Index: aoc2025.c
==================================================================
--- aoc2025.c
+++ aoc2025.c
@@ -4,10 +4,76 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include "aocdailies.h"
 #include "aocutils.h"
+
+#if 0
+/* === aocYYYYDD =======================================================
+===================================================================== */
+void aocYYYYDD(char *data, size_t len) {
+    (void)len; // unused argument
+}
+#endif
+
+/* === aoc202504 =======================================================
+===================================================================== */
+static unsigned evolve(struct RectangularMap *rm, unsigned src) {
+    unsigned changes = 0;
+    unsigned dst = 1 - src;
+    for (int row = 0; row < rm->rows; row++) {
+        for (int col = 0; col < rm->cols; col++) {
+            char cc = RMcharat(rm + src, col, row);
+            char *destin = RMcharptr(rm + dst, col, row);
+            if (cc == '@') {
+                unsigned nearby = 0;
+                for (int deltarow = -1; deltarow <= 1; deltarow++) {
+                    for (int deltacol = -1; deltacol <= 1; deltacol++) {
+                        nearby += (RMcharat(rm + src, col+deltacol,
+                                            row+deltarow) == '@');
+                    }
+                }
+                nearby -= 1; // remove self
+                if (nearby < 4) {
+                    *destin = '.';
+                    changes++;
+                } else {
+                    *destin = '@';
+                }
+            } else if (cc != 0) {
+                *destin = '.';
+            } else {
+                *destin = 0; // shouldn't happen
+            }
+        }
+    }
+    return changes;
+}
+
+void aoc202504(char *data, size_t len) {
+    (void)len; // unused argument
+    struct RectangularMap rm[2] = {0};
+    while (*data) {
+        char *data2 = data;
+        while (*data != '\n') data++;
+        *data = 0; // erase newline
+        RMaddline(rm + 0, data2);
+        RMaddline(rm + 1, data2); // set both rm's to the same thing
+        *data++ = '\n'; // unerase newline and skip it
+    }
+    unsigned part1 = evolve(rm, 0); // evolve from rm[0] to rm[1]
+    unsigned part2 = part1, src = 1;
+    for (;;) {
+        unsigned tmp = evolve(rm, src); // keep evolving
+        if (tmp == 0) break; // STOP
+        src = 1 - src;                  // back and forth
+        part2 += tmp; // add changes in this round
+    }
+    printf("P1: %u; P2: %u\n", part1, part2);
+    RMfree(rm);
+    RMfree(rm + 1);
+}
 
 /* === aoc202503 =======================================================
 ===================================================================== */
 static long long unsigned maxj(const unsigned char *b, unsigned blen, unsigned nlen) {
     if (nlen == 0) return 0;

Index: aocdailies.c
==================================================================
--- aocdailies.c
+++ aocdailies.c
@@ -4,10 +4,11 @@
 aocfunc *aocselect(unsigned y, unsigned d) {
     aocfunc *p;
     switch (y * 100 + d) {
         default: p = NULL; break;
 
+        case 202504: p = aoc202504; break;
         case 202503: p = aoc202503; break;
         case 202502: p = aoc202502; break;
         case 202501: p = aoc202501; break;
 
         //   YYYYdd ==>  aocYYYYdd

Index: aocdailies.h
==================================================================
--- aocdailies.h
+++ aocdailies.h
@@ -4,10 +4,11 @@
 #include <stddef.h>
 
 typedef void aocfunc(char *, size_t);
 aocfunc *aocselect(unsigned, unsigned);
 
+aocfunc aoc202504;
 aocfunc aoc202503;
 aocfunc aoc202502;
 aocfunc aoc202501;
 
 aocfunc aoc202422;

Index: aocutils.c
==================================================================
--- aocutils.c
+++ aocutils.c
@@ -2,10 +2,62 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include "aocutils.h"
+
+char RMcharat(struct RectangularMap *sm, int col, int row) {
+    if (col < 0) return 0;
+    if (row < 0) return 0;
+    if (col >= sm->cols) return 0;
+    if (row >= sm->rows) return 0;
+    return sm->data[row*sm->cols + col];
+}
+
+char *RMcharptr(struct RectangularMap *sm, int col, int row) {
+    if (col < 0) return NULL;
+    if (row < 0) return NULL;
+    if (col >= sm->cols) return NULL;
+    if (row >= sm->rows) return NULL;
+    return sm->data + row*sm->cols + col;
+}
+
+void RMcopy(struct RectangularMap *restrict dst,
+            const struct RectangularMap *restrict src) {
+    size_t size = (size_t)src->cols * (size_t)src->rows;
+    char *tmp = realloc(dst->data, size);
+    if (tmp == NULL) {
+        fprintf(stderr, "not enough memory\n");
+        exit(EXIT_FAILURE);
+    }
+    dst->data = tmp;
+    memcpy(dst->data, src->data, size);
+}
+
+void RMaddline(struct RectangularMap *sm, const char *lin) {
+    int linlen = strlen(lin);
+    if (sm->cols && (sm->cols != linlen)) {
+        // error
+        fprintf(stderr, "wrong line length!\n");
+        exit(EXIT_FAILURE);
+    }
+    if (sm->cols == 0) sm->cols = linlen;
+    char *tmp = realloc(sm->data, (size_t)(sm->rows+1) * (size_t)sm->cols);
+    if (tmp == NULL) {
+        fprintf(stderr, "not enough memory\n");
+        exit(EXIT_FAILURE);
+    }
+    sm->data = tmp;
+    memcpy(sm->data + sm->rows*sm->cols, lin, (size_t)sm->cols);
+    sm->rows += 1;
+}
+
+void RMfree(struct RectangularMap *sm) {
+    free(sm->data);
+    sm->data = NULL;
+    sm->cols = sm->rows = 0;
+}
 
 static uint32_t f(uint32_t x, uint32_t y, uint32_t z) {
     return (x & y) | (~x & z);
 }
 

Index: aocutils.h
==================================================================
--- aocutils.h
+++ aocutils.h
@@ -6,10 +6,21 @@
 struct TextGrid {
     unsigned cols, rows;
     char *data; // may have '\n' at end of rows
 };
 
+struct RectangularMap {
+    int cols, rows;
+    char *data;
+};
+char RMcharat(struct RectangularMap *sm, int col, int row);
+char *RMcharptr(struct RectangularMap *sm, int col, int row);
+void RMcopy(struct RectangularMap *restrict dst,
+            const struct RectangularMap *restrict src);
+void RMaddline(struct RectangularMap *sm, const char *lin);
+void RMfree(struct RectangularMap *sm);
+
 long long unsigned upow(unsigned base, unsigned exponent);
 
 void md5mini(unsigned char *dstarr, const char *src);
 
 bool TGvalid(struct TextGrid *tg, unsigned col, unsigned row);