Adds hex_to_string helper function to format_utils. (#707)

* Adds hex_to_string helper function to format_utils.

* Adds format utils test.

* Fix whitespace.

* Change API to return nonnegative integer.
Fix review comment.

src/utils/format_utils.cxx

@@ -242,6 +242,57 @@ string string_to_hex(const string &arg)
     return ret;
 }
 
+static uint8_t get_nibble(char b)
+{
+    if ('0' <= b && b <= '9')
+    {
+        return b - '0';
+    }
+    if ('a' <= b && b <= 'f')
+    {
+        return b - 'a' + 10;
+    }
+    if ('A' <= b && b <= 'F')
+    {
+        return b - 'A' + 10;
+    }
+    return 0xff;
+}
+
+size_t hex_to_string(
+    const char *input, size_t len, string *output, bool ignore_nonhex)
+{
+    uint8_t b = 0;         // current byte
+    bool next_high = true; // next nibble is high of the byte
+    for (size_t ofs = 0; ofs < len; ++ofs)
+    {
+        uint8_t nib = get_nibble(input[ofs]);
+        if (nib == 0xff)
+        {
+            if (!ignore_nonhex)
+            {
+                return ofs;
+            }
+            continue;
+        }
+        b |= nib & 0xf;
+        if (next_high)
+        {
+            b <<= 4;
+            next_high = false;
+            continue;
+        }
+        else
+        {
+            output->push_back(b);
+            b = 0;
+            next_high = true;
+            continue;
+        }
+    }
+    return len;
+}
+
 string mac_to_string(uint8_t mac[6], char colon)
 {
     string ret;

src/utils/format_utils.cxxtest

@@ -0,0 +1,35 @@
+#include "utils/format_utils.hxx"
+
+#include "utils/test_main.hxx"
+
+TEST(HexToString, basic)
+{
+    string input = "303132";
+    string output;
+    EXPECT_EQ(6u, hex_to_string(input.data(), input.size(), &output));
+    EXPECT_EQ("012", output);
+
+    output = "xx";
+    EXPECT_EQ(6u, hex_to_string(input.data(), input.size(), &output));
+    EXPECT_EQ("xx012", output);
+
+    output.clear();
+    input = "0A0BfC";
+    EXPECT_EQ(6u, hex_to_string(input.data(), input.size(), &output));
+    EXPECT_EQ("\x0a\x0b\xfc", output);
+
+    output.clear();
+    input = "30.31.32";
+    EXPECT_EQ(2u, hex_to_string(input.data(), input.size(), &output));
+    EXPECT_EQ("0", output);
+
+    output.clear();
+    EXPECT_EQ(8u, hex_to_string(input.data(), input.size(), &output, true));
+    EXPECT_EQ("012", output);
+
+    output.clear();
+    input = "303132F";
+    EXPECT_EQ(7u, hex_to_string(input.data(), input.size(), &output));
+    // lost one nibble, this is ok according to the contract
+    EXPECT_EQ("012", output);
+}

src/utils/format_utils.hxx

@@ -130,6 +130,21 @@ string int64_to_string_hex(int64_t value, unsigned padding = 0);
 /// original data.
 string string_to_hex(const string& arg);
 
+/// Converts hex bytes to binary representation.
+///
+/// @param input points to the input hexadecimal string
+/// @param len how many bytes are there
+/// @param output parsed byte data will be appended here
+/// @param ignore_nonhex if true, jumps over all nonhex characters. If false,
+/// stops at the first nonhex character.
+///
+/// @return number of ascii bytes consumed from the input. For example 0 if the
+/// first byte was not hex and ignore_nonhex=false. If the number of hex digits
+/// is not even, there will be data loss.
+///
+size_t hex_to_string(
+    const char *input, size_t len, string *output, bool ignore_nonhex = false);
+
 /// Formats a MAC address to string. Works both for Ethernet addresses as well
 /// as for OpenLCB node IDs.
 ///