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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’.
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