如何在 Perl 中获取已打包项目的数量而不实际拆包？

发布于 2024-09-25 18:48:48 字数 340 浏览 3 评论 0原文

我有一个 packed 值的字符串，它是使用以下内容按顺序创建的：

while (...) {
 ...
 $packed .= pack( 'L', $val );
}

在另一个程序中，在加载 $packed 后，我希望找出实际有多少个值打包。我知道解压后该怎么做：

my @vals = unpack( 'L*', $packed );
print scalar(@vals);

但这真的有必要吗？如果我只关心值的数量，我可以做得更好并跳过解包吗？

原文

I have a string of packed values which was created sequentially using something like:

while (...) {
 ...
 $packed .= pack( 'L', $val );
}

In another program, after I load $packed, I wish to find out how many values were actually packed. I know how to do that after unpacking:

my @vals = unpack( 'L*', $packed );
print scalar(@vals);

But is it really necessary? If I only care about the number of values, can I do better and skip the unpacking?

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生生漫 2024-10-02 18:48:48

由于您知道打包值的大小（L 是一个无符号 32 位 int，或 4 个字节），只需将长度除以大小即可：

my $count = length($packed) / 4;

如果您不想对大小进行硬编码，您还可以打包一个样本值来计算它。（请注意，Perl 的编译时常量折叠不适用于 pack，至少不适用于 5.10.1，因此您只需执行该计算一次。）

my $size = length(pack('L', 0));
...
my $count = length($packed) / $size;

Since you know the size of a packed value (L is an unsigned 32-bit int, or 4 bytes), just divide the length by the size:

my $count = length($packed) / 4;

If you don't want to hard code the size, you could also pack a sample value to calculate it. (Note that Perl's compile-time constant folding doesn't work with pack, at least not with 5.10.1, so you'd want to do that calculation only once.)

my $size = length(pack('L', 0));
...
my $count = length($packed) / $size;

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陈甜 2024-10-02 18:48:48

由于 L 只是 32 位值的组，因此您可以简单地计算字节数并除以 4。

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脱离于你 2024-10-02 18:48:48

use constant LLength => length(pack("L", 0));
...
print length($packed)/LLength;

检查LLength是否确实是常量：

$ perl -MO=Deparse,-d -e'use constant L => length(pack("L", 0));print L, "\n";'
sub L () { 4 }
use constant ("L", length pack("L", 0));
print 4, "\n";
-e syntax OK

use constant LLength => length(pack("L", 0));
...
print length($packed)/LLength;

Check if LLength is really constant:

$ perl -MO=Deparse,-d -e'use constant L => length(pack("L", 0));print L, "\n";'
sub L () { 4 }
use constant ("L", length pack("L", 0));
print 4, "\n";
-e syntax OK

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