Add log_10() function to calculator

Test Plan:
Loaded http://exercises.ka.local/test and watched tests pass
Loaded http://exercises.ka.local/exercises/law_of_cosines.html
Calculator looked reasonable.
Clicking "log" button inserted "log("
Entered "log(10)" and got 1
Entered "log(1000)" and got 3
Entered "log(0)" and got an error
Entered "log(0.001)" and got -3
Entered "log(e)" and got something irrational looking (0.434294482)
Entered "ln(e)" and got 1

Reviewers: charlie, joel

Reviewed By: joel

Subscribers: joel, justin

Maniphest Tasks: T2681

Differential Revision: http://phabricator.khanacademy.org/D10486

build/calculator/calculator-tail.js

@@ -92,6 +92,13 @@ window.Calculator = (function(parser) {
                             throw new CalculatorError($._("undefined"));
                         }
                         return ans;
+                    },
+                    log: function(a) {
+                        var ans = Math.log(a) / Math.LN10;
+                        if (isNaN(ans) || !isFinite(ans)) {
+                            throw new CalculatorError($._("undefined"));
+                        }
+                        return ans;
                     }
                 };
 

exercises/khan-exercise.html

@@ -81,10 +81,10 @@
                     <a href="#" data-text="sin(">sin</a><a href="#" data-text="cos(">cos</a><a href="#" data-text="tan(">tan</a><a href="#" data-text="sqrt(">√</a><a href="#" data-text="^">x<sup>y</sup></a>
                     </div>
                     <div class="calc-row">
-                    <a href="#" data-text="e^">e<sup>x</sup></a><a href="#" data-text="ln(">ln</a><a href="#" data-text="pi">&pi;</a><a href="#">(</a><a href="#">)</a>
+                    <a href="#" data-text="e^">e<sup>x</sup></a><a href="#" data-text="ln(">ln</a><a href="#" data-text="log(">log</a><a href="#" data-text="pi">&pi;</a>
                     </div>
                     <div class="calc-row">
-                    <a href="#" class="dark">7</a><a href="#" class="dark">8</a><a href="#" class="dark">9</a>
+                    <a href="#" class="dark">7</a><a href="#" class="dark">8</a><a href="#" class="dark">9</a><a href="#">(</a><a href="#">)</a>
                     </div>
                     <div class="calc-row">
                     <a href="#" class="dark">4</a><a href="#" class="dark">5</a><a href="#" class="dark">6</a><a href="#" data-text="*">×</a><a href="#" data-text="/">÷</a>

test/calculator.js

@@ -54,13 +54,16 @@
     });
 
 
-    asyncTest("pi, euler and natural logarithm", 6, function() {
+    asyncTest("pi, euler and logarithms", 9, function() {
         calculateStartsWith('pi', 3.1415);
         calculateStartsWith('2*pi', 6.2831);
         calculateStartsWith('e', 2.7182);
         calculateStrictEqual('ln(e^2)', 2);
         checkError('ln(-1)', /undefined/, 'ln for x < 0 is undefined (for x=-1)');
         checkError('ln(0)', /undefined/, 'ln for 0 is -infinity (undefined?)');
+        calculateStrictEqual('log(10000)', 4);
+        checkError('log(-1)', /undefined/, 'log for x < 0 is undefined (for x=-1)');
+        checkError('log(0)', /undefined/, 'log for 0 is -infinity (undefined?)');
         start();
     });