default signature uses comment hash instead of random

src/main/java/com/worksap/nlp/sudachi/dictionary/Description.java

@@ -30,7 +30,6 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Locale;
-import java.util.Random;
 
 /**
  * Description of the dictionary blocks, in-memory representation. Basically, an
@@ -39,13 +38,12 @@
 public class Description {
     private Instant creationTime = Instant.now();
     private String comment = "";
-    private String signature = defaultSignature(creationTime);
+    private String signature = defaultSignature(creationTime, comment);
     private String reference = "";
     private List<Block> blocks = new ArrayList<>();
     private long flags;
     private int numTotalEntries;
     private int numIndexedEntries;
-    private Random random = new Random();
 
     /**
      * Return a slice of the full dictionary with the provided name
@@ -220,10 +218,10 @@ private static void checkLegacyDictionaryFormat(ByteBuffer raw) {
         }
     }
 
-    private String defaultSignature(Instant date) {
+    private String defaultSignature(Instant date, String comment) {
         DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyyMMddHHmmss", Locale.US);
         return String.format("%s-%08x", formatter.format(LocalDateTime.ofInstant(date, ZoneId.systemDefault())),
-                random.nextLong());
+                comment.hashCode());
     }
 
     public Instant getCreationTime() {
@@ -258,6 +256,14 @@ public void setSignature(String signature) {
         this.signature = signature;
     }
 
+    /**
+     * Overwrite signature by the default value with the current creationTime and
+     * comment.
+     */
+    public void setDefaultSignature() {
+        this.signature = defaultSignature(creationTime, comment);
+    }
+
     public String getReference() {
         return reference;
     }

src/main/java/com/worksap/nlp/sudachi/dictionary/DictionaryBuilder.java

@@ -53,12 +53,10 @@ private static void build(Path matrixPath, String description, String posPath, S
         if (posPath != null) {
             builder = builder.posTable(Paths.get(posPath));
         }
-        if (signature != null) {
-            builder = builder.signature(signature);
-        }
         for (String lexiconPath : lexiconPaths) {
             builder = builder.lexicon(Paths.get(lexiconPath));
         }
+        builder = builder.signature(signature);
 
         try (SeekableByteChannel ch = Files.newByteChannel(outputPath, StandardOpenOption.WRITE,
                 StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING)) {

src/main/java/com/worksap/nlp/sudachi/dictionary/build/DicBuilder.java

@@ -202,17 +202,21 @@ private System readMatrix(String name, IOSupplier<InputStream> input, long size)
         }
 
         /**
-         * Set the system dictionary signature to the provided string. By default, it is
-         * current timestamp and a random 8 hexadecimal characters.
+         * Set the system dictionary signature to the provided string.
+         * 
+         * If null is provided, set the default value that consists of current timestamp
+         * and a 8 hexadecimal hashcode calculated from the comment.
          * 
          * @param signature
          *            provided dictionary signature. Can not be empty.
          * @return current object
          */
         public System signature(String signature) {
             if (signature == null) {
-                throw new IllegalArgumentException("signature can not be null");
+                description.setDefaultSignature();
+                return this;
             }
+
             if (signature.isEmpty()) {
                 throw new IllegalArgumentException("signature can not be empty");
             }