now*_*w_m 4 php quicksort usort
我有一个 usort() 示例，我添加了一些 echo 语句来查看代码的工作原理：
<?php
function list_cmp($a, $b) {
global $order;
echo "\$a=$a, \$b=$b </br>";
foreach ($order as $key => $value) {
echo "\$value=$value </br>";
if ($a == $value) {
echo "\$a=\$value, returing 0. </br>";
return 0;
}
if ($b == $value) {
echo "\$b=\$value, returing 1. </br>";
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;
$array[6] = 2;
usort($array, "list_cmp");
?>
代码的输出是这样的：
$a=2, $b=1
$value=1
$b=$value, returing 1.
$a=2, $b=3
$value=1
$value=3
$b=$value, returing 1.
$a=1, $b=3
$value=1
$a=$value, returing 0.
$a=2, $b=4
$value=1
$value=3
$value=4
$b=$value, returing 1.
$a=3, $b=4
$value=1
$value=3
$a=$value, returing 0.
$a=2, $b=2
$value=1
$value=3
$value=4
$value=2
$a=$value, returing 0.
$a=2, $b=1
$value=1
$b=$value, returing 1.
$a=2, $b=1
$value=1
$b=$value, returing 1.
$a=4, $b=1
$value=1
$b=$value, returing 1.
$a=3, $b=1
$value=1
$b=$value, returing 1.
$a=1, $b=1
$value=1
$a=$value, returing 0.
$a=2, $b=2
$value=1
$value=3
$value=4
$value=2
$a=$value, returing 0.
创建 12 个 $a-$b 对的机制是什么 - 2-1, 2-3, 1-3, 2-4, 3-4, 2-2, 2-1, 2-1 (再次相同?)、4-1、3-1、1-1、2-2。上面的代码返回 1,1,0,1,0,0,1,1,1,1,0,0。以及根据返回的值对数组进行排序的机制是什么？我试图了解 usort() 机制是如何工作的。
谢谢。
我不确定这是否是问题的一部分，但要清楚比较器的工作原理：您有一个由有序列表指定的订单$order和一个特殊的比较器回调 list_cmp，它（应该）返回参数
$a小于$b（return -1或一个值< 0）$a大于$b（return 1或一个值> 0）$a等于$b( return 0)list_cmp这是通过查找其订单表并检查是否
$a具有较小（或相等）的顺序，在这种情况下，循环提前退出，return 0如果$b具有较小的顺序，在这种情况下循环提前退出return 1。请注意，根据 PHP 文档，这是错误的，该文档指出它希望正/负/0 作为返回值。这仅在您知道内部仅检查是否正确时才是正确的comparator($a,$b) > 0，这意味着它仅检查是否$b小于和不等于$a，从而进行比较order of $a <= order of $b。如果代码开始检查$a小于和不等于 ，它很容易中断$b。
首先，我假设您使用的是 PHP 7 或更高版本。在这种情况下，您遇到了 6-15 个元素大小的数组的特殊情况。PHP 7+ 似乎没有对短列表使用快速排序，而是使用插入排序变体（由于缓存/代码局部性等硬件相关的东西，它很可能更快）。可以在Github PHP Mirror上查看排序源码fe （例如：PHP 7.0 Zend/Zend_sort.c Line 177-198）。
该代码分 3 步工作：
array[j]和array[j+1]，如果array[j] <= array[j+1]继续前进，否则，转至2。array[j] > array[j+1]，向后扫描以找到array[x] < array[j+1] <= array[x+1]for的点x < j（显然只有直到x命中开始）x+1 ... j向上移动一个，使其成为x+2 ... j+1并在位置插入前一个元素x+1如果您将该代码应用于配对 (2-1, 2-3, 1-3, 2-4, 3-4, 2-2, 2-1, 2-1, 4-1, 3-1, 1 -1, 2-2) 代码的作用一目了然。
-- [2,1],3,4,2,1,2 -> 1./2./3. compare [2,1], find and insert 1 before 2
-- 1,[2,3],4,2,1,2 -> 1./2. compare [2,3], find insert point for 3 (since order of 3 < order of 2)
-- [1,3],2,4,2,1,2 -> 3. compare [1,3], found insert point for 3 before 2
-- 1,3,[2,4],2,1,2 -> 1./2. compare [2,4], find insert point for 4 (since order of 4 < order of 2)
-- 1,[3,4],2,2,1,2 -> 3. compare [3,4], found insert point for 4 before 2
-- 1,3,4,[2,2],1,2 -> 1. compare [2,2], skip
-- 1,3,4,2,[2,1],2 -> 1./2. compare [2,1], find insert point for 1
-- 1,3,4,[2,1],2,2 -> 2. compare [2,1], find insert point for 1
-- 1,3,[4,1],2,2,2 -> 2. compare [4,1], find insert point for 1
-- 1,[3,1],4,2,2,2 -> 2. compare [3,1], find insert point for 1
-- [1,1],3,4,2,2,2 -> 3. compare [1,1], fond insert point for 1 before 3
-- 1,1,3,4,2,[2,2] -> 1. compare [2,2], skip
-- sorted: 1,1,3,4,2,2,2
PS：在这里你已经看到，通过比较模式推导出即使是一个简单的排序算法（22行代码）的工作也是相当复杂的。PHP 7 快速排序实现的代码行数大约是其 10 倍，并且有一些奇怪的优化（除了由于枢轴选择和递归导致的正常疯狂之外）。
大多数情况下，最好忽略深入的实现细节，只将其减少到需要的东西。对排序算法的典型问题是，如果它是稳定/不稳定，在执行O(log n)与O(n)内存消耗。有更简单的方法可以学习这些优化实现背后的核心算法，例如Quicksort Dance或任何其他可视化或带有示例的旧（电子）书或网页。
-- 已编辑
添加了一个（糟糕的、未优化的、不安全的）用于插入排序的 php 实现，以对其工作方式进行另一种可视化：
<?php
function my_usort($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");
输出现在完成，当前排序的数组，位置：
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]