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

typedef struct memcursor {
  char* start;
  char* pos;
  char* end; // IMPORTANT: Must point to the last byte, not the byte after it!
} memcursor_t;

memcursor_t* memcursor_append(memcursor_t*restrict cur, size_t size, char data[size]){
  if(cur->end - cur->pos < size)
    size = cur->end - cur->pos;
  memcpy(cur->pos, data, size);
  cur->pos += size;
  *cur->pos = 0;
  return cur;
}

memcursor_t* memcursor_printf(memcursor_t*restrict cur, const char* format, ...){
  va_list args;
  va_start(args, format);
  int ret = vsnprintf(cur->pos, cur->end-cur->pos+1, format, args);
  va_end(args);
  if(ret < 0)
    return cur;
  if(ret > cur->end-cur->pos)
    ret = cur->end-cur->pos;
  cur->pos += ret;
  return cur;
}

bool memcursor_eof(memcursor_t*restrict cur){
  return cur->pos == cur->end;
}

#define S(X) strlen((X)), (X)

int main(){
  char out_buf[29];
  out_buf[0] = 0;
  memcursor_t cur = { out_buf, out_buf, out_buf+sizeof(out_buf)-1 };

  memcursor_printf(&cur, "Test %d", 123);

  printf("data: %s\n", out_buf);
  printf("eof: %s\n", memcursor_eof(&cur)?"true":"false");

  memcursor_append(&cur, S(("\n" "The quick brown fox jumps over the lazy dog")) );

  printf("data: %s\n", out_buf);
  printf("eof: %s\n", memcursor_eof(&cur)?"true":"false");
}