Index: aoc2024.c
==================================================================
--- aoc2024.c
+++ aoc2024.c
@@ -9,14 +9,48 @@
 
 /* === aoc202406 =======================================================
    Hmmm, I've done something like this already this year... on day 4
    I need rcmap() moved to aocutils ... done; now it's called `linearize2d`
 ===================================================================== */
+
+static bool validpos(unsigned s, int r, int c) {
+    if (r < 0) return false;
+    if (c < 0) return false;
+    if ((unsigned)r >= s - 1) return false;
+    if ((unsigned)c >= s - 1) return false;
+    return true;
+}
+
+static void rightturn(int *drow, int *dcol) {
+    if (*drow) { *dcol = -*drow; *drow = 0; }
+    else       { *drow =  *dcol; *dcol = 0; }
+}
 
 void aoc202406(char *data, size_t len) {
-    (void)data;
     (void)len;
+    // assume data is well-formatted and has the same number of rows and columns
+    unsigned size = 1 + strchr(data, '\n') - data;
+    unsigned linear_pos = strchr(data, '^') - data;
+    int row_pos = (int)(linear_pos / size);
+    int col_pos = (int)(linear_pos % size);
+    int deltarow = -1, deltacol = 0;
+    //printf("character at [%u, %u] is '%c'\n", row_pos, col_pos, data[linearize2d(size, row_pos, col_pos)]);
+    unsigned visited = 1;
+    data[linearize2d(size, (unsigned)row_pos, (unsigned)col_pos)] = 'X'; // initial place visited
+    while (validpos(size, row_pos + deltarow, col_pos + deltacol)) {
+        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*/
+            case 'X': row_pos += deltarow;
+                      col_pos += deltacol;
+                      break;
+        }
+    }
+    printf("The guard visits %u positions before leaving the area.\n", visited);
 }
 
 /* === aoc202405 =======================================================
 ===================================================================== */