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+{
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+   "metadata": {},
+   "source": [
+    "### Advent of Code 2022\n",
+    "#### Day 3: Rucksack Reorganization\n",
+    "\n",
+    "**Part 1.** Every elf has a rucksack with two compartments.  What's the total priority value of the items that appear in both compartments of each sack?\n",
+    "\n",
+    "**Part 2.** Each group of three elves is carrying exactly one item in common.  What's the total priority value of those common items?\n",
+    "\n",
+    "To ease the conversion between strings, lists and sets I bring in some utility functions from `relation`. "
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "vscode": {
+     "languageId": "racket"
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+   "outputs": [],
+   "source": [
+    "(require racket\n",
+    "         advent-of-code\n",
+    "         threading\n",
+    "         (only-in relation ->list ->set ->char))\n"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "The items in each elf's inventory are represented by a string of alphabetic (case-sensitive) characters, each with a \"priority value\":\n",
+    "* Lowercase item types `a` through `z` have priorities 1 through 26.\n",
+    "* Uppercase item types `A` through `Z` have priorities 27 through 52."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {
+    "vscode": {
+     "languageId": "racket"
+    }
+   },
+   "outputs": [],
+   "source": [
+    "(define priority\n",
+    "  (for/hash ([i (in-naturals 1)]\n",
+    "             [c (in-sequences (inclusive-range 97 122) (inclusive-range 65 90))])\n",
+    "    (values (->char c) i)))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "##### Part 1\n",
+    "\n",
+    "The front half and back half of the rucksack have one item in common we need to identify.  For each rucksack in the inventory, we apply the following steps:\n",
+    "\n",
+    "1. Split the bag into the two halves,\n",
+    "2. find the common item in both halves (the intersection of the sets),\n",
+    "3. then look up that common item's value. "
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {
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+     "languageId": "racket"
+    }
+   },
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+       "<code>7845</code>"
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+     },
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+   ],
+   "source": [
+    "(define raw-inventory (~> (fetch-aoc-input (find-session) 2022 3) string-split))\n",
+    "\n",
+    "(for/sum ([bag (in-list raw-inventory)])\n",
+    "         (define len (/ (string-length bag) 2))\n",
+    "         (~>> bag\n",
+    "              ->list\n",
+    "              ((λ (xs) (list (take xs len) (drop xs len)))) ; step 1\n",
+    "              (apply set-intersect _) ; step 2\n",
+    "              set-first\n",
+    "              (hash-ref priority))) ; step 3\n"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "##### Part 2\n",
+    "\n",
+    "Now we need to take three rucksacks at a time and find the common item they all share.  The procedure is simpler than Part 1's, especially with the `in-slice` function that automatically forms groups of three."
+   ]
+  },
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+   "execution_count": 4,
+   "metadata": {
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+   ],
+   "source": [
+    "(for/sum ([bags (in-slice 3 raw-inventory)])\n",
+    "         (~>> bags (map ->set) (apply set-intersect) set-first (hash-ref priority)))\n"
+   ]
+  }
+ ],
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