Index: aoc2024.c
==================================================================
--- aoc2024.c
+++ aoc2024.c
@@ -5,19 +5,65 @@
 #include <stdlib.h>
 #include <string.h>
 
 #include "aocdailies.h"
 #include "aocutils.h"
+
+static size_t rcmap(unsigned size, unsigned row, unsigned col) {
+    return row * (size + 1) + col;
+}
+
+static bool masat(char *data, unsigned size, unsigned row, unsigned col, int drow, int dcol) {
+    int maxrow = (int)row + 3*drow;
+    int maxcol = (int)col + 3*dcol;
+    if ((0 <= maxrow) && (maxrow < (int)size) && (0 <= maxcol) && (maxcol < (int)size)) {
+        if (data[rcmap(size, (unsigned)((int)row + drow), (unsigned)((int)col + dcol))] != 'M') return false;
+        if (data[rcmap(size, (unsigned)((int)row + 2*drow), (unsigned)((int)col + 2*dcol))] != 'A') return false;
+        if (data[rcmap(size, (unsigned)((int)row + 3*drow), (unsigned)((int)col + 3*dcol))] != 'S') return false;
+    } else {
+        return false;
+    }
+    return true;
+}
+
+static unsigned xmasat(char *data, unsigned size, unsigned row, unsigned col) {
+    unsigned finds = 0;
+    if (data[rcmap(size, row, col)] != 'X') return 0;
+    if (masat(data, size, row, col, -1, -1)) finds++;
+    if (masat(data, size, row, col, -1,  0)) finds++;
+    if (masat(data, size, row, col, -1, +1)) finds++;
+    if (masat(data, size, row, col,  0, -1)) finds++;
+    if (masat(data, size, row, col,  0, +1)) finds++;
+    if (masat(data, size, row, col, +1, -1)) finds++;
+    if (masat(data, size, row, col, +1,  0)) finds++;
+    if (masat(data, size, row, col, +1, +1)) finds++;
+    return finds;
+}
 
 void aoc202404(char *data, size_t len) {
     (void)len; // unused argument
     // assume data is well-formatted and has the same number of rows and columns
-    unsigned cols = strchr(data, '\n') - data, rows = cols;
+    unsigned size = strchr(data, '\n') - data;
+    #if 0 // check assumption
+    printf("data has %u cols, first three rows start with %c%c, %c%c, and %c%c.\n",
+          size, data[0], data[1],
+          data[1*(size+1)], data[1*(size+1)+1],
+          data[2*(size+1)], data[2*(size+1)+1]);
+    printf("ends with %c%c.\n", data[(size-1)*(size+1) + size-2], data[(size-1)*(size+1)+size-1]);
     printf("data has %u cols, first three rows start with %c%c, %c%c, and %c%c.\n",
-          rows, data[0], data[1],
-          data[1*(cols+1)], data[1*(cols+1)+1],
-          data[2*(cols+1)], data[2*(cols+1)+1]);
+          size, data[rcmap(size, 0, 0)], data[rcmap(size, 0, 1)],
+          data[rcmap(size, 1, 0)], data[rcmap(size, 1, 1)],
+          data[rcmap(size, 2, 0)], data[rcmap(size, 2, 1)]);
+    printf("ends with %c%c.\n", data[rcmap(size, size-1, size-2)], data[rcmap(size, size-1, size-1)]);
+    #endif
+    unsigned xmascount = 0;
+    for (unsigned row = 0; row < size; row++) {
+        for (unsigned col = 0; col < size; col++) {
+            xmascount += xmasat(data, size, row, col);
+        }
+    }
+    printf("XMAS appears %u times in the little Elf's word search.\n", xmascount);
 }
 
 void aoc202403(char *data, size_t len) {
     (void)len; // unused argument
     unsigned sumproducts = 0, sumproducts2 = 0, term[2];