From 5f4f553c3588f3ef3cb704c9d3c6db6bb78ccfa9 Mon Sep 17 00:00:00 2001
From: m-holger <m-holger@kubitscheck.org>
Date: Thu, 23 Mar 2023 18:51:28 +0000
Subject: Refactor QUtil::hex_decode

---
 libqpdf/QUtil.cc | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/libqpdf/QUtil.cc b/libqpdf/QUtil.cc
index bae067b6..03301d9d 100644
--- a/libqpdf/QUtil.cc
+++ b/libqpdf/QUtil.cc
@@ -783,28 +783,25 @@ std::string
 QUtil::hex_decode(std::string const& input)
 {
     std::string result;
-    size_t pos = 0;
+    // We know result.size() <= 0.5 * input.size() + 1. However, reserving
+    // string space for this upper bound has a negative impact.
+    bool first = true;
+    char decoded;
     for (auto ch: input) {
-        bool skip = false;
-        if ((ch >= 'A') && (ch <= 'F')) {
-            ch = QIntC::to_char(ch - 'A' + 10);
-        } else if ((ch >= 'a') && (ch <= 'f')) {
-            ch = QIntC::to_char(ch - 'a' + 10);
-        } else if ((ch >= '0') && (ch <= '9')) {
-            ch = QIntC::to_char(ch - '0');
-        } else {
-            skip = true;
-        }
-        if (!skip) {
-            if (pos == 0) {
-                result.push_back(static_cast<char>(ch << 4));
-                pos = 1;
+        ch = hex_decode_char(ch);
+        if (ch < '\20') {
+            if (first) {
+                decoded = static_cast<char>(ch << 4);
+                first = false;
             } else {
-                result[result.length() - 1] |= ch;
-                pos = 0;
+                result.push_back(decoded | ch);
+                first = true;
             }
         }
     }
+    if (!first) {
+        result.push_back(decoded);
+    }
     return result;
 }
 
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