Run Length Encoding

#include <stdio.h>
#include <string.h>
static int str_in(char *p, int size) {
  if (!fgets(p, size, stdin)) {
    return 0;
  }
  int len = 0;
  for (; *p; ++p, ++len) {
    if (*p == '\n') {
      *p = '\0';
      break;
    }
    if (*p < ' ' || ('0' <= *p && *p <= '9') || '~' < *p) {
      return 0;
    }
  }
  return len;
}
static void compress_sub(char **to, char prev, int *len, int c) {
  if (2 < c) {
    **to = '0' + c;
    ++*to;
    ++*len;
  } else {
    for (--c; c; --c, **to = prev, ++*to, ++*len) {
      ;
    }
  }
}
static int compress(char *from, char *to) {
  char prev = *to++ = *from++;
  int len = 1, c = 1;
  for (; *from; ++from) {
    if (prev == *from && c < 9) {
      ++c;
      continue;
    }
    compress_sub(&to, prev, &len, c);
    prev = *to++ = *from;
    ++len;
    c = 1;
  }
  compress_sub(&to, prev, &len, c);
  *to = '\0';
  return len;
}
static void decompress(char *from, char *to) {
  char prev = *to++ = *from++;
  int len = 1;
  for (; *from; ++from) {
    if (*from < '0' || '9' < *from) {
      prev = *to++ = *from;
      ++len;
      continue;
    }
    for (int c = *from - '0' - 1; c; --c, *to++ = prev, ++len) {
      ;
    }
  }
  *to = '\0';
  // return len;
}
int main(void) {
#define N 16
  char src[N], dest[N], cmp[N];
  if (!str_in(src, N)) {
    return 1;
  }
#undef N
  printf("|%s| --> ", src);
  compress(src, dest);
  printf("|%s| --> ", dest);
  decompress(dest, cmp);
  printf("|%s| ", cmp);
  printf("%d\n", strcmp(src, cmp));
}