         }
+        write_to( string.size() );
         ret += '"';
 
         return ret;