跳至主要内容

Implement isdigit

It is seems very easy to implement c library function isdigit, but for a library code, performance is very important. So we will try to implement it and make it faster.

Function

So, first we make it right.
int isdigit(char c) {
    return c >= '0' && c <= '9';
}

Improvements

One – Macro

When it comes to performance for c code, macro can always be tried.
#define isdigit(c) c >= '0' && c <= '9'

Two – Table

Upper version use two comparison and one logical operation, but we can do better with more space:
#define isdigit(c) table[c]
This works and faster, but somewhat wasteful. We need only one bit to represent true or false, but we use a int.
So what to do?
There are many similar functions like isalpha(), isupper ... in c header file, so we can combine them into one int and get result by table[c]&SOME_BIT, which is what source do.
Source code of ctype.h:
#  define _ISbit(bit)   (1 << (bit))

// mask of table
enum
{
  _ISupper = _ISbit (0),    /* UPPERCASE.  */
  _ISlower = _ISbit (1),    /* lowercase.  */
  _ISalpha = _ISbit (2),    /* Alphabetic.  */
  _ISdigit = _ISbit (3),    /* Numeric.  */
  _ISxdigit = _ISbit (4),   /* Hexadecimal numeric.  */
  _ISspace = _ISbit (5),    /* Whitespace.  */
  _ISprint = _ISbit (6),    /* Printing.  */
  _ISgraph = _ISbit (7),    /* Graphical.  */
  _ISblank = _ISbit (8),    /* Blank (usually SPC and TAB).  */
  _IScntrl = _ISbit (9),    /* Control character.  */
  _ISpunct = _ISbit (10),   /* Punctuation.  */
  _ISalnum = _ISbit (11)    /* Alphanumeric.  */
};

// __ctype_b_loc is the table with result
# define __isctype(c, type) \
  ((*__ctype_b_loc ())[(int) (c)] & 
      (unsigned short int) type)

# define isdigit(c) __isctype((c), _ISdigit)

Conclusion

When we deal with char/byte and require very fast implementation, use a table with result pre-computed is a good solution.
An example of this trick : How to reverse a byte’s bit as fast as possible? For example, convert ‘00010001’ to ‘10001000’.
Written with StackEdit.
标签：

评论

发表评论

此博客中的热门博文

Spring Boot: Customize Environment

Spring Boot: Customize Environment Environment variable is a very commonly used feature in daily programming: used in init script used in startup configuration used by logging etc In Spring Boot, all environment variables are a part of properties in Spring context and managed by Environment abstraction. Because Spring Boot can handle the parse of configuration files, when we want to implement a project which uses yml file as a separate config file, we choose the Spring Boot. The following is the problems we met when we implementing the parse of yml file and it is recorded for future reader. Bind to Class Property values can be injected directly into your beans using the @Value annotation, accessed via Spring’s Environment abstraction or bound to structured objects via @ConfigurationProperties. As the document says, there exists three ways to access properties in *.properties or *.yml : @Value : access single value Environment : can access multi...
5 条评论
阅读全文

LevelDB Source Reading (4): Concurrent Access

In this thread, we come to the issue of concurrent access of LevelDB. As a database, it can be concurrently accessed by users. But, it wouldn’t be easy to provide high throughput under product load. What effort does LevelDB make to achieve this goal both in design and implementation? Goal of Design From this github issue , we can see LevelDB is designed for not allowing multi-process access. this (supporting multiple processes) doesn’t seem like a good feature for LevelDB to implement. They believe let multiple process running would be impossible to share memory/buffer/cache, which may affect the performance of LevelDB. In the case of multiple read-only readers without altering the code base, you could simply copy the file for each reader. Yes, it will be inefficient (though not on file systems that dedupe data), but then again, so would having multiple leveldb processes running as they wouldn’t be able to share their memory/buffer/etc. They achieve it by adding a l...
发表评论
阅读全文

LevelDB Source Reading (3): Compaction

LevelDB Source Reading (3): Compaction In the last blog that analyzes read/write process of Leveldb, we can see writing only happens to log file and memory table, then it relies on the compaction process to move the new updates into persistent sorted table for future use. So the compaction is a crucial part for the design, and we will dive into it in this blog. Compaction LevelDB compacts its underlying storage data in the background to improve read performance. The upper sentence is cited from the document of Leveldb , and we will see how it is implemented via code review. Background compaction // db_impl.cc void DBImpl :: MaybeScheduleCompaction ( ) { // use background thread to run compaction env_ - > Schedule ( & DBImpl :: BGWork , this ) ; } Two main aspects // arrange background compaction when Get, Open, Write void DBImpl :: BackgroundCompaction ( ) { // compact memtable CompactMemTable ( ) ; // compact ...
发表评论
阅读全文