fix(simplify): Correct regression in simplify (#2394)

* fix(simplify): Correct regression in simplify

  Also adds a 'debugConsole' option to simplify() so that it is possible
  to see the effect of each rule. This was critical to identifying the problem,
  which was that recent changes ended up with `simplifyConstant` in the
  wrong position in the list of rules. Correcting that also removed the need
  for the two rules coalescing negations with constants.

  Resolves #2393.

* fix(simplify): Correct another regression based on rule ordering

  Disccovered that `x - (y-y+x)` had also stopped simplifying due
  to recent changes, again because of re-ordering of the rules. So
  added it to the tests, and fixed the rule ordering (adding a more
  extensive comment about it). A big part of the reason that rule
  ordering is so sensitive is that the reduction engine only checks
  once in each pass for each rule whether it matches. So an alternate
  fix to changing the rule ordering back would have been to re-check
  each rule after it's applied (in case its application created new
  instances of the rule) but since the re-ordering worked, that seemed
  simpler as a fix for now.

src/function/algebra/simplify.js

@@ -140,11 +140,14 @@ export const createSimplify = /* #__PURE__ */ factory(name, dependencies, (
    * - [Symbolic computation - Simplification (Wikipedia)](https://en.wikipedia.org/wiki/Symbolic_computation#Simplification)
    *
    *  An optional `options` argument can be passed as last argument of `simplify`.
-   *  There is currently one option available:
-   *  - `exactFractions`: a boolean which is `true` by default.
-   *  - `fractionsLimit`: when `exactFractions` is true, a fraction will be returned
-   *    only when both numerator and denominator are smaller than `fractionsLimit`.
-   *    Default value is 10000.
+   *  Currently available options (defaults in parentheses):
+   *  - `consoleDebug` (false): whether to write the expression being simplified
+        and any changes to it, along with the rule responsible, to console
+   *  - `exactFractions` (true): whether to try to convert all constants to
+        exact rational numbers.
+   *  - `fractionsLimit` (10000): when `exactFractions` is true, constants will
+        be expressed as fractions only when both numerator and denominator
+        are smaller than `fractionsLimit`.
    *
    * Syntax:
    *
@@ -225,6 +228,7 @@ export const createSimplify = /* #__PURE__ */ factory(name, dependencies, (
     },
 
     'Node, Array, Map, Object': function (expr, rules, scope, options) {
+      const debug = options.consoleDebug
       rules = _buildRules(rules)
       let res = resolve(expr, scope)
       res = removeParens(res)
@@ -233,12 +237,26 @@ export const createSimplify = /* #__PURE__ */ factory(name, dependencies, (
       while (!visited[str]) {
         visited[str] = true
         _lastsym = 0 // counter for placeholder symbols
+        let laststr = str
+        if (debug) console.log('Working on: ', str)
         for (let i = 0; i < rules.length; i++) {
+          let rulestr = ''
           if (typeof rules[i] === 'function') {
             res = rules[i](res, options)
+            if (debug) rulestr = rules[i].name
           } else {
             flatten(res)
             res = applyRule(res, rules[i])
+            if (debug) {
+              rulestr = `${rules[i].l.toString()} -> ${rules[i].r.toString()}`
+            }
+          }
+          if (debug) {
+            const newstr = res.toString({ parenthesis: 'all' })
+            if (newstr !== laststr) {
+              console.log('Applying', rulestr, 'produced', newstr)
+              laststr = newstr
+            }
           }
           unflattenl(res) // using left-heavy binary tree here since custom rule functions may expect it
         }
@@ -309,11 +327,7 @@ export const createSimplify = /* #__PURE__ */ factory(name, dependencies, (
     { 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' },
+    simplifyConstant,
 
     // expand nested exponentiation
     { l: '(n ^ n1) ^ n2', r: 'n ^ (n1 * n2)' },
@@ -330,9 +344,15 @@ export const createSimplify = /* #__PURE__ */ factory(name, dependencies, (
     { l: 'n3*n1 + n3*n2', r: 'n3*(n1+n2)' }, // All sub-monomials tried there.
     { l: 'n*c + c', r: '(n+1)*c' },
 
-    simplifyConstant,
+    // remove parenthesis in the case of negating a quantity
+    // (It might seem this rule should precede collecting like terms,
+    // but putting it after gives another chance of noticing like terms,
+    // and any new like terms produced by this will be collected
+    // on the next pass through all the rules.)
+    { l: 'n1 + (n2 + n3)*(-1)', r: 'n1 + n2*(-1) + n3*(-1)' },
 
-    { l: '(-n)*n1', r: '-(n*n1)' }, // make factors positive (and undo 'make non-constant terms positive')
+    // make factors positive (and undo 'make non-constant terms positive')
+    { l: '(-n)*n1', r: '-(n*n1)' },
 
     // final ordering of constants
     { l: 'c+v', r: 'v+c', context: { add: { commutative: false } } },

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

@@ -270,12 +270,14 @@ describe('simplify', function () {
     simplifyAndCompare('x^2+x+3+x^2', '2*x^2+x+3')
     simplifyAndCompare('x+1+2x', '3*x+1')
     simplifyAndCompare('x-1+x', '2*x-1')
+    simplifyAndCompare('2-(x+1)', '1-x') // #2393
     simplifyAndCompare('x-1-2x+2', '1-x')
   })
 
   it('should collect like terms that are embedded in other terms', function () {
     simplifyAndCompare('10 - (x - 2)', '12 - x')
     simplifyAndCompare('x - (y + x)', '-y')
+    simplifyAndCompare('x - (y - y + x)', '0')
     simplifyAndCompare('x - (y - (y - x))', '0')
     simplifyAndCompare('5 + (5 * x) - (3 * x) + 2', '2*x+7')
   })