From a19235c14b3dc02de30b5d769de29d1752c23dbd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?L=C5=91rinc?= <lorinc.pap@gmail.com>
Date: Sun, 25 Feb 2024 17:50:25 +0100
Subject: [PATCH] Preallocate result in `TryParseHex` to avoid resizing

Running `make && ./src/bench/bench_bitcoin -filter=HexParse` a few times results in:
```
|           ns/base16 |            base16/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|                0.68 |    1,465,555,976.27 |    0.8% |      0.01 | `HexParse`
|                0.68 |    1,472,962,920.18 |    0.3% |      0.01 | `HexParse`
|                0.68 |    1,476,159,423.00 |    0.3% |      0.01 | `HexParse`
```
---
 src/util/strencodings.cpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/util/strencodings.cpp b/src/util/strencodings.cpp
index a54f408496..b51b283a69 100644
--- a/src/util/strencodings.cpp
+++ b/src/util/strencodings.cpp
@@ -81,6 +81,8 @@ template <typename Byte>
 std::optional<std::vector<Byte>> TryParseHex(std::string_view str)
 {
     std::vector<Byte> vch;
+    vch.reserve(str.size() / 2); // two hex characters form a single byte
+
     auto it = str.begin();
     while (it != str.end()) {
         if (IsSpace(*it)) {