Length of the longest valid parenthesis substring.

parens_substring.c

#include <stdbool.h>
#include <stdio.h>

int main(int argc, char **argv) {
  int length, maxlength, stacksize;

  length = maxlength = stacksize = 0;

  while (true) {
    int c;
    c = getc(stdin);
    if (c == '\n') {
      /* Remove any left parenthesis that haven't been closed and check if that makes it the longest substring. */
      if (length - stacksize > maxlength) {
        maxlength = length - stacksize;
      }

      printf("%d\n", maxlength);
      length = maxlength = stacksize = 0;
    } else if (c == EOF) {
      putchar('\n');
      break;
    } else if (c == '(') {
      /* Substring gets longer and deeper. */
      length++;
      stacksize++;
    } else if (c == ')') {
      /* Substring gets longer but shallower. */
      length++;
      stacksize--;
      
      if (stacksize == 0 && length > maxlength) {
        /* Longest parenthesis substring completely closed, store its current length and continue counting. */
        maxlength = length;
      } else if (stacksize < 0) {
        /* Found an unmatched closing parenthesis, reset all counters. */
        length = 0;
        stacksize = 0;
      }
    }
  }
}