- 比较器是如何工作的?
我不确定这是否是问题的一部分,但为了明确比较器的工作原理:
您有一个由有序列表$order
指定的顺序和一个特殊的比较回调list_cmp
,它(应该)返回参数
$a
小于$b
(return -1
或值<0
)
$a
大于$b
(return 1
或值>0
)
$a
等于$b
(return 0
)
list_cmp
通过查找其顺序表来执行此操作,并检查是否
$a
具有更小的(或相同的)顺序,在这种情况下,循环使用return 0
提前退出,或者如果
$b
具有更小的顺序,在这种情况下,循环使用return 1
提前退出。
请注意,根据PHP文档,这是错误的,因为它希望返回正/负/0作为返回值。只有当您知道内部仅检查comparator($a,$b) > 0
时,这才是正确的,这意味着它只检查$b
是否小于而不等于$a
,使得比较$a<= $b的顺序
。如果代码开始检查$a
是否小于且不等于$b
,它很容易出错。
快速排序(quicksort)排序是如何工作的?
首先,我假设您正在使用PHP 7或更高版本。在这种情况下,对于6-15个元素大小的数组,您遇到了一个特殊情况。PHP 7+似乎不使用快速排序来排序短列表,而是使用插入排序变体(由于硬件相关的东西,如缓存/代码局部性,它可能更快)。您可以在Github PHP Mirror中检查排序源代码(例如:PHP 7.0 Zend/Zend_sort.c Line 177-198)。
该代码分为3个步骤:
- 比较:比较相邻元素
array[j]
和array[j+1]
,如果array[j] <= array[j+1]
则继续,否则转到步骤2。
- 查找插入点:如果现在
array[j] > array[j+1]
,向后扫描以找到一个位置,在该位置之前的x
满足array[x] < array[j+1] <= array[x+1]
(很明显只需要向前扫描直到x
达到起始位置)
- 插入:将元素
x+1 ... j
上移一位,使其变为x+2 ... j+1
,并将原来的元素插入到位置x+1
如果将该代码应用于配对(2-1, 2-3, 1-3, 2-4, 3-4, 2-2, 2-1, 2-1, 4-1, 3-1, 1-1, 2-2),就会清楚地知道该代码的作用。
-- [2,1],3,4,2,1,2 -> 1./2./3. compare [2,1], find and insert 1 before 2
-- 1,[2,3],4,2,1,2 -> 1./2. compare [2,3], find insert point for 3 (since order of 3 < order of 2)
-- [1,3],2,4,2,1,2 -> 3. compare [1,3], found insert point for 3 before 2
-- 1,3,[2,4],2,1,2 -> 1./2. compare [2,4], find insert point for 4 (since order of 4 < order of 2)
-- 1,[3,4],2,2,1,2 -> 3. compare [3,4], found insert point for 4 before 2
-- 1,3,4,[2,2],1,2 -> 1. compare [2,2], skip
-- 1,3,4,2,[2,1],2 -> 1./2. compare [2,1], find insert point for 1
-- 1,3,4,[2,1],2,2 -> 2. compare [2,1], find insert point for 1
-- 1,3,[4,1],2,2,2 -> 2. compare [4,1], find insert point for 1
-- 1,[3,1],4,2,2,2 -> 2. compare [3,1], find insert point for 1
-- [1,1],3,4,2,2,2 -> 3. compare [1,1], fond insert point for 1 before 3
-- 1,1,3,4,2,[2,2] -> 1. compare [2,2], skip
-- sorted: 1,1,3,4,2,2,2
注意:这里可以看到,即使是一个简单的排序算法(22行代码),也很难通过比较模式来推导出它的工作方式。PHP 7快速排序实现的代码行数约为原始版本的10倍,并且有一些疯狂的优化(除了由于选择枢轴和递归而产生的正常疯狂)。
大多数情况下,最好忽略深度实现细节,只减少到所需的内容。对于排序算法的典型问题是它是否稳定/不稳定,并以O(log n)
的性能和O(n)
内存消耗运行。有更简单的方法来学习那些优化实现背后核心算法,例如“Quicksort Dance”或任何其他可视化效果,或者具有示例的好书或网页。
-- 编辑
添加了插入排序的(差劣的,未经过优化的,不安全的)PHP实现,以便进行另一种可视化效果的展示:
<?php
function my_usort($A, $comparator) {
// Start .. End Positions
$current_pos = 0;
$last_pos = count($A)-1;
// Outer Loop: each step checks that A[0] up to A[current_pos] is sorted.
// When the algorithm finishes we know that A[0] ... A[last_pos] is sorted
while($current_pos < $last_pos) {
echo "Sorted Subarray from \$A is " . json_encode(array_slice($A, 0, $current_pos+1)) . "<br>\n";
echo "\$A looks like this now: " . json_encode($A) .
", comparing [" . $A[$current_pos] . "," . $A[$current_pos +1] . "] (verify step)<br>\n";
// "Verification Step"
// At this point A[0] ... A[current_pos] is sorted.
// Check A[current_pos] <= A[current_pos +1]
if($comparator($A[$current_pos], $A[$current_pos +1]) > 0) {
// nope, A[current_pos] > A[current_pos +1] (list_cmp/comparator returns value > 0)
// "Insertion Step" start, find the correct position for A[current_pos+1] in the already
// sorted list A[0] ... A[current_pos]
$insert_point = $current_pos;
// Swap the missmatching Neighbor pair
echo "swapping: \$A[" . $insert_point . "] and \$A[" . ($insert_point+1) . "]<br>\n";
$tmp = $A[$insert_point +1];
$A[$insert_point +1] = $A[$insert_point];
$A[$insert_point] = $tmp;
$sorted_up_to_current_pos = false;
// Inner Loop: find correct insertion point
while($insert_point > 0 && !$sorted_up_to_current_pos) {
echo "\$A looks like this now: " . json_encode($A) .
", comparing [" . $A[$insert_point-1] . "," . $A[$insert_point] . "] (insertion step)<br>\n";
// "Insertion Step", Swap the missmatching Neighbor pairs until A[0] ... A[current_pos] is sorted again
if($comparator($A[$insert_point-1], $A[$insert_point]) > 0) {
// Swap the missmatching Neighbor pair
echo "swapping: \$A[" . ($insert_point-1) . "] and \$A[" . $insert_point . "]<br>\n";
$tmp = $A[$insert_point];
$A[$insert_point] = $A[$insert_point-1];
$A[$insert_point-1] = $tmp;
// goto new pair
$insert_point = $insert_point -1;
} else {
// found correct spot, done
$sorted_up_to_current_pos = true;
}
}
$A[$insert_point] = $tmp;
echo "\$A looks like this now: " . json_encode($A) . ", insertion done<br>\n";
}
$current_pos = $current_pos + 1;
}
echo "Sorted Array \$A is " . json_encode(array_slice($A, 0, $current_pos+1)) . "<br>\n";
}
function list_cmp($a, $b) {
global $order;
//echo "\$a=$a, \$b=$b </br>\n";
foreach ($order as $key => $value) {
//echo "\$value=$value </br>\n";
if ($a == $value) {
echo "\$a=\$value, returing 0. </br>\n";
return 0;
}
if ($b == $value) {
echo "\$b=\$value, returing 1. </br>\n";
return 1;
}
}
}
$order[0] = 1;
$order[1] = 3;
$order[2] = 4;
$order[3] = 2;
$array[0] = 2;
$array[1] = 1;
$array[2] = 3;
$array[3] = 4;
$array[4] = 2;
$array[5] = 1;
$array[6] = 2;
my_usort($array, "list_cmp");
输出已完成,当前排序数组的位置如下:
Sorted Subarray from $A is [2]
$A looks like this now: [2,1,3,4,2,1,2], comparing [2,1] (verify step)
$b=$value, returing 1.
swapping: $A[0] and $A[1]
$A looks like this now: [1,2,3,4,2,1,2], insertion done
Sorted Subarray from $A is [1,2]
$A looks like this now: [1,2,3,4,2,1,2], comparing [2,3] (verify step)
$b=$value, returing 1.
swapping: $A[1] and $A[2]
$A looks like this now: [1,3,2,4,2,1,2], comparing [1,3] (insertion step)
$a=$value, returing 0.
$A looks like this now: [1,3,2,4,2,1,2], insertion done
Sorted Subarray from $A is [1,3,2]
$A looks like this now: [1,3,2,4,2,1,2], comparing [2,4] (verify step)
$b=$value, returing 1.
swapping: $A[2] and $A[3]
$A looks like this now: [1,3,4,2,2,1,2], comparing [3,4] (insertion step)
$a=$value, returing 0.
$A looks like this now: [1,3,4,2,2,1,2], insertion done
Sorted Subarray from $A is [1,3,4,2]
$A looks like this now: [1,3,4,2,2,1,2], comparing [2,2] (verify step)
$a=$value, returing 0.
Sorted Subarray from $A is [1,3,4,2,2]
$A looks like this now: [1,3,4,2,2,1,2], comparing [2,1] (verify step)
$b=$value, returing 1.
swapping: $A[4] and $A[5]
$A looks like this now: [1,3,4,2,1,2,2], comparing [2,1] (insertion step)
$b=$value, returing 1.
swapping: $A[3] and $A[4]
$A looks like this now: [1,3,4,1,2,2,2], comparing [4,1] (insertion step)
$b=$value, returing 1.
swapping: $A[2] and $A[3]
$A looks like this now: [1,3,1,4,2,2,2], comparing [3,1] (insertion step)
$b=$value, returing 1.
swapping: $A[1] and $A[2]
$A looks like this now: [1,1,3,4,2,2,2], comparing [1,1] (insertion step)
$a=$value, returing 0.
$A looks like this now: [1,1,3,4,2,2,2], insertion done
Sorted Subarray from $A is [1,1,3,4,2,2]
$A looks like this now: [1,1,3,4,2,2,2], comparing [2,2] (verify step)
$a=$value, returing 0.
Sorted Array $A is [1,1,3,4,2,2,2]
sort()
函数使用的是快速排序算法,文档似乎暗示了 PHP 中的大多数其他排序方法也使用相同的算法。 - apokryfos