有没有办法让这个哈希查找任何更快吗？ [英] Is there a way to make this hash lookup any faster?
问题描述
我有（非常）快速处理范围有限的字符串，理货他们的价值观的要求。输入文件的格式为：
I have a requirement to (very) quickly process strings of a limited range, tallying their values. The input file is of the form:
January 7
March 22
September 87
March 36
和等等。因为线路宽度是相同的，我可以简单地用 FREAD A线相当快看完了，我已经开发了一个完美的哈希函数这工作，但我想看看是否任何人都可以提供如何使它更快的任何建议。我会每个配置文件建议，看看怎么回事。
and so forth. Because the line widths are identical, I can simply read in a line with fread reasonably fast, and I've developed a perfect hashing function which works, but I wanted to see if anyone could offer any advice on how to make it even faster. I'll profile each suggestion to see how it goes.
散列函数是基于月份名称以允许值到水桶的快速分配。这里和我一起承担。我先想出个字符的最小数为一个完美的哈希值：
The hashing function is based on the month name to allow fast allocation of the value to a bucket. Bear with me here. I first figured out the minimal number of characters for a perfect hash:
January
February
March
April
May
June
July
August
September
October
November
December
请记住，在几个月的所有的九个字符由于这一事实，我有整个输入线。
Keep in mind that the months are all nine characters due to the fact I have the entire input line.
不幸的是，没有任何的单的列，以纪念一个月唯一的。第1列重复Ĵ，第2列重复 A ，第3列重复研究，第4列重复 U 和列5起重复＆lt;空＆GT; （还有其他的重复但有足以prevent单列散列键）。
Unfortunately, there is no single column to mark a month unique. Column 1 duplicates J, column 2 duplicates a, column 3 duplicates r, column 4 duplicates u and columns 5 onwards duplicate <space> (there are other duplicates but one is enough to prevent a single-column hash key).
但是，通过使用第一和第四列，我得到的值菊， R ，麦当劳，艾， M＆lt;空＆GT; ，济， Jy进行，金， ST ，哦哦，氖和德，这是独一无二的。将有此文件中没有无效值，所以我不担心输入数据不正确桶。
However, by using the first and fourth column, I get the values Ju, Fr, Mc, Ai, M<space>, Je, Jy, Au, St, Oo, Ne and De, which are unique. There will be no invalid values in this file so I don't have to worry about incorrect buckets for the input data.
通过查看十六进制codeS的人物，我发现我可以只得到较低的独特价值与运算与战略价值：
By viewing the hex codes for the characters, I found I could get low unique values by just ANDing with strategic values:
FirstChar Hex Binary &0x0f
--------- --- --------- -----
A x41 0100 0001 1
D x44 0100 0100 4
F x46 0100 0110 6
J x4a 0100 1010 10
M x4d 0100 1101 13
N x4e 0100 1110 14
O x4f 0100 1111 15
S x53 0101 0011 3
SecondChar Hex Binary &0x1f
---------- --- --------- -----
<space> x20 0010 0000 0
c x63 0110 0011 3
e x65 0110 0101 5
i x69 0110 1001 9
o x6f 0110 1111 15
r x72 0111 0010 18
t x74 0111 0100 20
u x75 0111 0101 21
y x79 0111 1001 25
这让我建立一个静态数组来创建一个（希望）令人眼花缭乱的快速散列函数：
and this allowed me to set up a static array to create a (hopefully) blindingly-fast hash function:
#define __ -1
static unsigned int hash (const char *str) {
static unsigned char bucket[] = {
// A S D F J M N O
__, __, __, __, __, __, __, __, __, __, __, __, __, 4, __, __, // space
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, 2, __, __, // c
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, 11, __, __, __, __, __, 5, __, __, __, 10, __, // e
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, 3, __, __, __, __, __, __, __, __, __, __, __, __, __, __, // i
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, 9, // o
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, 1, __, __, __, __, __, __, __, __, __, // r
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, 8, __, __, __, __, __, __, __, __, __, __, __, __, // t
__, 7, __, __, __, __, __, __, __, __, 0, __, __, __, __, __, // u
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, __, __, __, __, __, __, //
__, __, __, __, __, __, __, __, __, __, 6, __, __, __, __, __ // y
};
return bucket[((unsigned int)(str[3]&0x1f)<<4)|(str[0]&0xf)];
}
测试，与code：
Testing that with the code:
#include <stdio.h>
#include <string.h>
// Hash function here.
static char *months[] = {
"January ", "February ", "March ", "April ", "May ", "June ",
"July ", "August ", "September", "October ", "November ", "December "
};
int main (void) {
int i;
for (i = 0; i < sizeof(months)/sizeof(*months); i++)
printf ("%-10s -> %2d\n", months[i], hash(months[i]));
return 0;
}
显示，它的功能正确的：
shows that it's functionally correct:
January -> 0
February -> 1
March -> 2
April -> 3
May -> 4
June -> 5
July -> 6
August -> 7
September -> 8
October -> 9
November -> 10
December -> 11
但我想知道它是否可以进行得更快。
but I want to know if it can be made faster.
任何建议了吗？我接受任何简单的优化，甚至完全重写，如果有一些固有的坏我的散列函数。
Any suggestions out there? I'm open to any simple optimisations or even a total rewrite if there's something inherently bad with my hashing function.
我不认为这是重要的，但最终版本将使用EBCDIC。该理论将仍然站立，但由于角色有不同的code点和操作可能略有变化。我会很高兴，只有在任何的建议是提供将转化还好EBCDIC ASCII码前，因为我相信任何帮助。
I don't think this is that important but the final version will be using EBCDIC. The theory will still stand but the AND operation may change slightly since the characters have different code points. I'll be happy with any assistance only on the ASCII front since I'm confident whatever advice is offered will translate okay to EBCDIC.
推荐答案
下面是我能找到的最小序列的 EBCDIC美国
Here's the smallest sequence I could find for EBCDIC-US:
它具有在桶24的元素，并且只使用2操作来计算索引
It has 24 elements in the bucket and uses only 2 operations to compute the index:
static unsigned int hash (const char *str)
{
static unsigned char tab[] = {
11, 4,__, 7,__,__, 9, 1,
__,__,__,__,__,__,__,__,
3, 5, 2,10, 8,__, 0, 6
};
return tab[0x17 & (str[ 1 ] + str[ 2 ])];
}
退而求其次，25个项目有异：
Second best, 25 items with xor:
static unsigned int hash(const char *str)
{
static unsigned char tab[] = {
9,__,__, 7,__,__,11, 1,
__, 4,__,__,__,__, 3,__,
__, 5, 8,10, 0,__,__, 6, 2
};
return tab[0x1f & (str[ 1 ] ^ str[ 2 ])];
}
（实际上，标签[]应该是32项长在这里，因为0x1F的可以产生不正确的输入的溢出）。
(Actually, tab[] should be 32 entries long here, because 0x1f can generate an overflow for incorrect inputs).
从大同更新：对于它的价值，第一个选项非常完美的EBCDIC code页500：
Update from Pax: For what it's worth, the first option worked perfectly for EBCDIC code page 500:
## Month str[1] str[2] Lookup
-- --------- ------ ------ ------
0 January a (81) n (95) 0
1 February e (85) b (82) 1
2 March a (81) r (99) 2
3 April p (97) r (99) 3
4 May a (81) y (a8) 4
5 June u (a4) n (95) 5
6 July u (a4) l (93) 6
7 August u (a4) g (87) 7
8 September e (85) p (97) 8
9 October c (83) t (a3) 9
10 November o (96) v (a5) 10
11 December e (85) c (83) 11
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