Add support for hash chaining to detect modifications in postings

doc/ledger3.texi

@@ -6702,9 +6702,15 @@ Record the hash of each transaction in a @var{Hash} metadata value,
 according to the hashing algorithm given by the @var{ALGO}
 argument. Note that if a @var{Hash} metadata value is explicitly
 provided and does not match what would have been generated, an error is
-reported. Hashes depend on previous entries, such that setting a single
-hash value is sufficient to guarantee the shape of the entire history
-leading up to that entry.
+reported. This provided value can also just be a prefix, at whatever
+length the user wishes, in which case only this amount of text is
+compared to the generated hash. Printing the entry with hashes enabled
+will display both this explicit metadata setting and the internally
+generated hash value.
+
+Hashes depend on previous entries, such that setting a single hash value
+is sufficient to guarantee the shape of the entire history leading up to
+that entry.
 
 @item --head @var{INT}
 @itemx --first @var{INT}

src/item.cc

@@ -133,13 +133,17 @@ item_t::set_tag(const string&            tag,
 
   string_map::iterator i = metadata->find(tag);
   if (i == metadata->end()) {
+    DEBUG("item.meta", "Setting new metadata value");
     std::pair<string_map::iterator, bool> result
       = metadata->insert(string_map::value_type(tag, tag_data_t(data, false)));
     assert(result.second);
     return result.first;
   } else {
-    if (overwrite_existing)
+    DEBUG("item.meta", "Found old metadata value");
+    if (overwrite_existing) {
+      DEBUG("item.meta", "Overwriting old metadata value");
       (*i).second = tag_data_t(data, false);
+    }
     return i;
   }
 }

src/item.h

@@ -199,10 +199,6 @@ class item_t : public flags::supports_flags<uint_least16_t>, public scope_t
     return _state;
   }
 
-  virtual string hash(string _nonce) const {
-    return "";
-  }
-
   virtual void define(const symbol_t::kind_t, const string&,
                       expr_t::ptr_op_t);
   virtual expr_t::ptr_op_t lookup(const symbol_t::kind_t kind,

src/post.cc

@@ -670,30 +670,6 @@ void post_t::set_reported_account(account_t * acct)
   acct->xdata().reported_posts.push_back(this);
 }
 
-extern "C" unsigned char *SHA512(
-  void *data, unsigned int data_len, unsigned char *digest);
-
-namespace {
-  std::string bufferToHex(const unsigned char* buffer, std::size_t size) {
-      std::ostringstream oss;
-      oss << std::hex << std::setfill('0');
-      for(std::size_t i = 0; i < size; ++i)
-          oss << std::setw(2) << static_cast<int>(buffer[i]);
-      return oss.str();
-  }
-}
-
-string post_t::hash(string nonce) const {
-  unsigned char data[128];
-  std::ostringstream repr;
-  repr << nonce;
-  repr << account->fullname();
-  repr << amount.to_string();
-  string repr_str(repr.str());
-  SHA512((void *)repr_str.c_str(), repr_str.length(), data);
-  return bufferToHex(data, 64 /*SHA512_DIGEST_LENGTH*/);
-}
-
 void extend_post(post_t& post, journal_t& journal)
 {
   commodity_t& comm(post.amount.commodity());

src/post.h

@@ -245,8 +245,6 @@ class post_t : public item_t
     return const_cast<post_t *>(this)->reported_account();
   }
 
-  virtual string hash(string nonce) const;
-
   friend class xact_t;
 
   struct compare_by_date_and_sequence

src/textual.cc

@@ -2027,10 +2027,12 @@ xact_t * instance_t::parse_xact(char *          line,
                  previous_xact->get_tag("Hash")->to_string() : "");
     if (xact->has_tag("Hash")) {
       string current_hash = xact->get_tag("Hash")->to_string();
-      if (expected_hash != current_hash) {
+      if (! std::equal(expected_hash.begin(),
+                       expected_hash.begin() +
+                       std::min(expected_hash.size(), current_hash.size()),
+                       current_hash.begin()))
         throw_(parse_error, _f("Expected hash %1% != %2%") %
                expected_hash % current_hash);
-      }
     } else {
       xact->set_tag("Hash", string_value(expected_hash));
     }

src/xact.cc

@@ -581,19 +581,47 @@ bool xact_t::valid() const
   return true;
 }
 
+extern "C" unsigned char *SHA512(
+  void *data, unsigned int data_len, unsigned char *digest);
+
+namespace {
+  std::string bufferToHex(const unsigned char* buffer, std::size_t size) {
+      std::ostringstream oss;
+      oss << std::hex << std::setfill('0');
+      for(std::size_t i = 0; i < size; ++i)
+          oss << std::setw(2) << static_cast<int>(buffer[i]);
+      return oss.str();
+  }
+}
+
 string xact_t::hash(string nonce) const {
   std::ostringstream repr;
   repr << nonce;
   repr << date();
   repr << aux_date();
   repr << code;
   repr << payee;
-  string hash(repr.str());
   posts_list all_posts(posts.begin(), posts.end());
+  std::vector<std::string> strings;
   foreach (post_t * post, all_posts) {
-    hash = post->hash(hash);
+    std::ostringstream posting;
+    posting << post->account->fullname();
+    if (! post->amount.is_null())
+      posting << post->amount.to_fullstring();
+    if (post->cost)
+      posting << post->cost->to_fullstring();
+    posting << post->checkin;
+    posting << post->checkout;
+    strings.push_back(posting.str());
+  }
+  std::sort(strings.begin(), strings.end());
+  foreach (string& str, strings) {
+    repr << str;
   }
-  return hash;
+  unsigned char data[128];
+  string repr_str(repr.str());
+  SHA512((void *)repr_str.c_str(), repr_str.length(), data);
+  return bufferToHex(data, 64 /*SHA512_DIGEST_LENGTH*/);
 }
 
 namespace {

src/xact.h

@@ -128,7 +128,7 @@ class xact_t : public xact_base_t
 
   virtual bool valid() const;
 
-  virtual string hash(string nonce) const;
+  string hash(string nonce) const;
 };
 
 class auto_xact_t : public xact_base_t