aoc2024: day 9 part 1

1 files changed, 89 insertions, 0 deletions

diff --git a/aoc24/src/day9.zig b/aoc24/src/day9.zig
new file mode 100644
index 0000000..520391d
--- /dev/null
+++ b/aoc24/src/day9.zig
@@ -0,0 +1,89 @@
+const std = @import("std");
+
+const Input = struct {
+    compressed: []Entry,
+    uncompressed: std.ArrayList(?u32),
+
+    const Entry = struct { taken: u8, free: u8 };
+};
+
+pub fn parse(allocator: std.mem.Allocator, data: []const u8) !Input {
+    var compressed = std.ArrayList(Input.Entry).init(allocator);
+    var i: usize = 0;
+    while (i < data.len) : (i += 2) {
+        try compressed.append(.{ .taken = data[i] - '0', .free = if (i + 1 == data.len or data[i + 1] == '\n') 0 else data[i + 1] - '0' });
+    }
+
+    var uncompressed = std.ArrayList(?u32).init(allocator);
+    for (0.., compressed.items) |j, entry| {
+        try uncompressed.appendNTimes(@intCast(j), entry.taken);
+        try uncompressed.appendNTimes(null, entry.free);
+    }
+    return .{ .compressed = compressed.items, .uncompressed = uncompressed };
+}
+
+const test_input = "2333133121414131402";
+
+test "part1" {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+
+    const output = try part1(arena.allocator(), try parse(arena.allocator(), test_input));
+    std.debug.print("got {}\n", .{output});
+    std.debug.assert(output == 1928);
+}
+
+pub fn part1(allocator: std.mem.Allocator, input: Input) !u64 {
+    var filesystem = std.ArrayList(u32).init(allocator);
+    var unmoved: u32 = 0;
+    var moved: u32 = @intCast(input.compressed.len);
+    var moved_left: u8 = 0;
+    while (unmoved < moved) {
+        try filesystem.appendNTimes(unmoved, input.compressed[unmoved].taken);
+        for (0..input.compressed[unmoved].free) |_| {
+            while (moved_left == 0) {
+                moved -= 1;
+                moved_left = input.compressed[moved].taken;
+            }
+            try filesystem.append(moved);
+            moved_left -= 1;
+        }
+        unmoved += 1;
+    }
+    try filesystem.appendNTimes(moved, moved_left);
+
+    // for (filesystem.items) |item| {
+    //     std.debug.print("{}", .{item});
+    // }
+    // std.debug.print("\n", .{});
+
+    var sum: u64 = 0;
+    for (0.., filesystem.items) |pos, id| {
+        sum += pos * @as(u64, @intCast(id));
+    }
+
+    return sum;
+}
+
+test "part2" {
+    var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
+    defer arena.deinit();
+
+    const output = try part2(arena.allocator(), try parse(arena.allocator(), test_input));
+    std.debug.print("got {}\n", .{output});
+    std.debug.assert(output == 2858);
+}
+
+pub fn part2(allocator: std.mem.Allocator, input: Input) !u32 {
+    var filesystem = std.ArrayList(u32).init(allocator);
+
+    _ = .{ filesystem, input };
+    filesystem = undefined;
+
+    var sum: u64 = 0;
+    for (0.., filesystem.items) |pos, id| {
+        sum += pos * @as(u64, @intCast(id));
+    }
+
+    return sum;
+}