day 15, part 2

day15-hard/build.zig

@@ -0,0 +1,70 @@
+const std = @import("std");
+
+// Although this function looks imperative, note that its job is to
+// declaratively construct a build graph that will be executed by an external
+// runner.
+pub fn build(b: *std.Build) void {
+    // Standard target options allows the person running `zig build` to choose
+    // what target to build for. Here we do not override the defaults, which
+    // means any target is allowed, and the default is native. Other options
+    // for restricting supported target set are available.
+    const target = b.standardTargetOptions(.{});
+
+    // Standard optimization options allow the person running `zig build` to select
+    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall. Here we do not
+    // set a preferred release mode, allowing the user to decide how to optimize.
+    const optimize = b.standardOptimizeOption(.{});
+
+    const exe = b.addExecutable(.{
+        .name = "day15-hard",
+        // In this case the main source file is merely a path, however, in more
+        // complicated build scripts, this could be a generated file.
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+
+    // This declares intent for the executable to be installed into the
+    // standard location when the user invokes the "install" step (the default
+    // step when running `zig build`).
+    b.installArtifact(exe);
+
+    // This *creates* a Run step in the build graph, to be executed when another
+    // step is evaluated that depends on it. The next line below will establish
+    // such a dependency.
+    const run_cmd = b.addRunArtifact(exe);
+
+    // By making the run step depend on the install step, it will be run from the
+    // installation directory rather than directly from within the cache directory.
+    // This is not necessary, however, if the application depends on other installed
+    // files, this ensures they will be present and in the expected location.
+    run_cmd.step.dependOn(b.getInstallStep());
+
+    // This allows the user to pass arguments to the application in the build
+    // command itself, like this: `zig build run -- arg1 arg2 etc`
+    if (b.args) |args| {
+        run_cmd.addArgs(args);
+    }
+
+    // This creates a build step. It will be visible in the `zig build --help` menu,
+    // and can be selected like this: `zig build run`
+    // This will evaluate the `run` step rather than the default, which is "install".
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+
+    // Creates a step for unit testing. This only builds the test executable
+    // but does not run it.
+    const unit_tests = b.addTest(.{
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+
+    const run_unit_tests = b.addRunArtifact(unit_tests);
+
+    // Similar to creating the run step earlier, this exposes a `test` step to
+    // the `zig build --help` menu, providing a way for the user to request
+    // running the unit tests.
+    const test_step = b.step("test", "Run unit tests");
+    test_step.dependOn(&run_unit_tests.step);
+}

day15-hard/sample.in

@@ -0,0 +1 @@
+rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7

day15-hard/src/main.zig

@@ -0,0 +1,111 @@
+const std = @import("std");
+
+const SEVEN_BYTES: u64 = (1 << 56) - 1;
+
+fn solveLine(line: []const u8) u64 {
+    var boxes = std.mem.zeroes([256][1024]usize);
+
+    var i: usize = 0;
+    while (i < line.len) {
+        var lens_hash: u16 = 0;
+        var lens_name: u64 = 0;
+        while (i < line.len and line[i] != '-' and line[i] != '=') : (i += 1) {
+            lens_hash += line[i];
+            lens_hash *= 17;
+            lens_hash %= 256;
+
+            lens_name *= 256;
+            lens_name += line[i];
+        }
+
+        if (lens_name | SEVEN_BYTES != SEVEN_BYTES) {
+            unreachable;
+        }
+
+        var box = &boxes[lens_hash];
+
+        switch (line[i]) {
+            '-' => {
+                //std.debug.print("Going to remove {x} from box {d}\n", .{ lens_name, lens_hash });
+                for (box, 0..) |*slot, slot_index| {
+                    _ = slot_index;
+                    if (slot.* & SEVEN_BYTES == lens_name) {
+                        //std.debug.print("Removed from {d}\n", .{slot_index});
+                        slot.* = 0;
+                    }
+                }
+
+                i += 1;
+            },
+            '=' => {
+                i += 1;
+
+                switch (line[i]) {
+                    '0'...'9' => {
+                        const focus = line[i] - '0';
+                        const new_slot_value = lens_name | (@as(u64, focus) << 56);
+                        //std.debug.print("Going to insert {x} into box {d}\n", .{ new_slot_value, lens_hash });
+
+                        var slot_index: usize = 0;
+                        while (slot_index < boxes.len) : (slot_index += 1) {
+                            if (box[slot_index] & SEVEN_BYTES == lens_name) {
+                                //std.debug.print("Inserted at {d}\n", .{slot_index});
+                                box[slot_index] = new_slot_value;
+                                break;
+                            }
+                        } else while (slot_index > 0) : (slot_index -= 1) {
+                            if (box[slot_index - 1] != 0) {
+                                //std.debug.print("Inserted at {d}\n", .{slot_index});
+                                box[slot_index] = new_slot_value;
+                                break;
+                            }
+                        } else {
+                            //std.debug.print("Inserted at {d}\n", .{0});
+                            box[0] = new_slot_value;
+                        }
+
+                        i += 1;
+                    },
+                    else => unreachable,
+                }
+            },
+            else => unreachable,
+        }
+
+        if (i < line.len and line[i] != ',') {
+            unreachable;
+        }
+
+        i += 1;
+    }
+
+    var result: u64 = 0;
+    for (boxes, 0..) |box, box_number| {
+        //std.debug.print("Computing result for box {d}\n", .{box_number});
+        var actual_slot_number: usize = 1;
+        for (box) |slot_value| {
+            if (slot_value != 0) {
+                const power = (box_number + 1) * actual_slot_number * (slot_value >> 56);
+                //std.debug.print("Found value in slot {d}: {x}; adding {d} to result\n", .{ actual_slot_number, slot_value, power });
+                result += power;
+                actual_slot_number += 1;
+            }
+        }
+    }
+    return result;
+}
+
+pub fn main() !void {
+    const stdout = std.io.getStdOut().writer();
+
+    const raw_in = std.io.getStdIn();
+    var buffered_reader = std.io.bufferedReader(raw_in.reader());
+    var reader = buffered_reader.reader();
+
+    var result: u64 = 0;
+    var line_buffer: [30000]u8 = undefined;
+    while (try reader.readUntilDelimiterOrEof(&line_buffer, '\n')) |line| {
+        result += solveLine(line);
+    }
+    try stdout.print("{d}\n", .{result});
+}