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#include <ctype.h>
#include <stdlib.h>
#include <assert.h>
int ctoi(char c) { return c - '0'; }
double strtod(const char *restrict nptr, char **restrict endptr) {
double r = 0;
// An initial, possibly empty, sequence of white-space characters (as
// specified by isspace())
for (; isspace(*nptr); nptr++)
;
// A subject sequence interpreted as a floating-point constant or representing
// infinity or NaN
{
// The expected form of the subject sequence is an optional '+' or '-' sign
int sign = 0;
int exp_sign = 0;
if ('+' == *nptr) {
sign = 0;
nptr++;
} else if ('-' == *nptr) {
sign = 1;
nptr++;
}
// A non-empty sequence of decimal digits optionally containing a radix
// character
double exp = 0;
for (; isdigit(*nptr); nptr++) {
r *= 10;
r += ctoi(*nptr);
}
if ('.' == *nptr) {
double div = 10;
for (; isdigit(*nptr); nptr++) {
r += ctoi(*nptr) / div;
div *= 10;
}
}
r *= (sign) ? (-1) : (1);
// then an optional exponent part consisting of the character 'e' or
// the character 'E'
if ('e' == tolower(*nptr)) {
// optionally followed by a '+' or '-' character
if ('+' == *nptr) {
exp_sign = 0;
nptr++;
} else if ('-' == *nptr) {
exp_sign = 1;
nptr++;
}
// and then followed by one or more decimal digits
for (; isdigit(*nptr); nptr++) {
exp *= 10;
exp += ctoi(*nptr);
}
exp *= (exp_sign) ? (-1) : (1);
}
assert(0 == exp); // TODO
}
// A final string of one or more unrecognized characters, including the
// terminating NUL character of the input string
;
return r;
}
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