为什么使用随机字符串的代码打印出“hello world”?
以下打印声明将打印“hello world”。 有谁能解释这个吗?
System.out.println(randomString(-229985452) + " " + randomString(-147909649));
和randomString()
看起来像这样:
public static String randomString(int i)
{
Random ran = new Random(i);
StringBuilder sb = new StringBuilder();
while (true)
{
int k = ran.nextInt(27);
if (k == 0)
break;
sb.append((char)('`' + k));
}
return sb.toString();
}
当使用特定的种子参数(在本例中为-229985452
或-147909649
)构造java.util.Random
的实例时,它遵循从该种子值开始的随机数生成算法。
使用相同种子构建的每个Random
数将每次生成相同的数字模式。
其他答案解释了为什么,但这里是如何。
给定一个Random
的实例:
Random r = new Random(-229985452)
r.nextInt(27)
生成的前6个数字是:
8
5
12
12
15
0
和r.nextInt(27)
生成的前6个数字给定Random r = new Random(-147909649)
为:
23
15
18
12
4
0
然后,将这些数字添加到字符`
(它是96)的整数表示形式:
8 + 96 = 104 --> h
5 + 96 = 101 --> e
12 + 96 = 108 --> l
12 + 96 = 108 --> l
15 + 96 = 111 --> o
23 + 96 = 119 --> w
15 + 96 = 111 --> o
18 + 96 = 114 --> r
12 + 96 = 108 --> l
4 + 96 = 100 --> d
我会把它留在这里。 如果你已经掌握了一些fork-join-fu来让这个东西烧掉所有的CPU核心(只是线程很无聊,对吗?),请分享你的代码。 我将不胜感激。
public static void main(String[] args) {
long time = System.currentTimeMillis();
generate("stack");
generate("over");
generate("flow");
generate("rulez");
System.out.println("Took " + (System.currentTimeMillis() - time) + " ms");
}
private static void generate(String goal) {
long[] seed = generateSeed(goal, Long.MIN_VALUE, Long.MAX_VALUE);
System.out.println(seed[0]);
System.out.println(randomString(seed[0], (char) seed[1]));
}
public static long[] generateSeed(String goal, long start, long finish) {
char[] input = goal.toCharArray();
char[] pool = new char[input.length];
label:
for (long seed = start; seed < finish; seed++) {
Random random = new Random(seed);
for (int i = 0; i < input.length; i++)
pool[i] = (char) random.nextInt(27);
if (random.nextInt(27) == 0) {
int base = input[0] - pool[0];
for (int i = 1; i < input.length; i++) {
if (input[i] - pool[i] != base)
continue label;
}
return new long[]{seed, base};
}
}
throw new NoSuchElementException("Sorry :/");
}
public static String randomString(long i, char base) {
System.out.println("Using base: '" + base + "'");
Random ran = new Random(i);
StringBuilder sb = new StringBuilder();
for (int n = 0; ; n++) {
int k = ran.nextInt(27);
if (k == 0)
break;
sb.append((char) (base + k));
}
return sb.toString();
}
输出:
-9223372036808280701
Using base: 'Z'
stack
-9223372036853943469
Using base: 'b'
over
-9223372036852834412
Using base: 'e'
flow
-9223372036838149518
Using base: 'd'
rulez
Took 7087 ms
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