adding an index.js file for factoring and exposing it to top level of the mathsteps module

Author: aelnaiem

index.js

@@ -1,8 +1,10 @@
 const ChangeTypes = require('./lib/ChangeTypes');
+const factor = require('./lib/factor');
 const simplifyExpression = require('./lib/simplifyExpression');
 const solveEquation = require('./lib/solveEquation');
 
 module.exports = {
+  factor,
   simplifyExpression,
   solveEquation,
   ChangeTypes,

lib/factor/factorQuadratic.js

@@ -3,9 +3,7 @@ const math = require('mathjs');
 const ConstantFactors = require('./ConstantFactors');
 
 const ChangeTypes = require('../ChangeTypes');
-const checks = require('../checks');
 const evaluate = require('../util/evaluate');
-const flatten = require('../util/flattenOperands');
 const Negative = require('../Negative');
 const Node = require('../node');
 
@@ -31,11 +29,6 @@ const FACTOR_FUNCTIONS = [
 //        requires us simplify the following only within the parens:
 //        a(x - (-b + sqrt(b^2 - 4ac)) / 2a)(x - (-b - sqrt(b^2 - 4ac)) / 2a)
 function factorQuadratic(node) {
-  node = flatten(node);
-  if (!checks.isQuadratic(node)) {
-    return Node.Status.noChange(node);
-  }
-
   // get a, b and c
   let symbol, aValue = 0, bValue = 0, cValue = 0;
   for (const term of node.args) {

lib/factor/index.js

@@ -0,0 +1,19 @@
+const math = require('mathjs');
+const stepThrough = require('./stepThrough');
+
+function factorString(expressionString, debug=false) {
+  let node;
+  try {
+    node = math.parse(expressionString);
+  }
+  catch (err) {
+    return [];
+  }
+
+  if (node) {
+    return stepThrough(node, debug);
+  }
+  return [];
+}
+
+module.exports = factorString;

lib/factor/stepThrough.js

@@ -0,0 +1,37 @@
+const checks = require('../checks');
+
+const factorQuadratic = require('./factorQuadratic');
+
+const flattenOperands = require('../util/flattenOperands');
+const removeUnnecessaryParens = require('../util/removeUnnecessaryParens');
+
+// Given a mathjs expression node, steps through factoring the expression.
+// Currently only supports factoring quadratics.
+// Returns a list of details about each step.
+function stepThrough(node, debug=false) {
+  if (debug) {
+    // eslint-disable-next-line
+    console.log('\n\nFactoring: ' + print(node, false, true));
+  }
+
+  if (checks.hasUnsupportedNodes(node)) {
+    return [];
+  }
+
+  let nodeStatus;
+  const steps = [];
+
+  node = flattenOperands(node);
+  node = removeUnnecessaryParens(node, true);
+  if (checks.isQuadratic(node)) {
+    nodeStatus = factorQuadratic(node);
+    if (nodeStatus.hasChanged()) {
+      steps.push(nodeStatus);
+    }
+  }
+  // Add factoring higher order polynomials...
+
+  return steps;
+}
+
+module.exports = stepThrough;

lib/simplifyExpression/stepThrough.js

@@ -43,8 +43,10 @@ function stepThrough(node, debug=false) {
       logSteps(nodeStatus);
     }
     steps.push(removeUnnecessaryParensInStep(nodeStatus));
-    const nextNode = Status.resetChangeGroups(nodeStatus.newNode);
-    nodeStatus = step(nextNode);
+
+    node = Status.resetChangeGroups(nodeStatus.newNode);
+    nodeStatus = step(node);
+
     if (iters++ === MAX_STEP_COUNT) {
       // eslint-disable-next-line
       console.error('Math error: Potential infinite loop for expression: ' +

test/factor/factor.test.js

@@ -0,0 +1,33 @@
+const assert = require('assert');
+const factor = require('../../lib/factor');
+const print = require('../../lib/util/print');
+
+const NO_STEPS = 'no-steps';
+
+function testFactor(expressionString, outputStr, debug=false) {
+  const steps = factor(expressionString, debug);
+  let lastStep;
+  if (steps.length === 0) {
+    lastStep = NO_STEPS;
+  }
+  else {
+    lastStep = print(steps[steps.length -1].newNode);
+  }
+  it(expressionString + ' -> ' + outputStr, (done) => {
+    assert.equal(lastStep, outputStr);
+    done();
+  });
+}
+
+describe('factor expressions', function () {
+  const tests = [
+    ['x^2', NO_STEPS],
+    ['x^2 + 2x', 'x * (x + 2)'],
+    ['x^2 - 4', '(x + 2) * (x - 2)'],
+    ['x^2 + 4', NO_STEPS],
+    ['x^2 + 2x + 1', '(x + 1)^2'],
+    ['x^2 + 3x + 2', '(x + 1) * (x + 2)'],
+  ];
+  tests.forEach(t => testFactor(t[0], t[1], t[2]));
+});
+

test/factor/factorQuadratic.test.js

@@ -29,6 +29,7 @@ describe('factorQuadratic', function () {
     ['-x^2 - 3x - 2', '-(x + 1) * (x + 2)'],
     ['-x^2 + 1', '-(x + 1) * (x - 1)'],
     ['-x^2 - 1', '-x^2 - 1'],
+    //['2x^2 + 5x + 3','(x + 3) * (2x - 1)'], // TODO(ael)
   ];
   tests.forEach(t => testFactorQuadratic(t[0], t[1]));
 });