Add median-of-3 for Array.sort (1/3 of the compares for pre-sorted data)

Author: gfwilliams

ChangeLog

@@ -24,6 +24,7 @@
             I2C: Fix readReg to ensure it sends I2C STOP after the read
             Puck.js: E.getTemperature(true) will now use the uC's internal temp sensor (previously only external was possible)
             Add fast path to xIntArray.indexOf
+            Add median-of-3 for Array.sort (1/3 of the compares for pre-sorted data)
 
      2v27 : nRF5x: Ensure Bluetooth notifications work correctly when two separate connections use the same handle for their characteristics
             nRF5x: Remove handlers from our handlers array when a device is disconnected

src/jswrap_array.c

@@ -822,6 +822,40 @@ NO_INLINE static void _jswrap_array_sort(JsvIterator *head, int n, JsVar *compar
 
   JsvIterator pivot;
   jsvIteratorClone(&pivot, head);
+#ifndef SAVE_ON_FLASH
+  if (n>16) {
+    /* if we have enough elements where using the wrong pivot may be a problem, try a median of 3
+    to find the best pivot - this can be slower as we have to iterate over all the items in the list */
+    int midIndex = n>>1;
+    JsvIterator mid;
+    jsvIteratorClone(&mid, head);
+    for (int i=0; i<midIndex; i++) jsvIteratorNext(&mid);
+    JsvIterator high;
+    jsvIteratorClone(&high, &mid);
+    for (int i=midIndex; i<n-1; i++) jsvIteratorNext(&high);
+
+    JsVar *lowValue = jsvIteratorGetValue(&pivot);
+    JsVar *midValue = jsvIteratorGetValue(&mid);
+    JsVar *highValue = jsvIteratorGetValue(&high);
+
+    // now find the median of these three values
+    JsVarInt lowMid = _jswrap_array_sort_compare(lowValue, midValue, compareFn);
+    JsVarInt midHigh = _jswrap_array_sort_compare(midValue, highValue, compareFn);
+    JsVarInt lowHigh = _jswrap_array_sort_compare(lowValue, highValue, compareFn);
+    if ((lowMid<=0 && midHigh<=0) || (lowHigh<=0 && midHigh>=0)) {
+      // mid is median
+      jsvIteratorSetValue(&pivot, midValue);
+      jsvIteratorSetValue(&mid, lowValue);
+    } else if ((lowMid>=0 && lowHigh<=0) || (midHigh>=0 && lowHigh>=0)) {
+      // high is median
+      jsvIteratorSetValue(&pivot, highValue);
+      jsvIteratorSetValue(&high, lowValue);
+    } // else low is median, do nothing
+    jsvUnLock3(lowValue, midValue, highValue);
+    jsvIteratorFree(&mid);
+    jsvIteratorFree(&high);
+  }
+#endif
   bool pivotLowest = true; // is the pivot the lowest value in here?
   JsVar *pivotValue = jsvIteratorGetValue(&pivot);
   /* We're just going to use the first entry (head) as the pivot...
@@ -830,7 +864,7 @@ NO_INLINE static void _jswrap_array_sort(JsvIterator *head, int n, JsVar *compar
 
   int nlo = 0, nhigh = 0;
   JsvIterator it;
-  jsvIteratorClone(&it, head); //
+  jsvIteratorClone(&it, head);
   jsvIteratorNext(&it);