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      1 /*
      2  * Copyright (C) 2014 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *   http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package com.android.calculator2;
     18 
     19 import org.javia.arity.Symbols;
     20 import org.javia.arity.SyntaxException;
     21 import org.javia.arity.Util;
     22 
     23 public class CalculatorExpressionEvaluator {
     24 
     25     /**
     26      * The maximum number of significant digits to display.
     27      */
     28     private static final int MAX_DIGITS = 12;
     29 
     30     /**
     31      * A {@link Double} has at least 17 significant digits, we show the first {@link #MAX_DIGITS}
     32      * and use the remaining digits as guard digits to hide floating point precision errors.
     33      */
     34     private static final int ROUNDING_DIGITS = Math.max(17 - MAX_DIGITS, 0);
     35 
     36     private final Symbols mSymbols;
     37     private final CalculatorExpressionTokenizer mTokenizer;
     38 
     39     public CalculatorExpressionEvaluator(CalculatorExpressionTokenizer tokenizer) {
     40         mSymbols = new Symbols();
     41         mTokenizer = tokenizer;
     42     }
     43 
     44     public void evaluate(CharSequence expr, EvaluateCallback callback) {
     45         evaluate(expr.toString(), callback);
     46     }
     47 
     48     public void evaluate(String expr, EvaluateCallback callback) {
     49         expr = mTokenizer.getNormalizedExpression(expr);
     50 
     51         // remove any trailing operators
     52         while (expr.length() > 0 && "+-/*".indexOf(expr.charAt(expr.length() - 1)) != -1) {
     53             expr = expr.substring(0, expr.length() - 1);
     54         }
     55 
     56         try {
     57             if (expr.length() == 0 || Double.valueOf(expr) != null) {
     58                 callback.onEvaluate(expr, null, Calculator.INVALID_RES_ID);
     59                 return;
     60             }
     61         } catch (NumberFormatException e) {
     62             // expr is not a simple number
     63         }
     64 
     65         try {
     66             double result = mSymbols.eval(expr);
     67             if (Double.isNaN(result)) {
     68                 callback.onEvaluate(expr, null, R.string.error_nan);
     69             } else {
     70                 // The arity library uses floating point arithmetic when evaluating the expression
     71                 // leading to precision errors in the result. The method doubleToString hides these
     72                 // errors; rounding the result by dropping N digits of precision.
     73                 final String resultString = mTokenizer.getLocalizedExpression(
     74                         Util.doubleToString(result, MAX_DIGITS, ROUNDING_DIGITS));
     75                 callback.onEvaluate(expr, resultString, Calculator.INVALID_RES_ID);
     76             }
     77         } catch (SyntaxException e) {
     78             callback.onEvaluate(expr, null, R.string.error_syntax);
     79         }
     80     }
     81 
     82     public interface EvaluateCallback {
     83         public void onEvaluate(String expr, String result, int errorResourceId);
     84     }
     85 }
     86