statistical encoding

brouter-codec/src/main/java/btools/codec/NoisyDiffCoder.java

@@ -0,0 +1,92 @@
+package btools.codec;
+
+/**
+ * Encoder/Decoder for signed integers that automatically detects the typical
+ * range of these numbers to determine a noisy-bit count as a very simple
+ * dictionary
+ * 
+ * Adapted for 3-pass encoding (counters -> statistics -> encoding )
+ * but doesn't do anything at pass1
+ */
+public final class NoisyDiffCoder
+{
+  private int tot;
+  private int[] freqs;
+  private int noisybits;
+  private StatCoderContext bc;
+  private int pass;
+
+  /**
+   * Create a decoder and read the noisy-bit count from the gibe context
+   */
+  public NoisyDiffCoder( StatCoderContext bc )
+  {
+    noisybits = bc.decodeVarBits();
+    this.bc = bc;
+  }
+
+  /**
+   * Create an encoder for 3-pass-encoding
+   */
+  public NoisyDiffCoder()
+  {
+  }
+
+  /**
+   * encodes a signed int (pass3 only, stats collection in pass2)
+   */
+  public void encodeSignedValue( int value )
+  {
+    if ( pass == 3 )
+    {
+      bc.encodeNoisyDiff( value, noisybits );
+    }
+    else if ( pass == 2 )
+    {
+      count( value < 0 ? -value : value );
+    }
+  }
+
+  /**
+   * decodes a signed int
+   */
+  public int decodeSignedValue()
+  {
+    return bc.decodeNoisyDiff( noisybits );
+  }
+
+  /**
+   * Starts a new encoding pass and (in pass3) calculates the noisy-bit count
+   * from the stats collected in pass2 and writes that to the given context
+   */
+  public void encodeDictionary( StatCoderContext bc )
+  {
+    if ( ++pass == 3 )
+    {
+      // how many noisy bits?
+      for ( noisybits = 0; noisybits < 14 && tot > 0; noisybits++ )
+      {
+        if ( freqs[noisybits] < ( tot >> 1 ) )
+          break;
+      }
+      bc.encodeVarBits( noisybits );
+    }
+    this.bc = bc;
+  }
+
+  private void count( int value )
+  {
+    if ( freqs == null )
+      freqs = new int[14];
+    int bm = 1;
+    for ( int i = 0; i < 14; i++ )
+    {
+      if ( value < bm )
+        break;
+      else
+        freqs[i]++;
+      bm <<= 1;
+    }
+    tot++;
+  }
+}