1<!doctype html> 2<html> 3<head> 4<title>Calculator Implementation Overview</title> 5<meta charset="UTF-8"> 6</head> 7<h1>M Calculator Implementation Overview</h1> 8<p>Although the appearance of the calculator has changed little from Lollipop, and some of the UI 9code is indeed the same, the rest of the code has changed substantially. Unsurprisingly, 10<b>Calculator.java</b> implements the main UI. The other major parts of the implementation 11are:</p> 12 13<p><b>CR.java</b> in <b>external/crcalc</b> provides the underlying demand-driven ("constructive 14real") arithmetic implementation. Numbers are represented primarily as objects with a method that 15can compute arbitrarily precise approximations. The actual arithmetic performed by these methods 16is based on Java's <tt>java.util.BigInteger</tt> arithmetic, with appropriate implicit 17scaling.</p> 18 19<p><b>BoundedRational.java</b> is a rational arithmetic package that is used to provide finite 20exact answers in "easy" cases. It is used primarily to determine when an approximation provided 21by CR.java is actually exact. This is used in turn both to limit the length of displayed results 22and scrolling, as well as to identify errors such as division by zero, that would otherwise result 23in timeouts during computations. It is in some sense not needed to produce correct results, but 24it significantly improves the usability of the calculator. It is also used for the "display as 25fraction" option in the overflow menu.</p> 26 27<p><b>CalculatorExpr.java</b> implements calculator arithmetic expressions. It supports editing, 28saving, restoring, and evaluation of expressions. Evaluation produces a constructive real (CR) 29and possibly a BoundedRational result. Unlike the "arity" library used in earlier versions, the 30underlying expression is represented as a sequence of "tokens", many of which are represented by 31Button ids, not as a character string.</p> 32 33<p><b>Evaluator.java</b> implements much of the actual calculator logic, particularly background 34expression evaluation. Expression evaluation here includes both using CalculatorExpr.java to 35evaluate the expression, and then invoking the resulting CR value to actually produce finite 36approximations and convert them to decimal. Two types of expression evaluation are supported: 37(1) Initial evaluation of the expression and producing an initial decimal approximation, and (2) 38reevaluation to higher precision. (1) is invoked directly from the Calculator UI, while (2) is 39invoked from the calculator display, commonly in response to scrolling. When the display requests 40a result, a "result" is immediately returned, though it may contains blank placeholders. The 41display is then notified when the real result becomes available.</p> 42 43<p><b>CalculatorText.java</b> is the TextView subclass used to display the formula.</p> 44 45<p><b>CalculatorResult.java</b> is the TextView subclass used to display the result. It handles 46result formatting, scrolling, etc. After the user hits "=", the CalculatorResult widget moves 47into the top position, replacing the formula display. Currently it remains in that position until 48the formula is again modified.</p> 49</body> 50</html> 51 52