To paraphrase the puzzle, Santa takes a walk with an Elf, and they play a game involving a bag of colorful cubes. In each game, there is an unknown number of each colored cubes in the bag, and the Elf pulls out a number of cubes, for example:

Game 1: 3 blue, 4 red; 1 red, 2 green, 6 blue; 2 green
Game 2: 1 blue, 2 green; 3 green, 4 blue, 1 red; 1 green, 1 blue
Game 3: 8 green, 6 blue, 20 red; 5 blue, 4 red, 13 green; 5 green, 1 red
Game 4: 1 green, 3 red, 6 blue; 3 green, 6 red; 3 green, 15 blue, 14 red
Game 5: 6 red, 1 blue, 3 green; 2 blue, 1 red, 2 green

Part 1

In part 1, we want to know which games would have been possible if the bag contained only 12 red cubes, 13 green cubes, and 14 blue cubes. The answer is the sum of the eligible game ids, in the above example 1 + 2 + 5 = 8.

#!/usr/bin/env bash
#
# Solver for https://adventofcode.com/2023/day/2 part 1
# Redirect the input file to this script, for example day2part1.sh < path/to/input.txt
#

set -euo pipefail

max_red=12
max_green=13
max_blue=14

is_eligible_cubes() {
  local count color
  count=$1
  color=$2

  case $color in
    red) ((max_red < count)) && return 1 ;;
    green) ((max_green < count)) && return 1 ;;
    blue) ((max_blue < count)) && return 1 ;;
  esac

  return 0
}

is_eligible_game() {
  local iterations count color
  iterations=$1

  while [ "$iterations" ]; do
    [[ $iterations =~ ([0-9]+)\ (red|green|blue) ]] || break
    count=${BASH_REMATCH[1]}
    color=${BASH_REMATCH[2]}
    iterations=${iterations:${#BASH_REMATCH[0]}}
    iterations=${iterations#*[,;]}
    is_eligible_cubes "$count" "$color" || return 1
  done

  return 0
}

solve_day2_part1() {
  local sum game_id iterations
  sum=0

  while read _ game_id iterations; do
    game_id=${game_id%:}
    is_eligible_game "$iterations" && ((sum += game_id))
  done

  echo "$sum"
}

solve_day2_part1

Part 2

To paraphrase the change in part 2, for each game, we're looking for the maximum number of cubes per color that were drawn, multiply them together, and then sum this number for all games. With the example above this would be 48 + 12 + 1560 + 630 + 36 = 2286.

#!/usr/bin/env bash
#
# Solver for https://adventofcode.com/2023/day/2 part 2
# Redirect the input file to this script, for example day2part2.sh < path/to/input.txt
#

set -euo pipefail

solve_day2_part2() {
  local sum game_id iterations count color red green blue
  sum=0

  while read _ game_id iterations; do
    game_id=${game_id%:}
    red=0
    green=0
    blue=0

    while [ "$iterations" ]; do
      [[ $iterations =~ ([0-9]+)\ (red|green|blue) ]] || break
      count=${BASH_REMATCH[1]}
      color=${BASH_REMATCH[2]}
      iterations=${iterations:${#BASH_REMATCH[0]}}
      iterations=${iterations#*[,;]}
      case $color in
        red) ((red < count)) && red=$count ;;
        green) ((green < count)) && green=$count ;;
        blue) ((blue < count)) && blue=$count ;;
      esac
    done

    ((sum += red * green * blue))

  done

  echo "$sum"
}

solve_day2_part2

Review request

What would you do differently?