Index: aoc2024.c
==================================================================
--- aoc2024.c
+++ aoc2024.c
@@ -7,10 +7,11 @@
 #include "aocdailies.h"
 #include "aocutils.h"
 
 /* === aoc202407 =======================================================
    Part one looks easy
+   Part two also easy
 ===================================================================== */
 
 static unsigned long long concat(unsigned long long a, unsigned long long b) {
     unsigned long long r = a, t = b;
     while (b) {
@@ -18,33 +19,29 @@
         b /= 10;
     }
     return r + t;
 }
 
-static bool canbetrue(unsigned long long v, unsigned long long *a, size_t n) {
-    if (n == 1) return (v == *a);
-    unsigned long long tmp = a[1];
-    a[1] = a[0] + tmp;
-    if (canbetrue(v, a+1, n-1)) { a[1] = tmp; return true; }
-    a[1] = a[0] * tmp;
-    if (canbetrue(v, a+1, n-1)) { a[1] = tmp; return true; }
-    a[1] = tmp;
-    return false;
-}
-
-static bool canbetrue3(unsigned long long v, unsigned long long *a, size_t n) {
-    // still, let's assume no unsigned long long overflowing
-    if (n == 1) return (v == *a);
-    unsigned long long tmp = a[1];
-    a[1] = a[0] + tmp;
-    if (canbetrue3(v, a+1, n-1)) { a[1] = tmp; return true; }
-    a[1] = a[0] * tmp;
-    if (canbetrue3(v, a+1, n-1)) { a[1] = tmp; return true; }
-    a[1] = concat(a[0], tmp);
-    if (canbetrue3(v, a+1, n-1)) { a[1] = tmp; return true; }
-    a[1] = tmp;
-    return false;
+static int operatorsrequired(unsigned long long v, unsigned long long *a, size_t n, int minop) {
+    // assumes no unsigned long long overflowing
+    if (n == 1) {
+        if (v == *a) return minop;
+        return 0;
+    }
+    unsigned long long tmp = a[1];
+    a[1] = a[0] + tmp;
+    int p = operatorsrequired(v, a+1, n-1, (minop > 2)?3:2);
+    a[1] = tmp;
+    if (p) return p;
+    a[1] = a[0] * tmp;
+    p = operatorsrequired(v, a+1, n-1, (minop > 2)?3:2);
+    a[1] = tmp;
+    if (p) return p;
+    a[1] = concat(a[0], tmp);
+    p = operatorsrequired(v, a+1, n-1, 3);
+    a[1] = tmp;
+    return p;
 }
 
 void aoc202407(char *data, size_t len) {
     (void)len; // unused argument
     unsigned long long calibrationtotal = 0, calibration3total = 0;
@@ -56,16 +53,25 @@
         err += 1; // skip ':'
         nnumber = 0;
         while (*err == ' ') {
             number[nnumber++] = strtoul(err, &err, 10);
         }
+        switch (operatorsrequired(testvalue, number, nnumber, 0)) {
+            default: break;
+            case 2: calibrationtotal += testvalue;
+                    __attribute__((fallthrough));
+            case 3: calibration3total += testvalue;
+                    break;
+        }
+        #if 0
         if (canbetrue(testvalue, number, nnumber)) {
             calibrationtotal += testvalue;
         }
         if (canbetrue3(testvalue, number, nnumber)) {
             calibration3total += testvalue;
         }
+        #endif
         line = strtok(NULL, "\n");
     }
     printf("The calibration total is {%llu}.\n", calibrationtotal);
     printf("The calibration total with three operators is {%llu}.\n", calibration3total);
 }
@@ -107,11 +113,11 @@
         switch (data[linearize2d(size, (unsigned)(row_pos+deltarow), (unsigned)(col_pos+deltacol))]) {
             default: exit(EXIT_FAILURE); // does not happen
             case '#': rightturn(&deltarow, &deltacol); break;
             case '.': visited++;
                       data[linearize2d(size, (unsigned)(row_pos+deltarow), (unsigned)(col_pos+deltacol))] = 'X';
-                      /*fallthrough*/
+                      __attribute__((fallthrough));
             case 'X': row_pos += deltarow;
                       col_pos += deltacol;
                       break;
         }
     }
@@ -134,11 +140,11 @@
                     case '#': if (data[linearize2d(size, (unsigned)(row_pos), (unsigned)(col_pos))] == '4') looping = true;
                               rightturn(&deltarow, &deltacol);
                               break;
                     case '4': looping = true; break;
                     case '.': data[linearize2d(size, (unsigned)(row_pos+deltarow), (unsigned)(col_pos+deltacol))] = '0';
-                              /*fallthrough*/
+                              __attribute__((fallthrough));
                     case '1':
                     case '2':
                     case '3': data[linearize2d(size, (unsigned)(row_pos+deltarow), (unsigned)(col_pos+deltacol))]++;
                               row_pos += deltarow;
                               col_pos += deltacol;