From 46ad473c826149a1c36f668779aaa7f8d72229d3 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <code@jdkaplan.dev>
Date: Sun, 8 Dec 2024 22:29:31 -0800
Subject: [PATCH] 2024 Day 9 Part 1

---
 2024/src/day09.zig | 160 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 160 insertions(+)
 create mode 100644 2024/src/day09.zig

diff --git a/2024/src/day09.zig b/2024/src/day09.zig
new file mode 100644
index 0000000..0db140c
--- /dev/null
+++ b/2024/src/day09.zig
@@ -0,0 +1,160 @@
+const std = @import("std");
+const aoc = @import("aoc.zig");
+
+const Allocator = std.mem.Allocator;
+const ArrayList = std.ArrayList;
+
+pub fn main() !void {
+    var gpa: std.heap.GeneralPurposeAllocator(.{}) = .{};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+
+    const file = try std.fs.cwd().openFile("input/day09.txt", .{});
+    defer file.close();
+
+    const text = try file.readToEndAlloc(allocator, comptime 1 << 30);
+    defer allocator.free(text);
+
+    var disk = try Disk.parse(allocator, text);
+    defer disk.deinit();
+
+    const stdout_file = std.io.getStdOut().writer();
+    var bw = std.io.bufferedWriter(stdout_file);
+    const stdout = bw.writer();
+
+    try stdout.print("{d}\n", .{try part1(allocator, disk)});
+    try bw.flush();
+}
+
+fn part1(_: Allocator, start: Disk) !u64 {
+    var disk = start;
+    while (try disk.defrag_step()) {}
+    return disk.checksum();
+}
+
+const File = struct {
+    id: usize,
+    len: u64,
+};
+
+const Run = union(enum) {
+    file: File,
+    empty: struct {
+        len: u64,
+    },
+
+    fn empty(len: u64) Run {
+        return Run{ .empty = .{ .len = len } };
+    }
+
+    fn file(id: usize, len: u64) Run {
+        return Run{ .file = .{ .id = id, .len = len } };
+    }
+};
+
+const Disk = struct {
+    runs: ArrayList(Run),
+
+    fn deinit(self: *Disk) void {
+        self.runs.deinit();
+    }
+
+    fn parse(allocator: Allocator, text: []const u8) !Disk {
+        var runs = ArrayList(Run).init(allocator);
+
+        var id: usize = 0;
+        var isFile = true;
+        for (text) |char| {
+            if (char == '\n') {
+                break;
+            }
+
+            const len = char - '0';
+            if (isFile) {
+                try runs.append(Run{ .file = .{ .id = id, .len = len } });
+                id += 1;
+            } else {
+                try runs.append(Run{ .empty = .{ .len = len } });
+            }
+
+            isFile = !isFile;
+        }
+
+        return .{ .runs = runs };
+    }
+
+    fn defrag_step(self: *Disk) !bool {
+        var last: File = undefined;
+
+        while (true) {
+            switch (self.runs.pop()) {
+                .file => |file| {
+                    last = file;
+                    break;
+                },
+                .empty => continue,
+            }
+        }
+
+        var i: usize = 0;
+        while (i < self.runs.items.len) : (i += 1) {
+            const want = switch (self.runs.items[i]) {
+                .file => continue,
+                .empty => |empty| empty.len,
+            };
+
+            if (last.len == want) {
+                self.runs.items[i] = Run.file(last.id, want);
+            } else if (last.len > want) {
+                self.runs.items[i] = Run.file(last.id, want);
+                try self.runs.append(Run.file(last.id, last.len - want));
+            } else if (last.len < want) {
+                self.runs.items[i] = Run.file(last.id, last.len);
+                try self.runs.insert(i + 1, Run.empty(want - last.len));
+            }
+
+            return true;
+        }
+
+        try self.runs.append(Run{ .file = last });
+
+        return false;
+    }
+
+    fn checksum(self: Disk) u64 {
+        var sum: u64 = 0;
+        var i: usize = 0;
+        for (self.runs.items) |run| {
+            switch (run) {
+                .file => |file| {
+                    for (0..file.len) |j| {
+                        sum += file.id * (i + j);
+                    }
+                    i += file.len;
+                },
+                .empty => unreachable,
+            }
+        }
+        return sum;
+    }
+
+    pub fn format(
+        self: Disk,
+        comptime _: []const u8,
+        _: std.fmt.FormatOptions,
+        writer: anytype,
+    ) !void {
+        for (self.runs.items) |run| {
+            switch (run) {
+                .file => |file| {
+                    for (0..file.len) |_| {
+                        try writer.print("{}", .{file.id});
+                    }
+                },
+                .empty => |empty| for (0..empty.len) |_| {
+                    try writer.print(".", .{});
+                },
+            }
+        }
+    }
+};