Day 9

Sources/AdventOfCode2024/AdventOfCode2024.swift

@@ -10,6 +10,7 @@ public struct AdventOfCode2024: Year {
         Day6(),
         Day7(),
         Day8(),
+        Day9(),
         // {DAYS}
     ]
 

Sources/AdventOfCode2024/Day9/Day9.swift

@@ -0,0 +1,110 @@
+import Algorithms
+import Foundation
+import Utilities
+
+public struct Day9: Day {
+    public func part1Solution(for input: Input) -> Int {
+        var blockRanges: [BlockRange] = []
+        for (offset, length) in input.characters.integers.enumerated() {
+            let id = offset.isMultiple(of: 2) ? offset / 2 : nil
+            blockRanges.append(.init(id: id, length: length))
+        }
+
+        var insertableRanges = blockRanges.filter { $0.id != nil }
+        var insertingRange = insertableRanges.removeLast()
+
+        var compressedBlockRanges: [BlockRange] = []
+
+        for blockRange in blockRanges {
+            if let id = blockRange.id {
+                guard id < insertingRange.id! else { break }
+
+                compressedBlockRanges.append(blockRange)
+                continue
+            }
+
+            var remainingLength = blockRange.length
+            while remainingLength > 0 {
+                let length = min(remainingLength, insertingRange.length)
+                remainingLength -= length
+
+                var insertedRange = insertingRange
+                insertedRange.length = length
+
+                compressedBlockRanges.append(insertedRange)
+
+                insertingRange.length -= length
+                if insertingRange.length == 0 {
+                    insertingRange = insertableRanges.removeLast()
+                }
+            }
+        }
+
+        if insertingRange.length > 0 {
+            compressedBlockRanges.append(insertingRange)
+        }
+
+        var index = 0
+        var checksum = 0
+        for blockRange in compressedBlockRanges {
+            checksum += blockRange.checksum(at: index)
+            index += blockRange.length
+        }
+        return checksum
+    }
+
+    public func part2Solution(for input: Input) -> Int {
+        var blockRanges: [BlockRange] = []
+        for (offset, length) in input.characters.integers.enumerated().reversed() {
+            let id = offset.isMultiple(of: 2) ? offset / 2 : nil
+            blockRanges.append(.init(id: id, length: length))
+        }
+
+        var i = 0
+        while i < blockRanges.count {
+            defer { i += 1 }
+
+            let blockRange = blockRanges[i]
+            guard blockRange.id != nil else {
+                continue
+            }
+
+            for (j, emptyRange) in blockRanges.enumerated().dropFirst(i + 1).reversed() {
+                guard emptyRange.id == nil,
+                      emptyRange.length >= blockRange.length
+                else { continue }
+
+                if emptyRange.length == blockRange.length {
+                    blockRanges[j] = blockRange
+                } else {
+                    blockRanges[j].length -= blockRange.length
+                    blockRanges.insert(blockRange, at: j+1)
+                }
+                blockRanges[i].id = nil
+                break
+            }
+        }
+
+        var index = 0
+        var checksum = 0
+        for blockRange in blockRanges.reversed() {
+            checksum += blockRange.checksum(at: index)
+            index += blockRange.length
+        }
+        return checksum
+    }
+
+    struct BlockRange: CustomStringConvertible {
+        var id: Int?
+        var length: Int
+
+        var description: String {
+            String(repeating: id.map { "\($0)" } ?? " ", count: length)
+        }
+
+        func checksum(at index: Int) -> Int {
+            guard let id else { return 0}
+            return id * ((length - 1).gaussSum + index * length)
+        }
+    }
+}