mnemonics: fix prefix extraction with non ASCII text

src/mnemonics/electrum-words.cpp

@@ -99,7 +99,7 @@ namespace
       {
         if (has_checksum)
         {
-          trimmed_word = it2->substr(0, (*it1)->get_unique_prefix_length());
+          trimmed_word = Language::utf8prefix(*it2, (*it1)->get_unique_prefix_length());
           // Use the trimmed words and map
           if (trimmed_word_map.count(trimmed_word) == 0)
           {
@@ -144,7 +144,7 @@ namespace
     {
       if (it->length() > unique_prefix_length)
       {
-        trimmed_words += it->substr(0, unique_prefix_length);
+        trimmed_words += Language::utf8prefix(*it, unique_prefix_length);
       }
       else
       {
@@ -170,9 +170,9 @@ namespace
 
     std::string checksum = seed[create_checksum_index(seed, unique_prefix_length)];
 
-    std::string trimmed_checksum = checksum.length() > unique_prefix_length ? checksum.substr(0, unique_prefix_length) :
+    std::string trimmed_checksum = checksum.length() > unique_prefix_length ? Language::utf8prefix(checksum, unique_prefix_length) :
       checksum;
-    std::string trimmed_last_word = last_word.length() > unique_prefix_length ? last_word.substr(0, unique_prefix_length) :
+    std::string trimmed_last_word = last_word.length() > unique_prefix_length ? Language::utf8prefix(last_word, unique_prefix_length) :
       last_word;
     return trimmed_checksum == trimmed_last_word;
   }

src/mnemonics/language_base.h

@@ -45,6 +45,26 @@
  */
 namespace Language
 {
+  /*!
+   * \brief Returns a string made of (at most) the first count characters in s.
+   *        Assumes well formedness. No check is made for this.
+   * \param  s               The string from which to return the first count characters.
+   * \param  count           How many characters to return.
+   * \return                 A string consisting of the first count characters in s.
+   */
+  std::string utf8prefix(const std::string &s, size_t count)
+  {
+    std::string prefix = "";
+    const char *ptr = s.c_str();
+    while (count-- && *ptr)
+    {
+      prefix += *ptr++;
+      while (((*ptr) & 0xc0) == 0x80)
+        prefix += *ptr++;
+    }
+    return prefix;
+  }
+
   /*!
    * \class Base
    * \brief A base language class which all languages have to inherit from for
@@ -70,7 +90,7 @@ namespace Language
         (*word_map)[*it] = ii;
         if (it->length() > unique_prefix_length)
         {
-          (*trimmed_word_map)[it->substr(0, unique_prefix_length)] = ii;
+          (*trimmed_word_map)[utf8prefix(*it, unique_prefix_length)] = ii;
         }
         else
         {