Complete challenge 2023-03.

Author: Matthew Judy

Sources/2023/03/GearRatios.swift

@@ -61,26 +61,51 @@ import Shared
 /// Of course, the actual engine schematic is much larger. **What is the sum of
 /// all of the part numbers in the engine schematic?**
 ///
+/// # Part Two
+///
+/// The engineer finds the missing part and installs it in the engine! As the
+/// engine springs to life, you jump in the closest gondola, finally ready to
+/// ascend to the water source.
+///
+/// You don't seem to be going very fast, though. Maybe something is still
+/// wrong? Fortunately, the gondola has a phone labeled "help", so you pick it
+/// up and the engineer answers.
+///
+/// Before you can explain the situation, she suggests that you look out the
+/// window. There stands the engineer, holding a phone in one hand and waving
+/// with the other. You're going so slowly that you haven't even left the
+/// station. You exit the gondola.
+///
+/// The missing part wasn't the only issue - one of the gears in the engine is
+/// wrong. A **gear** is any `*` symbol that is adjacent to **exactly two part
+/// numbers**. Its **gear ratio** is the result of multiplying those two
+/// numbers together.
+///
+/// This time, you need to find the gear ratio of every gear and add them all
+/// up so that the engineer can figure out which gear needs to be replaced.
+///
+/// Consider the above engine schematic again.  In this schematic, there are
+/// **two** gears. The first is in the top left; it has part numbers
+/// `467` and `35`, so its gear ratio is `16345`. The second gear is in the
+/// lower right; its gear ratio is `451490`. (The `*` adjacent to 617 is not a
+/// gear because it is only adjacent to one part number.) Adding up all of the
+/// gear ratios produces **`467835`**.
+///
+/// **What is the sum of all of the gear ratios in your engine schematic?**
 ///
 @main
 struct GearRatios: AsyncParsableCommand
 {
-//    /// Adds a `--mode` option to the command, which allows the command's logic
-//    /// to branch and handle requirements of either "Part One" or "Part Two".
-//    /// The option must be followed by a value from this enumeration.
-//    enum Mode: String, ExpressibleByArgument, CaseIterable
-//    {
-//        case <#modeA#>
-//        case <#modeB#>
-//    }
-//    @Option(help: "'<#modeA#>' or '<#modeB#>'")
-//    var mode: Mode
-
-//    /// Adds a flag to the command, named for the behavioral difference in
-//    /// "Part Two."  This allows the command's logic to branch and handle the
-//    /// requirements of either "Part One" or "Part Two".
-//    @Flag(help: "Search for both cardinal values ('one', 'two', ...) and integers.")
-//    var <#partTwoDifference#>: Bool = false
+    /// Adds a "sub-command" argument to the command, which allows the logic
+    /// to branch and handle requirements of either "Part One" or "Part Two".
+    /// The argument must be a value from this enumeration.
+    enum Mode: String, ExpressibleByArgument, CaseIterable
+    {
+        case partNumberSum
+        case gearRatioSum
+    }
+
+    @Argument var mode: Mode
 }
 
 
@@ -95,6 +120,11 @@ struct Part
             var description : String { "(L:\(self.line), C:\(self.column)" }
         }
 
+        enum Glossary: Character
+        {
+            case gear = "*"
+        }
+
         let character : Character
         let location  : Location
     }
@@ -113,6 +143,8 @@ struct Part
         self.symbol  = symbol
         self.numbers = identifiers.map(\.number)
     }
+
+    var isGear: Bool { (self.symbol.character == Symbol.Glossary.gear.rawValue) && (self.numbers.count == 2) }
 }
 
 
@@ -122,7 +154,7 @@ extension GearRatios
 {
     mutating func run() async throws
     {
-        let input: AsyncLineSequence = URL.homeDirectory.appending(path: "Desktop/input.txt").lines
+        let input: AsyncLineSequence = FileHandle.standardInput.bytes.lines
         let lines = try await input.reduce(into: [])
         {
             $0.append($1)
@@ -174,9 +206,21 @@ extension GearRatios
             )
         }
 
-        let partNumberSum: Int = parts.flatMap(\.numbers).reduce(0, +)
-
-        print(partNumberSum)
+        switch self.mode
+        {
+            case .partNumberSum:
+                let partNumberSum = parts.reduce(0) { $0 + $1.numbers.reduce(0, +) }
+                print(partNumberSum)
+
+
+            case .gearRatioSum:
+                let gearRatioSum = parts.reduce(0)
+                { 
+                    return if ($1.isGear == false) { $0 }
+                        else { ($0 + $1.numbers.reduce(1, *)) }
+                }
+                print(gearRatioSum)
+        }
     }
 }