#include "asn1.h"

#ifdef UNITTEST
#undef UNITTEST
#include "scan_asn1tagint.c"
#define UNITTEST
#endif

size_t scan_asn1tag(const char* src,const char* max,enum asn1_tagclass* tc,enum asn1_tagtype* tt,unsigned long* tag) {
  if (max<=src)
    return 0;
  *tc=(*src&0xC0);
  *tt=(*src&0x20);
/* The lower 5 bits are the tag, unless it's 0x1f, in which case the
 * next bytes are the tag: always take the lower 7 bits; the last byte
 * in the sequence is marked by a cleared high bit */
  if ((*src & 0x1f) == 0x1f) {
    size_t res=scan_asn1tagint(src+1,max,tag);
    if (res && *tag < 0x1f)	// non-minimal encoding
      return 0;
    return res+!!res;	/* add 1 unless it's 0, then leave 0 */
  } else {
    *tag=*src&0x1f;
    return 1;
  }
}

#ifdef UNITTEST
#include <assert.h>
#include <string.h>

int main() {
  enum asn1_tagclass tc;
  enum asn1_tagtype tt;
  unsigned long tag;
  char buf[15];
  assert(scan_asn1tag(buf,buf,&tc,&tt,&tag)==0);	// empty input
  strcpy(buf,"\x01"); assert(scan_asn1tag(buf,buf+10,&tc,&tt,&tag)==1 && tc==UNIVERSAL && tt==PRIMITIVE && tag==BOOLEAN);
  /* incomplete input */
  strcpy(buf,"\x1f"); assert(scan_asn1tag(buf,buf+1,&tc,&tt,&tag)==0);
  /* long-form encoding when short-form would have sufficed */
  strcpy(buf,"\x1f\x1e");
  assert(scan_asn1tag(buf,buf+10,&tc,&tt,&tag)==0);
  /* OK */
  strcpy(buf,"\x1f\x1f");
  assert(scan_asn1tag(buf,buf+10,&tc,&tt,&tag)==2 && tc==UNIVERSAL && tt==PRIMITIVE && tag==0x1f);
  /* non-minimal encoding */
  strcpy(buf,"\x1f\x80\x01");
  assert(scan_asn1tag(buf,buf+10,&tc,&tt,&tag)==0);
  /* incomplete encoding */
  assert(scan_asn1tag(buf,buf+2,&tc,&tt,&tag)==0);
  strcpy(buf,"\x1f\x81\x00");
  assert(scan_asn1tag(buf,buf+10,&tc,&tt,&tag)==3 && tc==UNIVERSAL && tt==PRIMITIVE && tag==0x80);
  /* value not representable */
  memcpy(buf,"\x1f\xff\xff\xff\xff\xff\xff\xff\xff\xff\x7f",11);
  assert(scan_asn1tag(buf,buf+12,&tc,&tt,&tag)==0);
  memcpy(buf,"\x1f\x8f\xff\xff\xff\x7f",7);
  assert(scan_asn1tag(buf,buf+10,&tc,&tt,&tag)==6 && tc==UNIVERSAL && tt==PRIMITIVE && tag==0xffffffff);
}
#endif