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#include <stdbool.h>
static long long valueof(int c, int base)
{
long long result = 0;
if (base < 2 || base > 36)
{
// bases less than 2 or greater than 36 are invalid.
return -1;
}
// check if value is an ascii numeric character
if (c >= '0' && c <= '9')
{
// simple as
result = (long long)(c ^ 0x30);
}
// check if value is an uppercase ASCII alphabetical character
else if (
(c >= 'A' && c <= 'Z') ||
(c >= 'a' && c <= 'z'))
{
// mask in uppercase bit
c |= 0x20;
// value is 10 plus the alphabetical order of the character
result = (long long)(10 + c - 'a');
}
else
{
// character has no valid interpretation under base 36
return -1LL;
}
if (result > (base - 1))
{
// character is not valid under given base
return -1LL;
}
return result;
}
unsigned long long strtoull(
const char *restrict begin,
char **restrict end,
int base)
{
long result = 0;
const char *current = begin;
// check for empty string
if (*current == 0)
{
if (end)
{
*end = (char *)begin;
return result;
}
}
// skip initial whitespace
bool whitespace = true;
while (whitespace)
{
switch (*current)
{
case 0x9:
case 0xa:
case 0xb:
case 0xc:
case 0xd:
case 0x20:
current++;
break;
default:
whitespace = false;
}
}
bool negative = false;
// detect if negative sign is used
if (*current == '-')
{
negative = true;
current++;
}
// detect base from input
if (base == 0)
{
// base is non-decimal
if (current[0] == 0)
{
if (current[1] == 'b')
{
base = 2;
current += 2;
}
else if (current[1] == 'x')
{
base = 16;
current += 2;
}
else
{
base = 8;
current++;
}
}
// base is decimal otherwise
else
{
base = 10;
}
}
// do the actual conversion now
while (*current)
{
long long value = valueof(*current, base);
// we've reached the end of the conversion
if (value == -1)
{
break;
}
result *= base;
result += value;
current++;
}
// store the end pointer if needed
if (end)
{
*end = (char *)current;
}
// flip the result if we're meant to
if (negative)
{
result = -result;
}
return result;
}
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