update

Task/Entropy/Java/entropy.java

@@ -0,0 +1,51 @@
+import java.lang.Math;
+import java.util.Map;
+import java.util.HashMap;
+
+public class REntropy {
+
+  @SuppressWarnings("boxing")
+  public static double getShannonEntropy(String s) {
+    int n = 0;
+    Map<Character, Integer> occ = new HashMap<>();
+
+    for (int c_ = 0; c_ < s.length(); ++c_) {
+      char cx = s.charAt(c_);
+      if (occ.containsKey(cx)) {
+        occ.put(cx, occ.get(cx) + 1);
+      } else {
+        occ.put(cx, 1);
+      }
+      ++n;
+    }
+
+    double e = 0.0;
+    for (Map.Entry<Character, Integer> entry : occ.entrySet()) {
+      char cx = entry.getKey();
+      double p = (double) entry.getValue() / n;
+      e += p * log2(p);
+    }
+    return -e;
+  }
+
+  private static double log2(double a) {
+    return Math.log(a) / Math.log(2);
+  }
+  public static void main(String[] args) {
+    String[] sstr = {
+      "1223334444",
+      "1223334444555555555",
+      "122333",
+      "1227774444",
+      "aaBBcccDDDD",
+      "1234567890abcdefghijklmnopqrstuvwxyz",
+      "Rosetta Code",
+    };
+
+    for (String ss : sstr) {
+      double entropy = REntropy.getShannonEntropy(ss);
+      System.out.printf("Shannon entropy of %40s: %.12f%n", "\"" + ss + "\"", entropy);
+    }
+    return;
+  }
+}