Index: aoc2024.c
==================================================================
--- aoc2024.c
+++ aoc2024.c
@@ -4,10 +4,63 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include "aocdailies.h"
 #include "aocutils.h"
+
+/* === aoc202409 =======================================================
+   WOW! input consists of a 20,000 long string of numbers!
+===================================================================== */
+
+void aoc202409(char *data, size_t len) {
+    (void)len; // unused argument
+    int *disk = malloc(512 * sizeof *disk);
+    size_t rdisk = 512;
+    size_t ndisk = 0;
+    int fileid = 0;
+    while (*data) {
+        for (int k = 0; k < *data - '0'; k++) {
+            if (ndisk == rdisk) {
+                // assume it works
+                rdisk = (13*rdisk)/8;
+                disk = realloc(disk, rdisk * sizeof *disk);
+            }
+            disk[ndisk++] = fileid;
+        }
+        fileid++;
+        data++;
+        if (*data) {
+            for (int k = 0; k < *data - '0'; k++) {
+                if (ndisk == rdisk) {
+                    // assume it works
+                    rdisk = (13*rdisk)/8;
+                    disk = realloc(disk, rdisk * sizeof *disk);
+                }
+                disk[ndisk++] = -1;
+            }
+            data++;
+        }
+    }
+    int *left = disk, *right = disk + ndisk;
+    for (;;) {
+        while (*left != -1) left++;
+        if (left >= right) break;
+        while (right[-1] == -1) right--;
+        // swap *left and right[-1]
+        int tmp = *--right;
+        *right = *left;
+        *left++ = tmp;
+    }
+    unsigned long long chksum = 0;
+    size_t index = 0;
+    while (disk[index] >= 0) {
+        chksum += (size_t)disk[index] * index;
+        index++;
+    }
+    printf("%llu\n", chksum);
+    free(disk);
+}
 
 /* === aoc202408 =======================================================
    Oh! another square of text!
    Idea: for all points p with an antenna find all points q>p with
 a corresponding frequency. For each such pair calculate the 2 antinodes

Index: aocdailies.c
==================================================================
--- aocdailies.c
+++ aocdailies.c
@@ -5,10 +5,11 @@
     aocfunc *p;
     switch (y * 100 + d) {
         default: p = NULL; break;
 
         //   YYYYdd ==>  aocYYYYdd
+        case 202409: p = aoc202409; break;
         case 202408: p = aoc202408; break;
         case 202407: p = aoc202407; break;
         case 202406: p = aoc202406; break;
         case 202405: p = aoc202405; break;
         case 202404: p = aoc202404; break;

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 aoc202409;
 aocfunc aoc202408;
 aocfunc aoc202407;
 aocfunc aoc202406;
 aocfunc aoc202405;
 aocfunc aoc202404;