day 11, part 2

day11-hard/build.zig

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+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 = "day11-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);
+}

day11-hard/hard.in

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day11-hard/sample.in

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+...#......
+.......#..
+#.........
+..........
+......#...
+.#........
+.........#
+..........
+.......#..
+#...#.....

day11-hard/src/main.zig

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+const std = @import("std");
+
+const Char = enum(u2) {
+    Galaxy,
+    Empty,
+};
+
+fn solve(projections: [2][]const u32) u64 {
+    var result: u64 = 0;
+
+    for (projections) |projection| {
+        var i: usize = 0;
+        while (i < projection.len) : (i += 1) {
+            if (projection[i] == 0) {
+                continue;
+            }
+
+            var distance: u64 = 0;
+            var j = i + 1;
+            while (j < projection.len) : (j += 1) {
+                if (projection[j] == 0) {
+                    distance += 1000000;
+                } else {
+                    distance += 1;
+                    result += projection[i] * projection[j] * distance;
+                }
+            }
+        }
+    }
+
+    return result;
+}
+
+fn charLinesToProjections(allocator: std.mem.Allocator, char_lines: [][]const Char) ![2][]const u32 {
+    var rows = try allocator.alloc(u32, char_lines.len);
+    var columns = try allocator.alloc(u32, char_lines[0].len);
+    @memset(rows, 0);
+    @memset(columns, 0);
+    for (char_lines, 0..) |char_line, i| {
+        for (char_line, 0..) |char, j| {
+            if (char == .Galaxy) {
+                rows[i] += 1;
+                columns[j] += 1;
+            }
+        }
+    }
+
+    return .{ rows, columns };
+}
+
+fn readInput(allocator: std.mem.Allocator, reader: anytype) ![][]const Char {
+    var lines = std.ArrayList([]Char).init(allocator);
+    var line_buffer: [1000]u8 = undefined;
+    while (try reader.readUntilDelimiterOrEof(&line_buffer, '\n')) |line_ref| {
+        var line = try allocator.alloc(Char, line_ref.len);
+        for (line_ref, line) |raw_char, *char| {
+            char.* = switch (raw_char) {
+                '#' => .Galaxy,
+                '.' => .Empty,
+                else => unreachable,
+            };
+        }
+        try lines.append(line);
+    }
+
+    return lines.items;
+}
+
+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 allocator_buffer: [1_000_000]u8 = undefined;
+    var fba = std.heap.FixedBufferAllocator.init(&allocator_buffer);
+    var allocator = fba.allocator();
+
+    var lines_arena = std.heap.ArenaAllocator.init(allocator);
+    const lines_allocator = lines_arena.allocator();
+    const char_lines = try readInput(lines_allocator, reader);
+
+    var projections_arena = std.heap.ArenaAllocator.init(allocator);
+    const projections_allocator = projections_arena.allocator();
+    const projections = try charLinesToProjections(projections_allocator, char_lines);
+    lines_arena.deinit();
+
+    const result = solve(projections);
+    projections_arena.deinit();
+
+    try stdout.print("{d}\n", .{result});
+}