Add spellcheck and word suggestion to the prediction service

third_party/prediction/suggest/policyimpl/dictionary/utils/probability_utils.h

+/*
+ * Copyright (C) 2013, The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef LATINIME_PROBABILITY_UTILS_H
+#define LATINIME_PROBABILITY_UTILS_H
+
+#include "third_party/prediction/defines.h"
+
+namespace latinime {
+
+// TODO: Quit using bigram probability to indicate the delta.
+class ProbabilityUtils {
+ public:
+  static AK_FORCE_INLINE int backoff(const int unigramProbability) {
+    return unigramProbability;
+    // For some reason, applying the backoff weight gives bad results in tests.
+    // To apply the
+    // backoff weight, we divide the probability by 2, which in our storing
+    // format means
+    // decreasing the score by 8.
+    // TODO: figure out what's wrong with this.
+    // return unigramProbability > 8 ?
+    //         unigramProbability - 8 : (0 == unigramProbability ? 0 : 8);
+  }
+
+  static AK_FORCE_INLINE int computeProbabilityForBigram(
+      const int unigramProbability,
+      const int bigramProbability) {
+    // We divide the range [unigramProbability..255] in 16.5 steps - in other
+    // words, we want
+    // the unigram probability to be the median value of the 17th step from the
+    // top. A value of
+    // 0 for the bigram probability represents the middle of the 16th step from
+    // the top,
+    // while a value of 15 represents the middle of the top step.
+    // See makedict.BinaryDictEncoder#makeBigramFlags for details.
+    const float stepSize =
+        static_cast<float>(MAX_PROBABILITY - unigramProbability) /
+        (1.5f + MAX_BIGRAM_ENCODED_PROBABILITY);
+    return unigramProbability +
+           static_cast<int>(static_cast<float>(bigramProbability + 1) *
+                            stepSize);
+  }
+
+ private:
+  DISALLOW_IMPLICIT_CONSTRUCTORS(ProbabilityUtils);
+};
+}
+#endif /* LATINIME_PROBABILITY_UTILS_H */