+
+ elements = pool->elements;
+
+ /* We have exclusive access to the structure here, so we can reset
+ bottom and top counters, and prune invalid sparks. Contents are
+ copied in-place if they are valuable, otherwise discarded. The
+ routine uses "real" indices t and b, starts by computing them
+ as the modulus size of top and bottom,
+
+ Copying:
+
+ At the beginning, the pool structure can look like this:
+ ( bottom % size >= top % size , no wrap-around)
+ t b
+ ___________***********_________________
+
+ or like this ( bottom % size < top % size, wrap-around )
+ b t
+ ***********__________******************
+ As we need to remove useless sparks anyway, we make one pass
+ between t and b, moving valuable content to b and subsequent
+ cells (wrapping around when the size is reached).
+
+ b t
+ ***********OOO_______XX_X__X?**********
+ ^____move?____/
+
+ After this movement, botInd becomes the new bottom, and old
+ bottom becomes the new top index, both as indices in the array
+ size range.
+ */
+ // starting here
+ currInd = (pool->top) & (pool->moduloSize); // mod
+
+ // copies of evacuated closures go to space from botInd on
+ // we keep oldBotInd to know when to stop
+ oldBotInd = botInd = (pool->bottom) & (pool->moduloSize); // mod
+
+ // on entry to loop, we are within the bounds
+ ASSERT( currInd < pool->size && botInd < pool->size );
+
+ while (currInd != oldBotInd ) {
+ /* must use != here, wrap-around at size
+ subtle: loop not entered if queue empty
+ */
+
+ /* check element at currInd. if valuable, evacuate and move to
+ botInd, otherwise move on */
+ spark = elements[currInd];
+
+ /* if valuable work: shift inside the pool */
+ if ( closure_SHOULD_SPARK(spark) ) {
+ elements[botInd] = spark; // keep entry (new address)
+ botInd++;
+ n++;
+ } else {
+ pruned_sparks++; // discard spark
+ }
+ currInd++;
+
+ // in the loop, we may reach the bounds, and instantly wrap around
+ ASSERT( currInd <= pool->size && botInd <= pool->size );
+ if ( currInd == pool->size ) { currInd = 0; }
+ if ( botInd == pool->size ) { botInd = 0; }
+
+ } // while-loop over spark pool elements
+
+ ASSERT(currInd == oldBotInd);
+
+ pool->top = oldBotInd; // where we started writing
+ pool->topBound = pool->top;
+
+ pool->bottom = (oldBotInd <= botInd) ? botInd : (botInd + pool->size);
+ // first free place we did not use (corrected by wraparound)
+
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