{
 "cells": [
  {
   "cell_type": "markdown",
   "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"
    }
   },
   "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": {
    "vscode": {
     "languageId": "racket"
    }
   },
   "outputs": [
    {
     "data": {
      "text/html": [
       "<code>7845</code>"
      ],
      "text/plain": [
       "7845"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "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."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "vscode": {
     "languageId": "racket"
    }
   },
   "outputs": [
    {
     "data": {
      "text/html": [
       "<code>2790</code>"
      ],
      "text/plain": [
       "2790"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "(for/sum ([bags (in-slice 3 raw-inventory)])\n",
    "         (~>> bags (map ->set) (apply set-intersect) set-first (hash-ref priority)))\n"
   ]
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Racket (Trusted)",
   "language": "racket",
   "name": "racket-trusted"
  },
  "language_info": {
   "codemirror_mode": "scheme",
   "file_extension": ".rkt",
   "mimetype": "text/x-racket",
   "name": "Racket",
   "pygments_lexer": "racket",
   "version": "8.7"
  },
  "orig_nbformat": 4,
  "vscode": {
   "interpreter": {
    "hash": "7454d72389401259f8ab87ac90deac92d19baedf4a52e60301852b1f4c653c5c"
   }
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}