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Maxim Yegorushkin
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 Posted: Fri Jan 27, 2006 12:20 pm Post subject: Re: Advice required on my ascii to hex conversion C++ |
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[email]andrewfaseuk (AT) hotmail (DOT) com[/email] wrote:
[]
| Quote: | #include <stdlib.h
#include
#include
inline int is_hex(char c)
{
return (c >= '0' && c <= '9')
|| ((c | 0x20) >= 'a' && (c | 0x20) <= 'f')
;
}
inline unsigned char hex2bin(unsigned char h, unsigned char l)
{
h |= 0x20; // to lower
h -= 0x30;
h -= -(h > 9) & 0x27;
l |= 0x20;
l -= 0x30;
l -= -(l > 9) & 0x27;
return h << 4 | l;
}
any chance you could explain how this works ? i don't know what "|="
means not "return h << 4 | l;" to be honest the whole thing seems a bit
|
Check out your C/C++ textbook for operators.
| Quote: | confuising for me anyway
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hex2bin function was cut from an optimized parsing code. The function
is a bit tricky to understand because it was deliberately coded to
avoid branching (conditional jumps) in generated code.
Basically, converting a hex digit into a hex nibble (4 binary digits)
algorithm looks like;
char xdigit; // hex digit to convert [0-9A-Fa-f]
xdigit = tolower(xdigit); // make it lowercase [0-9a-f]
xdigit -= '0'; // if it was a [0-9] digit, it's the value now
if(xdigit > 9) // if it was a [a-f] digit, compensate for that
xdigit = xdigit + '0' - 'a';
The original code is just an optimization of the algorithm.
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