AoC 2023

aoc.c

#include <stdio.h>
#include <limits.h>

#define byte4_dup(c)        (((c)<<24u)|((c)<<16u)|((c)<<8u)|(c))
#define byte4_cmp_lt(x,n)   (((x)-~0UL/255u*(n))&~(x)&~0UL/255u*128u) // x>=0; 0<=n<=128 
#define byte4_cmp_gt(x,n)   (((x)+~0UL/255*(127-(n))|(x))&~0UL/255*128) // x>=0; 0<=n<=127 

#ifdef _MSC_VER
static int bit_ffs32(unsigned int u) {_BitScanForward(&u, u); return (int)(u);}
static int bit_fls32(unsigned int u) {_BitScanBackward(&u, u); return 31-(int)(u);}
#else /* GCC, CLANG */
#define bit_ffs32(u) __builtin_ctz(u)
#define bit_fls32(u) (31-__builtin_clz(u))
#endif

#define min(a,b) ((a) < (b) ? (a) : (b))
#define max(a,b) ((a) > (b) ? (a) : (b))

int main() {
  static const char *in[] = {
    "1abc2  ",
    "pqr3stu8vwx",
    "a1b2c3d4e5f",
    "treb7uchet ",
  };
  for (int i = 0; i < 4; ++i) {
    const char *s = in[i];
    int lhs = INT_MAX, rhs = 0;
    for (int at = 0;; at += 4) {
      /* check for number character */
      unsigned v = *(unsigned*)(s + at);
      unsigned n = v - byte4_dup('0');
      unsigned e = byte4_cmp_lt(n,10);
      if (e) {
        int ffs = at + (bit_ffs32(e) >> 3);
        int fls = at + (bit_fls32(e) >> 3);
        lhs = min(ffs, lhs);
        rhs = max(fls, rhs);
      }
      /* check for '\0' */
      unsigned c = (~v) & 0x80808080;
      if ((v - 0x01010101) & c) {
        break;
      }
    }
    int res = (s[lhs] - '0') * 10 + (s[rhs] - '0');
    printf("[%d] = %d\n", i, res);
  }
  return 0;
}