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