2024 Day 1

2024/.gitignore

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+zig-out
+.zig-cache
+
+/input

2024/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(.{});
+
+    // Creates a step for unit testing. This only builds the test executable
+    // but does not run it.
+    const lib_unit_tests = b.addTest(.{
+        .root_source_file = b.path("src/aoc.zig"),
+        .target = target,
+        .optimize = optimize,
+    });
+
+    const run_lib_unit_tests = b.addRunArtifact(lib_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_lib_unit_tests.step);
+}

2024/build.zig.zon

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+.{
+    // This is the default name used by packages depending on this one. For
+    // example, when a user runs `zig fetch --save <url>`, this field is used
+    // as the key in the `dependencies` table. Although the user can choose a
+    // different name, most users will stick with this provided value.
+    //
+    // It is redundant to include "zig" in this name because it is already
+    // within the Zig package namespace.
+    .name = "aoc",
+
+    // This is a [Semantic Version](https://semver.org/).
+    // In a future version of Zig it will be used for package deduplication.
+    .version = "0.0.0",
+
+    // This field is optional.
+    // This is currently advisory only; Zig does not yet do anything
+    // with this value.
+    //.minimum_zig_version = "0.11.0",
+
+    // This field is optional.
+    // Each dependency must either provide a `url` and `hash`, or a `path`.
+    // `zig build --fetch` can be used to fetch all dependencies of a package, recursively.
+    // Once all dependencies are fetched, `zig build` no longer requires
+    // internet connectivity.
+    .dependencies = .{
+        // See `zig fetch --save <url>` for a command-line interface for adding dependencies.
+        //.example = .{
+        //    // When updating this field to a new URL, be sure to delete the corresponding
+        //    // `hash`, otherwise you are communicating that you expect to find the old hash at
+        //    // the new URL.
+        //    .url = "https://example.com/foo.tar.gz",
+        //
+        //    // This is computed from the file contents of the directory of files that is
+        //    // obtained after fetching `url` and applying the inclusion rules given by
+        //    // `paths`.
+        //    //
+        //    // This field is the source of truth; packages do not come from a `url`; they
+        //    // come from a `hash`. `url` is just one of many possible mirrors for how to
+        //    // obtain a package matching this `hash`.
+        //    //
+        //    // Uses the [multihash](https://multiformats.io/multihash/) format.
+        //    .hash = "...",
+        //
+        //    // When this is provided, the package is found in a directory relative to the
+        //    // build root. In this case the package's hash is irrelevant and therefore not
+        //    // computed. This field and `url` are mutually exclusive.
+        //    .path = "foo",
+
+        //    // When this is set to `true`, a package is declared to be lazily
+        //    // fetched. This makes the dependency only get fetched if it is
+        //    // actually used.
+        //    .lazy = false,
+        //},
+    },
+
+    // Specifies the set of files and directories that are included in this package.
+    // Only files and directories listed here are included in the `hash` that
+    // is computed for this package. Only files listed here will remain on disk
+    // when using the zig package manager. As a rule of thumb, one should list
+    // files required for compilation plus any license(s).
+    // Paths are relative to the build root. Use the empty string (`""`) to refer to
+    // the build root itself.
+    // A directory listed here means that all files within, recursively, are included.
+    .paths = .{
+        "build.zig",
+        "build.zig.zon",
+        "src",
+        // For example...
+        //"LICENSE",
+        //"README.md",
+    },
+}

2024/src/aoc.zig

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+const std = @import("std");
+
+pub const Lines = struct {
+    buf_reader: std.io.BufferedReader(4096, std.fs.File.Reader),
+    line: std.ArrayList(u8),
+
+    pub fn init(allocator: std.mem.Allocator, file: std.fs.File) Lines {
+        return .{
+            .buf_reader = std.io.bufferedReader(file.reader()),
+            .line = std.ArrayList(u8).init(allocator),
+        };
+    }
+
+    pub fn deinit(self: Lines) void {
+        self.line.deinit();
+    }
+
+    pub fn next(self: *Lines) !?std.ArrayList(u8) {
+        const writer = self.line.writer();
+        const reader = self.buf_reader.reader();
+
+        self.line.clearRetainingCapacity();
+        if (reader.streamUntilDelimiter(writer, '\n', null)) {
+            return self.line;
+        } else |err| switch (err) {
+            error.EndOfStream => return null,
+            else => return err,
+        }
+    }
+};

2024/src/day01.zig

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+const std = @import("std");
+const aoc = @import("aoc.zig");
+
+pub fn main() !void {
+    var gpa: std.heap.GeneralPurposeAllocator(.{}) = .{};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+
+    const file = try std.fs.cwd().openFile("input/day01.txt", .{});
+    defer file.close();
+
+    const stdout_file = std.io.getStdOut().writer();
+    var bw = std.io.bufferedWriter(stdout_file);
+    const stdout = bw.writer();
+
+    var lines = aoc.Lines.init(allocator, file);
+    defer lines.deinit();
+
+    var left = std.ArrayList(u32).init(allocator);
+    defer left.deinit();
+
+    var right = std.ArrayList(u32).init(allocator);
+    defer right.deinit();
+
+    while (try lines.next()) |line| {
+        var it = std.mem.tokenizeScalar(u8, line.items, ' ');
+        const l = it.next().?;
+        const r = it.next().?;
+
+        try left.append(try std.fmt.parseInt(u32, l, 10));
+        try right.append(try std.fmt.parseInt(u32, r, 10));
+    }
+
+    std.mem.sort(u32, left.items, {}, comptime std.sort.asc(u32));
+    std.mem.sort(u32, right.items, {}, comptime std.sort.asc(u32));
+
+    try stdout.print("{d}\n", .{part1(left, right)});
+    try stdout.print("{d}\n", .{try part2(allocator, left, right)});
+    try bw.flush();
+}
+
+fn part1(left: std.ArrayList(u32), right: std.ArrayList(u32)) u32 {
+    var i: usize = 0;
+    var distance: u32 = 0;
+    while (i < left.items.len) {
+        distance += abs_diff(left.items[i], right.items[i]);
+        i += 1;
+    }
+
+    return distance;
+}
+
+fn abs_diff(a: u32, b: u32) u32 {
+    if (a > b) {
+        return a - b;
+    } else {
+        return b - a;
+    }
+}
+
+fn part2(allocator: std.mem.Allocator, left: std.ArrayList(u32), right: std.ArrayList(u32)) !u32 {
+    var counts = std.AutoHashMap(u32, u32).init(allocator);
+    defer counts.deinit();
+
+    for (right.items) |r| {
+        const v = counts.get(r) orelse 0;
+        try counts.put(r, v + 1);
+    }
+
+    var similarity: u32 = 0;
+    for (left.items) |l| {
+        similarity += l * (counts.get(l) orelse 0);
+    }
+
+    return similarity;
+}