Index: aoc2024.c
==================================================================
--- aoc2024.c
+++ aoc2024.c
@@ -7,10 +7,13 @@
 #include "aocdailies.h"
 #include "aocutils.h"
 
 /* === aoc202409 =======================================================
    WOW! input consists of a 20,000 long string of numbers!
+   Make an array of blocks, each with either -1 or the fileid
+   For Part Two: make an array of blocks with the encoded value of
+(fileid * 10) + length, or, when empty, the negative length
 ===================================================================== */
 
 void aoc202409(char *data, size_t len) {
     (void)len; // unused argument
     int *disk = malloc(512 * sizeof *disk);
@@ -22,11 +25,11 @@
             if (ndisk == rdisk) {
                 // assume it works
                 rdisk = (13*rdisk)/8;
                 disk = realloc(disk, rdisk * sizeof *disk);
             }
-            disk[ndisk++] = (fileid * 10) + (*data - '0'); // encode fileid and length
+            disk[ndisk++] = (fileid * 10) + (*data - '0'); // encoded
         }
         fileid++;
         data++;
         if (*data) {
             for (int k = 0; k < *data - '0'; k++) {
@@ -33,24 +36,25 @@
                 if (ndisk == rdisk) {
                     // assume it works
                     rdisk = (13*rdisk)/8;
                     disk = realloc(disk, rdisk * sizeof *disk);
                 }
-                disk[ndisk++] = -1 * (*data - '0');
+                disk[ndisk++] = -1 * (*data - '0'); // negative length
             }
             data++;
         }
     }
-    // copy disk
+    // copy disk for Part Two
     int *diskcopy = malloc(ndisk * sizeof *disk);
     memcpy(diskcopy, disk, ndisk * sizeof *disk);
 
     int *left = disk, *right = disk + ndisk;
     for (;;) {
         while (*left >= 0) left++;
         if (left >= right) break;
         while (right[-1] < 0) right--;
+        if (left >= right) break;
         // swap *left and right[-1]
         int tmp = *--right;
         *right = *left;
         *left++ = tmp;
     }
@@ -62,10 +66,50 @@
     }
     printf("%llu\n", chksum);
     free(disk);
 
     // part 2
+    for (int filetomove = fileid - 1; filetomove > 0; filetomove--) {
+        int *startoffile = diskcopy;
+        while (*startoffile / 10 != filetomove) {
+            if (*startoffile < 0) startoffile += -*startoffile;
+            else startoffile += *startoffile % 10;
+        }
+        int blocksneeded = *startoffile % 10;
+        int *bigblock = diskcopy;
+        while (bigblock < startoffile) {
+            if (*bigblock < 0) {
+                if (-*bigblock < blocksneeded) {
+                    bigblock += -*bigblock;
+                } else {
+                    // found a suitable block: move and quit tightest loop
+                    int bbsize = -*bigblock;
+                    int k;
+                    for (k = 0; k < blocksneeded; k++) {
+                        int tmp = startoffile[k];
+                        startoffile[k] = bigblock[k];
+                        bigblock[k] = tmp;
+                    }
+                    for (; k < bbsize; k++) {
+                        bigblock[k] += blocksneeded;
+                    }
+                    break;
+                }
+            } else {
+                bigblock += *bigblock % 10;
+            }
+        }
+    }
+
+    unsigned long long chksum2 = 0;
+    index = 0;
+    for (index = 0; index < ndisk; index++) {
+        if (diskcopy[index] > 0) {
+            chksum2 += ((size_t)diskcopy[index]/10) * index;
+        }
+    }
+    printf("%llu\n", chksum2);
     free(diskcopy);
 }
 
 /* === aoc202408 =======================================================
    Oh! another square of text!