c3c/lib/std/sort/quicksort.c3

module std::sort;
import std::sort::qs;

/**
 * Sort list using the quick sort algorithm.
 * @require is_searchable(list) "The list must be indexable and support .len or .len()"
 * @require !cmp || is_comparer(cmp, list) "Expected a comparison function which compares values"
 **/
macro quicksort(list, cmp = null) @builtin
{
	var $Type = $typeof(list);
	var $CmpType = $typeof(cmp);
	usz len = sort::@len_from_list(list);
	qs::qsort(<$Type, $CmpType>)(list, 0, (isz)len - 1, cmp);
}

module std::sort::qs(<Type, Comparer>);

def ElementType = $typeof(Type{}[0]);

struct StackElementItem @private
{
	isz low;
	isz high;
}

def Stack = StackElementItem[64] @private;

// Based on https://alienryderflex.com/quicksort by Darel Rex Finley, Public Domain.

fn void qsort(Type list, isz low, isz high, Comparer cmp)
{
	if (low >= 0 && high >= 0 && low < high)
	{
		Stack stack;
		stack[0].low = low;
		stack[0].high = high;
		isz i;
		isz l;
		isz h;
		while (i >= 0)
		{
		    l = stack[i].low;
		    h = stack[i].high;

		    if (l < h)
		    {
			ElementType pivot = list[l];
			while (l < h)
			{
				$switch
					$case $checks(cmp(list[0], list[0])):
						while (cmp(list[h], pivot) >= 0 && l < h) h--;
					$case $checks(cmp(&list[0], &list[0])):
						while (cmp(&list[h], &pivot) >= 0 && l < h) h--;
					$default:
						while (greater_eq(list[h], pivot) && l < h) h--;
				$endswitch
				if (l < h) list[l++] = list[h];
				$switch
					$case $checks(cmp(list[0], list[0])):
						while (cmp(list[l], pivot) <= 0 && l < h) l++;
					$case $checks(cmp(&list[0], &list[0])):
						while (cmp(&list[l], &pivot) <= 0 && l < h) l++;
					$default:
						while (less_eq(list[l], pivot) && l < h) l++;
				$endswitch
				if (l < h) list[h--] = list[l];
			}
			list[l] = pivot;
			stack[i + 1].low = l + 1;
			stack[i + 1].high = stack[i].high;
			stack[i++].high = l;
			if (stack[i].high - stack[i].low > stack[i - 1].high - stack[i - 1].low)
			{
				@swap(stack[i], stack[i - 1]);
			}
		    }
		    else
		    {
			i--;
		    }
		}
	}
}