lib: Add utility function to convert from hex to binary

src/lib/base/String.cpp

@@ -35,6 +35,42 @@
 namespace barrier {
 namespace string {
 
+namespace {
+
+// returns negative in case of non-matching character
+int hex_to_number(char ch)
+{
+    switch (ch) {
+        case '0': return 0;
+        case '1': return 1;
+        case '2': return 2;
+        case '3': return 3;
+        case '4': return 4;
+        case '5': return 5;
+        case '6': return 6;
+        case '7': return 7;
+        case '8': return 8;
+        case '9': return 9;
+
+        case 'a': return 10;
+        case 'b': return 11;
+        case 'c': return 12;
+        case 'd': return 13;
+        case 'e': return 14;
+        case 'f': return 15;
+
+        case 'A': return 10;
+        case 'B': return 11;
+        case 'C': return 12;
+        case 'D': return 13;
+        case 'E': return 14;
+        case 'F': return 15;
+    }
+    return -1;
+}
+
+} // namespace
+
 std::string
 format(const char* fmt, ...)
 {
@@ -196,6 +232,26 @@ std::string to_hex(const std::string& subject, int width, const char fill)
     return ss.str();
 }
 
+std::vector<std::uint8_t> from_hex(const std::string& data)
+{
+    if ((data.size() % 2) != 0) {
+        return {};
+    }
+
+    std::vector<std::uint8_t> result;
+    result.reserve(data.size() / 2);
+
+    for (std::size_t i = 0; i < data.size(); i += 2) {
+        auto high = hex_to_number(data[i]);
+        auto low = hex_to_number(data[i + 1]);
+        if (high < 0 || low < 0) {
+            return {};
+        }
+        result.push_back(high * 16 + low);
+    }
+    return result;
+}
+
 void
 uppercase(std::string& subject)
 {

src/lib/base/String.h

@@ -77,6 +77,9 @@ Convert each character in \c subject into hexdecimal form with \c width
 */
 std::string to_hex(const std::string& subject, int width, const char fill = '0');
 
+/// Convert binary data from hexadecimal
+std::vector<std::uint8_t> from_hex(const std::string& data);
+
 //! Convert to all uppercase
 /*!
 Convert each character in \c subject to uppercase

src/test/unittests/base/StringTests.cpp

@@ -62,6 +62,20 @@ TEST(StringTests, toHex_plaintext_hexString)
     EXPECT_EQ("666f6f626172", string::to_hex(subject, width));
 }
 
+TEST(StringTests, fromhex_plaintext_string)
+{
+    auto result = string::from_hex("666f6f626172");
+    std::string expected = "foobar";
+    EXPECT_EQ(result, std::vector<std::uint8_t>(expected.begin(), expected.end()));
+}
+
+TEST(StringTests, fromhex_binary_string)
+{
+    auto result = string::from_hex("01020304050600fff9");
+    auto expected = std::vector<std::uint8_t>{1, 2, 3, 4, 5, 6, 0, 0xff, 0xf9};
+    EXPECT_EQ(result, expected);
+}
+
 TEST(StringTests, uppercase_lowercaseInput_uppercaseOutput)
 {
     String subject = "12foo3BaR";