docs/implementation-overview.html - LeafOS-Project/android_packages_apps_ExactCalculator - Gitiles

 <!doctype html>
 <html>
 <head>
 <title>Calculator Implementation Overview</title>
 <meta charset="UTF-8">
 </head>
 <h1>M Calculator Implementation Overview</h1>
 <p>Although the appearance of the calculator has changed little from Lollipop, and some of the UI
 code is indeed the same, the rest of the code has changed substantially. Unsurprisingly,
 <b>Calculator.java</b> implements the main UI. The other major parts of the implementation
 are:</p>

 <p><b>CR.java</b> in <b>external/crcalc</b> provides the underlying demand-driven ("constructive
 real") arithmetic implementation. Numbers are represented primarily as objects with a method that
 can compute arbitrarily precise approximations. The actual arithmetic performed by these methods
 is based on Java's <tt>java.util.BigInteger</tt> arithmetic, with appropriate implicit
 scaling.</p>

 <p><b>BoundedRational.java</b> is a rational arithmetic package that is used to provide finite
 exact answers in "easy" cases. It is used primarily to determine when an approximation provided
 by CR.java is actually exact. This is used in turn both to limit the length of displayed results
 and scrolling, as well as to identify errors such as division by zero, that would otherwise result
 in timeouts during computations. It is in some sense not needed to produce correct results, but
 it significantly improves the usability of the calculator. It is also used for the "display as
 fraction" option in the overflow menu.</p>

 <p><b>CalculatorExpr.java</b> implements calculator arithmetic expressions. It supports editing,
 saving, restoring, and evaluation of expressions. Evaluation produces a constructive real (CR)
 and possibly a BoundedRational result. Unlike the "arity" library used in earlier versions, the
 underlying expression is represented as a sequence of "tokens", many of which are represented by
 Button ids, not as a character string.</p>

 <p><b>Evaluator.java</b> implements much of the actual calculator logic, particularly background
 expression evaluation. Expression evaluation here includes both using CalculatorExpr.java to
 evaluate the expression, and then invoking the resulting CR value to actually produce finite
 approximations and convert them to decimal. Two types of expression evaluation are supported:
 (1) Initial evaluation of the expression and producing an initial decimal approximation, and (2)
 reevaluation to higher precision. (1) is invoked directly from the Calculator UI, while (2) is
 invoked from the calculator display, commonly in response to scrolling. When the display requests
 a result, a "result" is immediately returned, though it may contains blank placeholders. The
 display is then notified when the real result becomes available.</p>

 <p><b>CalculatorText.java</b> is the TextView subclass used to display the formula.</p>

 <p><b>CalculatorResult.java</b> is the TextView subclass used to display the result. It handles
 result formatting, scrolling, etc. After the user hits "=", the CalculatorResult widget moves
 into the top position, replacing the formula display. Currently it remains in that position until
 the formula is again modified.</p>
 </body>
 </html>
	<!doctype html>
	<html>
	<head>
	<title>Calculator Implementation Overview</title>
	<meta charset="UTF-8">
	</head>
	<h1>M Calculator Implementation Overview</h1>
	<p>Although the appearance of the calculator has changed little from Lollipop, and some of the UI
	code is indeed the same, the rest of the code has changed substantially. Unsurprisingly,
	<b>Calculator.java</b> implements the main UI. The other major parts of the implementation
	are:</p>

	<p><b>CR.java</b> in <b>external/crcalc</b> provides the underlying demand-driven ("constructive
	real") arithmetic implementation. Numbers are represented primarily as objects with a method that
	can compute arbitrarily precise approximations. The actual arithmetic performed by these methods
	is based on Java's <tt>java.util.BigInteger</tt> arithmetic, with appropriate implicit
	scaling.</p>

	<p><b>BoundedRational.java</b> is a rational arithmetic package that is used to provide finite
	exact answers in "easy" cases. It is used primarily to determine when an approximation provided
	by CR.java is actually exact. This is used in turn both to limit the length of displayed results
	and scrolling, as well as to identify errors such as division by zero, that would otherwise result
	in timeouts during computations. It is in some sense not needed to produce correct results, but
	it significantly improves the usability of the calculator. It is also used for the "display as
	fraction" option in the overflow menu.</p>

	<p><b>CalculatorExpr.java</b> implements calculator arithmetic expressions. It supports editing,
	saving, restoring, and evaluation of expressions. Evaluation produces a constructive real (CR)
	and possibly a BoundedRational result. Unlike the "arity" library used in earlier versions, the
	underlying expression is represented as a sequence of "tokens", many of which are represented by
	Button ids, not as a character string.</p>

	<p><b>Evaluator.java</b> implements much of the actual calculator logic, particularly background
	expression evaluation. Expression evaluation here includes both using CalculatorExpr.java to
	evaluate the expression, and then invoking the resulting CR value to actually produce finite
	approximations and convert them to decimal. Two types of expression evaluation are supported:
	(1) Initial evaluation of the expression and producing an initial decimal approximation, and (2)
	reevaluation to higher precision. (1) is invoked directly from the Calculator UI, while (2) is
	invoked from the calculator display, commonly in response to scrolling. When the display requests
	a result, a "result" is immediately returned, though it may contains blank placeholders. The
	display is then notified when the real result becomes available.</p>

	<p><b>CalculatorText.java</b> is the TextView subclass used to display the formula.</p>

	<p><b>CalculatorResult.java</b> is the TextView subclass used to display the result. It handles
	result formatting, scrolling, etc. After the user hits "=", the CalculatorResult widget moves
	into the top position, replacing the formula display. Currently it remains in that position until
	the formula is again modified.</p>
	</body>
	</html>