adds R.stableSort and tests

src/stableSort.js

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+var _curry2 = require('./internal/_curry2');
+
+/**
+ * Returns a copy of the list, sorted according to the comparator function, which should accept two values at a
+ * time and return a negative number if the first value is smaller, a positive number if it's larger, and zero
+ * if they are equal, values of the same key retain their original order. Please note that this is a **copy** of the list.
+ *  It does not modify the original.
+ * @func
+ * @memberOf R
+ * @category List
+ * @sig (a,a -> Number) -> [a] -> [a]
+ * @param {Function} comparator A sorting function :: a -> b -> Int
+ * @param {Array} list The list to sort
+ * @return {Array} a new array with its elements sorted by the comparator function.
+ * @example
+ *
+ *      var diff = function(a, b) { return a - b; };
+ *      R.sort(diff, [4,2,7,5]); //=> [2, 4, 5, 7]
+ */
+module.exports = _curry2(function stableSort(comparator, list) {
+  var sorted = [];
+  var idx;
+
+  idx = 0;
+  while (idx < list.length) {
+    sorted.push([list[idx], idx]);
+    idx += 1;
+  }
+
+  sorted.sort(function(a, b) {
+    var order = comparator(a[0], b[0]);
+    return order === 0 ? a[1] - b[1] : order;
+  });
+
+  idx = 0;
+  while (idx < list.length) {
+    sorted[idx] = sorted[idx][0];
+    idx += 1;
+  }
+  return sorted;
+});

test/stableSort.js

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+var R = require('..');
+var eq = require('./shared/eq');
+
+
+describe('stableSort', function() {
+  it('sorts the elements of a list', function() {
+    eq(R.sort(function(a, b) {return a - b;}, [3, 1, 8, 1, 2, 5]), [1, 1, 2, 3, 5, 8]);
+  });
+
+  it('does not affect the list passed supplied', function() {
+    var list = [3, 1, 8, 1, 2, 5];
+    eq(R.sort(function(a, b) {return a - b;}, list), [1, 1, 2, 3, 5, 8]);
+    eq(list, [3, 1, 8, 1, 2, 5]);
+  });
+
+  it('is curried', function() {
+    var sortByLength = R.sort(function(a, b) {return a.length - b.length;});
+    eq(sortByLength(['one', 'two', 'three', 'four', 'five', 'six']),
+       ['one', 'two', 'six', 'four', 'five', 'three']);
+  });
+
+  it('maintains stability', function() {
+    var list = [
+      {type: 'Apple', price: 10},
+      {type: 'Pear', price: 100},
+      {type: 'Bannana', price: 10},
+      {type: 'Orange', price: 20}
+    ];
+
+    eq(R.stableSort(function(a, b) {return a.price - b.price;}, list),
+      [
+        {type: 'Apple', price: 10},
+        {type: 'Bannana', price: 10},
+        {type: 'Orange', price: 20},
+        {type: 'Pear', price: 100}
+      ]
+    );
+  });
+});