fix(simplify): Improve collection of nonconstant like terms (#2384)

The key is that the rule-matching engine is optimized for finding matches
  on the lefts of terms, but the central term-collection rule
  `n1*n3 + n2*n3 -> (n1+n2)*n3` was written with the key common term rightmost.
  Reversing this rule to `n3*n1 + n3*n2 -> n3*(n1+n2)` therefore does most
  of the work of improving like-term collection. It also better corresponds
  to typical mathematical presentation: common terms tend to be pulled out
  to the left in common practice.

  Floating constants to the right of a product initially (before they are
  moved back to the left for human-preferred output) and ensuring that
  negations are subsumed into constants whenever possible did the rest.

  Also, rule context was not being propagated into the simplification
  engine; this commit corrects that.

  Resolves #1179.
  Resolves #1290.

Co-authored-by: Jos de Jong <wjosdejong@gmail.com>

src/function/algebra/simplify.js

@@ -285,12 +285,20 @@ export const createSimplify = /* #__PURE__ */ factory(name, dependencies, (
     { l: 'log(e)', r: '1' },
 
     // temporary rules
+    // Note initially we tend constants to the right because like-term
+    // collection prefers the left, and we would rather collect nonconstants
     { l: 'n-n1', r: 'n+-n1' }, // temporarily replace 'subtract' so we can further flatten the 'add' operator
-    { l: '-(c*v)', r: '(-c) * v' }, // make non-constant terms positive
-    { l: '-v', r: '(-1) * v' },
+    { l: '-(c*v)', r: 'v * (-c)' }, // make non-constant terms positive
+    { l: '-v', r: 'v * (-1)' },
     { l: 'n/n1^n2', r: 'n*n1^-n2' }, // temporarily replace 'divide' so we can further flatten the 'multiply' operator
     { l: 'n/n1', r: 'n*n1^-1' },
 
+    // remove parenthesis in the case of negating a quantity
+    { l: 'n1 + (n2 + n3)*(-1)', r: 'n1 + n2*(-1) + n3*(-1)' },
+    // subsume resulting -1 into constants where possible
+    { l: '(-1) * c', r: '-c' },
+    { l: '(-1) * (-c)', r: 'c' },
+
     // expand nested exponentiation
     { l: '(n ^ n1) ^ n2', r: 'n ^ (n1 * n2)' },
 
@@ -302,17 +310,15 @@ export const createSimplify = /* #__PURE__ */ factory(name, dependencies, (
     // collect like terms
     { l: 'n+n', r: '2*n' },
     { l: 'n+-n', r: '0' },
-    { l: 'n1*n2 + n2', r: '(n1+1)*n2' },
-    { l: 'n1*n3 + n2*n3', r: '(n1+n2)*n3' },
-
-    // remove parenthesis in the case of negating a quantitiy
-    { l: 'n1 + -1 * (n2 + n3)', r: 'n1 + -1 * n2 + -1 * n3' },
+    { l: 'v*n + v', r: 'v*(n+1)' }, // NOTE: leftmost position is special:
+    { l: 'n3*n1 + n3*n2', r: 'n3*(n1+n2)' }, // All sub-monomials tried there.
+    { l: 'n*c + c', r: '(n+1)*c' },
 
     simplifyConstant,
 
     { l: '(-n)*n1', r: '-(n*n1)' }, // make factors positive (and undo 'make non-constant terms positive')
 
-    // ordering of constants
+    // final ordering of constants
     { l: 'c+v', r: 'v+c', context: { add: { commutative: false } } },
     { l: 'v*c', r: 'c*v', context: { multiply: { commutative: false } } },
 
@@ -376,7 +382,7 @@ export const createSimplify = /* #__PURE__ */ factory(name, dependencies, (
             r: removeParens(parse(rule.r))
           }
           if (rule.context) {
-            newRule.evaluate = rule.context
+            newRule.context = rule.context
           }
           if (rule.evaluate) {
             newRule.evaluate = parse(rule.evaluate)

test/unit-tests/function/algebra/simplify.test.js

@@ -231,7 +231,12 @@ describe('simplify', function () {
     simplifyAndCompare('x+x', '2*x')
     simplifyAndCompare('2x+x', '3*x')
     simplifyAndCompare('2(x+1)+(x+1)', '3*(x + 1)')
-    simplifyAndCompare('y*x^2+2*x^2', '(y+2)*x^2')
+    simplifyAndCompare('2(x+1)+x+1', '3*(x + 1)')
+    simplifyAndCompare('y*x^2+2*x^2', 'x^2*(y+2)')
+    simplifyAndCompare('x*y + y*x', '2*x*y')
+    simplifyAndCompare('x*y - y*x', '0')
+    simplifyAndCompare('x^2*y^3*z - y*z*y*x^2*y', '0')
+    simplifyAndCompare('x^2*y^3*z - y*z*x^2*y', 'x^2*z*(y^3-y^2)')
   })
 
   it('should collect separated like terms', function () {
@@ -246,6 +251,7 @@ describe('simplify', function () {
     simplifyAndCompare('10 - (x - 2)', '12 - x')
     simplifyAndCompare('x - (y + x)', '-y')
     simplifyAndCompare('x - (y - (y - x))', '0')
+    simplifyAndCompare('5 + (5 * x) - (3 * x) + 2', '2*x+7')
   })
 
   it('should collect separated like factors', function () {