71 lines
1.7 KiB
C
71 lines
1.7 KiB
C
#include <stdio.h>
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#include <string.h>
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#include <stdlib.h>
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/* Constraints: input is in the form of (\+|-)?[0-9]+
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* and without leading zero (0 itself can be as "0" or "+0", but not "-0");
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* input pointer is realloc'able and may change;
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* if input has leading + sign, return may or may not keep it.
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* The constranits conform to sprintf("%+d") and this function's own output.
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*/
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char * incr(char *s)
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{
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int i, begin, tail, len;
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int neg = (*s == '-');
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char tgt = neg ? '0' : '9';
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/* special case: "-1" */
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if (!strcmp(s, "-1")) {
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s[0] = '0', s[1] = '\0';
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return s;
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}
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len = strlen(s);
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begin = (*s == '-' || *s == '+') ? 1 : 0;
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/* find out how many digits need to be changed */
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for (tail = len - 1; tail >= begin && s[tail] == tgt; tail--);
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if (tail < begin && !neg) {
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/* special case: all 9s, string will grow */
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if (!begin) s = realloc(s, len + 2);
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s[0] = '1';
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for (i = 1; i <= len - begin; i++) s[i] = '0';
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s[len + 1] = '\0';
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} else if (tail == begin && neg && s[1] == '1') {
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/* special case: -1000..., so string will shrink */
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for (i = 1; i < len - begin; i++) s[i] = '9';
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s[len - 1] = '\0';
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} else { /* normal case; change tail to all 0 or 9, change prev digit by 1*/
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for (i = len - 1; i > tail; i--)
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s[i] = neg ? '9' : '0';
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s[tail] += neg ? -1 : 1;
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}
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return s;
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}
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void string_test(const char *s)
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{
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char *ret = malloc(strlen(s));
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strcpy(ret, s);
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printf("text: %s\n", ret);
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printf(" ->: %s\n", ret = incr(ret));
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free(ret);
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}
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int main()
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{
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string_test("+0");
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string_test("-1");
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string_test("-41");
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string_test("+41");
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string_test("999");
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string_test("+999");
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string_test("109999999999999999999999999999999999999999");
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string_test("-100000000000000000000000000000000000000000000");
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return 0;
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}
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