refactor: replace hardcoded numbers in ExpandHRP

Replace ExpandHRP with a PreparePolynomialCoefficients function. Instead
of using a hardcoded value for the size of the array (90 in this case)
and a hardcoded value for the checksum, use the actual values vector and
define checksum size as a constexpr.

Co-Authored-By Lőrinc <lorinc.pap@gmail.com>

src/bech32.cpp

@@ -19,6 +19,9 @@ namespace
 
 typedef std::vector<uint8_t> data;
 
+/** The Bech32 and Bech32m checksum size */
+constexpr size_t CHECKSUM_SIZE = 6;
+
 /** The Bech32 and Bech32m character set for encoding. */
 const char* CHARSET = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";
 
@@ -308,18 +311,18 @@ bool CheckCharacters(const std::string& str, std::vector<int>& errors)
     return errors.empty();
 }
 
-/** Expand a HRP for use in checksum computation. */
-data ExpandHRP(const std::string& hrp)
+std::vector<unsigned char> PreparePolynomialCoefficients(const std::string& hrp, const data& values, const size_t extra)
 {
     data ret;
-    ret.reserve(hrp.size() + 90);
-    ret.resize(hrp.size() * 2 + 1);
-    for (size_t i = 0; i < hrp.size(); ++i) {
-        unsigned char c = hrp[i];
-        ret[i] = c >> 5;
-        ret[i + hrp.size() + 1] = c & 0x1f;
-    }
-    ret[hrp.size()] = 0;
+    ret.reserve(hrp.size() + 1 + hrp.size() + values.size() + extra);
+
+    /** Expand a HRP for use in checksum computation. */
+    for (size_t i = 0; i < hrp.size(); ++i) ret.push_back(hrp[i] >> 5);
+    ret.push_back(0);
+    for (size_t i = 0; i < hrp.size(); ++i) ret.push_back(hrp[i] & 0x1f);
+
+    ret.insert(ret.end(), values.begin(), values.end());
+
     return ret;
 }
 
@@ -331,7 +334,8 @@ Encoding VerifyChecksum(const std::string& hrp, const data& values)
     // list of values would result in a new valid list. For that reason, Bech32 requires the
     // resulting checksum to be 1 instead. In Bech32m, this constant was amended. See
     // https://gist.github.com/sipa/14c248c288c3880a3b191f978a34508e for details.
-    const uint32_t check = PolyMod(Cat(ExpandHRP(hrp), values));
+    auto enc = PreparePolynomialCoefficients(hrp, values, 0);
+    const uint32_t check = PolyMod(enc);
     if (check == EncodingConstant(Encoding::BECH32)) return Encoding::BECH32;
     if (check == EncodingConstant(Encoding::BECH32M)) return Encoding::BECH32M;
     return Encoding::INVALID;
@@ -340,8 +344,8 @@ Encoding VerifyChecksum(const std::string& hrp, const data& values)
 /** Create a checksum. */
 data CreateChecksum(Encoding encoding, const std::string& hrp, const data& values)
 {
-    data enc = Cat(ExpandHRP(hrp), values);
-    enc.resize(enc.size() + 6); // Append 6 zeroes
+    auto enc = PreparePolynomialCoefficients(hrp, values, CHECKSUM_SIZE);
+    enc.insert(enc.end(), CHECKSUM_SIZE, 0x00);
     uint32_t mod = PolyMod(enc) ^ EncodingConstant(encoding); // Determine what to XOR into those 6 zeroes.
     data ret(6);
     for (size_t i = 0; i < 6; ++i) {
@@ -442,9 +446,10 @@ std::pair<std::string, std::vector<int>> LocateErrors(const std::string& str, Ch
     std::optional<Encoding> error_encoding;
     for (Encoding encoding : {Encoding::BECH32, Encoding::BECH32M}) {
         std::vector<int> possible_errors;
-        // Recall that (ExpandHRP(hrp) ++ values) is interpreted as a list of coefficients of a polynomial
+        // Recall that (expanded hrp + values) is interpreted as a list of coefficients of a polynomial
         // over GF(32). PolyMod computes the "remainder" of this polynomial modulo the generator G(x).
-        uint32_t residue = PolyMod(Cat(ExpandHRP(hrp), values)) ^ EncodingConstant(encoding);
+        auto enc = PreparePolynomialCoefficients(hrp, values, 0);
+        uint32_t residue = PolyMod(enc) ^ EncodingConstant(encoding);
 
         // All valid codewords should be multiples of G(x), so this remainder (after XORing with the encoding
         // constant) should be 0 - hence 0 indicates there are no errors present.