Reserve space to reduce memory operations while encoding barcode

In the test case from the bug, the majority of the time is being spent
resizing Widestring internal buffers, since += is being called in a
tight loop. Since the size of the input being mutated and stored is
known, this CL reserves the space before hand to lower thrashing. This
substantially improves runtime of this test case locally.

BUG=chromium:802242

Change-Id: I5176dabc94634b4d6bc3e9425fe6469a5bf35a41
Reviewed-on: https://pdfium-review.googlesource.com/30190
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/fxbarcode/pdf417/BC_PDF417HighLevelEncoder.cpp b/fxbarcode/pdf417/BC_PDF417HighLevelEncoder.cpp
index ffc057a5f6..6dd7ee5191 100644
--- a/fxbarcode/pdf417/BC_PDF417HighLevelEncoder.cpp
+++ b/fxbarcode/pdf417/BC_PDF417HighLevelEncoder.cpp
@@ -61,8 +61,9 @@ WideString CBC_PDF417HighLevelEncoder::encodeHighLevel(WideString wideMsg,
                                                        int32_t& e) {
   ByteString bytes;
   CBC_UtilCodingConvert::UnicodeToUTF8(wideMsg, bytes);
-  WideString msg;
   int32_t len = bytes.GetLength();
+  WideString msg;
+  msg.Reserve(len);
   for (int32_t i = 0; i < len; i++) {
     wchar_t ch = (wchar_t)(bytes[i] & 0xff);
     if (ch == '?' && bytes[i] != '?') {
@@ -72,8 +73,9 @@ WideString CBC_PDF417HighLevelEncoder::encodeHighLevel(WideString wideMsg,
     msg += ch;
   }
   std::vector<uint8_t> byteArr(bytes.begin(), bytes.end());
-  WideString sb;
   len = msg.GetLength();
+  WideString sb;
+  sb.Reserve(len);
   int32_t p = 0;
   int32_t textSubMode = SUBMODE_ALPHA;
   if (compaction == TEXT) {