From 7c4d518400682910090c492455d6b2a9280e55ba Mon Sep 17 00:00:00 2001
From: Inga <inga-lovinde@noreply.oomza.cutegay.software>
Date: Sun, 24 Dec 2023 16:42:01 +0000
Subject: [PATCH] day 24, part 1

---
 day24-easy/build.zig    |  70 ++++++++++
 day24-easy/easy.in      | 301 ++++++++++++++++++++++++++++++++++++++++
 day24-easy/sample.in    |   6 +
 day24-easy/src/main.zig | 274 ++++++++++++++++++++++++++++++++++++
 4 files changed, 651 insertions(+)
 create mode 100644 day24-easy/build.zig
 create mode 100644 day24-easy/easy.in
 create mode 100644 day24-easy/sample.in
 create mode 100644 day24-easy/src/main.zig

diff --git a/day24-easy/build.zig b/day24-easy/build.zig
new file mode 100644
index 0000000..9e5ab34
--- /dev/null
+++ b/day24-easy/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 = "day24-easy",
+        // 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);
+}
diff --git a/day24-easy/easy.in b/day24-easy/easy.in
new file mode 100644
index 0000000..50cf2ba
--- /dev/null
+++ b/day24-easy/easy.in
@@ -0,0 +1,301 @@
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+258839444803333, 359307272174162, 491040464305920 @ -101, -71, -189
+438852036816038, 234662011691490, 230447917164767 @ -154, 176, 303
+319019287398668, 116780954839674, 124625877397421 @ 99, 468, 488
+352917393750542, 374422600929240, 331383505608641 @ -152, -164, -50
+195344845860214, 301121886409204, 332964776613930 @ 38, -17, -25
+400522757703600, 308426702399630, 196988468040207 @ -161, -136, 257
+284703994903406, 285213238709084, 384924770280109 @ -36, -7, -141
+362232770762533, 530818901488834, 343940129727285 @ -217, -287, -22
+45157694746670, 320876742566838, 375111995820257 @ 138, -24, -50
+289981591630079, 268210148916372, 256460299333274 @ 7, 15, 79
+331311259361931, 87468879551434, 251328459377219 @ 7, 493, 88
+255647357313974, 117174591735492, 264243000879137 @ 144, 367, 58
+39001138288358, 114608685277716, 247611338464993 @ 216, 234, 96
+78345398144248, 340057542653440, 372356987549459 @ 128, -53, -56
+424107853080798, 240875316375644, 320063937315273 @ 101, 118, -909
+156344745449478, 302595795634244, 241698220535297 @ 31, -7, 103
+259179525889580, 255577897264032, 245657998679240 @ 176, 35, 103
+210437756820600, 308843950567838, 334302104162039 @ -29, -16, -11
+304722473056688, 167943309812562, 245775315224337 @ 27, 246, 102
+248122674393056, 235916327443281, 327786852013169 @ -120, 72, 14
+277043010072101, 123245559791018, 410107190763725 @ -105, 217, -110
+366478985554055, 303131976216915, 315679631866466 @ -93, -94, -85
+137462543579714, 193702341645138, 279591949608261 @ 24, 120, 60
+326574684867086, 300236980784568, 384613730429168 @ 30, -93, -287
+315741107573126, 240787447589859, 243709950017684 @ 54, 74, 109
+355983983336914, 231680004259508, 342129249097317 @ -119, 91, -105
+404485560780974, 268790246483668, 317108024873793 @ -159, -19, -138
+419149424470741, 226367239846582, 244911238158061 @ -95, 185, 122
+407961452161350, 418167010527684, 231986081894817 @ -88, -837, 183
+67044622101206, 292746298461236, 438291405083389 @ 90, 11, -111
+274658774358620, 321359106006246, 308958759350345 @ 67, -106, -38
+173007315404390, 264144818818980, 34813586299265 @ 164, 29, 470
+102839643978206, 304673187492884, 348410364511329 @ 81, -7, -22
+354380589155632, 388319481114849, 286851917289450 @ -152, -192, 27
+96907155993854, 281599092045428, 384838864876129 @ 39, 25, -46
+291523929178939, 384166057783565, 354070361522453 @ -13, -208, -110
+345463466774126, 269347673310984, 253030914846059 @ 34, -19, 81
+249599421151031, 394069474260792, 366619266165953 @ 5, -181, -98
+394927184083574, 236003527736340, 249133632375809 @ -44, 105, 94
+362710918466496, 490756620149074, 390801202288303 @ -175, -352, -145
+220642186771519, 81912509824171, 355772958887750 @ -47, 260, -35
+367491008763696, 250945847161850, 440745640166155 @ -175, 52, -247
+409446291207698, 330788391219960, 292713233046131 @ -12, -543, -213
+265825149596573, 244764814652484, 385353795717239 @ -39, 63, -116
+86994969742866, 55589210487038, 266168402419143 @ 80, 274, 75
+320921915321526, 254694248869940, 238822296842297 @ 20, 37, 121
+179360093396906, 300619763168811, 350320133311184 @ 10, -6, -31
+16801569357148, 350089851801349, 165789472544968 @ 148, -52, 185
+433009745308238, 267184125912896, 273312821311953 @ -48, -230, -222
+309970609487918, 492679271429412, 562469959691093 @ -162, -225, -270
+330256938537206, 202170127764104, 344108272059359 @ -114, 137, -71
+213152807168420, 273421736762004, 323641629860561 @ 7, 23, -10
+172814593254919, 102501683328829, 247297979068407 @ -23, 218, 96
+219063685290956, 245608985586714, 349589973549179 @ -51, 62, -25
+430972008921002, 276609779270530, 253785144061591 @ -198, -115, 68
+141649807653506, 439100473497764, 78375461552199 @ 108, -209, 333
+312663199862950, 493743864927560, 363529412231925 @ -98, -344, -93
+423775394632430, 316536435784716, 224041240411841 @ -163, -326, 230
+327003532119926, 456110225504124, 400015395503489 @ -100, -318, -178
+435439277730906, 246965061962354, 245922460626269 @ -8, 26, 148
+283854002218376, 234052132379684, 271898082205509 @ -75, 79, 60
+208829340415406, 226635123507144, 482805207137969 @ -86, 81, -138
+349118117289530, 297778463181064, 357393083055225 @ -141, -32, -100
+299777008641782, 223081277864628, 246108022591457 @ -23, 106, 100
+438922841832386, 250584839183004, 267893924213549 @ -77, -31, -209
+438166800949636, 330151804608232, 277338384714133 @ -245, -379, -52
+420579599545694, 243376742146548, 280885790949095 @ -134, 71, -89
+258938745011974, 234495963857860, 323177451263361 @ 32, 82, -43
+36448776939491, 418742104071604, 231340500503399 @ 93, -113, 112
+404233472314214, 421003477346676, 157922394665657 @ -102, -735, 507
+418557987222777, 382699811698471, 419900143401830 @ -161, -584, -707
+232576226848342, 10003025240182, 203381334758715 @ 262, 689, 216
+255015067009391, 261956436770199, 352283365082099 @ -63, 40, -44
+169534023774211, 286707269739714, 329134981894044 @ 37, 9, -9
+299376592913146, 402426294254590, 407612010835957 @ -140, -130, -100
+215005453311763, 469310214941092, 228082501098752 @ -91, -161, 115
+65509617858912, 478414643265824, 223736911874359 @ 53, -167, 119
+290296833488420, 405978755186600, 456496601688775 @ -63, -199, -243
+83587437987463, 176759381379671, 129955793868008 @ 221, 164, 271
+304597538450668, 82725932045064, 153572240068869 @ 31, 453, 324
+228877113530886, 334430535336004, 330141973131265 @ 35, -82, -37
+353863067391134, 267957912015969, 257969098038089 @ 126, -42, 54
+206869829642681, 263886870857978, 400371713273807 @ -45, 41, -80
+132676076258186, 168321498105084, 311987025981449 @ 49, 153, 20
+411199564272509, 257136439666427, 222473651711960 @ -81, -8, 247
+201361581214124, 268475095057634, 370530268050177 @ -63, 38, -34
\ No newline at end of file
diff --git a/day24-easy/sample.in b/day24-easy/sample.in
new file mode 100644
index 0000000..94d8c9a
--- /dev/null
+++ b/day24-easy/sample.in
@@ -0,0 +1,6 @@
+7 27 7 27
+19, 13, 30 @ -2,  1, -2
+18, 19, 22 @ -1, -1, -2
+20, 25, 34 @ -2, -2, -4
+12, 31, 28 @ -1, -2, -1
+20, 19, 15 @  1, -5, -3
\ No newline at end of file
diff --git a/day24-easy/src/main.zig b/day24-easy/src/main.zig
new file mode 100644
index 0000000..364135f
--- /dev/null
+++ b/day24-easy/src/main.zig
@@ -0,0 +1,274 @@
+const std = @import("std");
+
+fn StackList(comptime T: type, comptime capacity_type: type, comptime capacity: capacity_type) type {
+    return struct {
+        const Self = @This();
+        mem: [capacity]T,
+        length: capacity_type,
+
+        fn add(self: *Self, value: T) void {
+            self.mem[self.length] = value;
+            self.length += 1;
+        }
+
+        fn has(self: *Self, needle: T) bool {
+            for (0..self.length) |i| {
+                if (self.mem[i] == needle) {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        fn getMutableSlice(self: *Self) []T {
+            return (&self.mem)[0..self.length];
+        }
+
+        fn getSlice(self: *const Self) []const T {
+            return self.mem[0..self.length];
+        }
+
+        fn init() Self {
+            return Self{
+                .mem = undefined,
+                .length = 0,
+            };
+        }
+    };
+}
+
+fn readNumber(comptime T: type, line: []const u8, index: *usize) T {
+    var result: T = 0;
+    while (index.* < line.len and line[index.*] == ' ') : (index.* += 1) {}
+
+    var is_negative = false;
+    if (index.* < line.len and line[index.*] == '-') {
+        is_negative = true;
+        index.* += 1;
+    }
+
+    std.debug.assert(index.* < line.len);
+
+    while (index.* < line.len) : (index.* += 1) {
+        const char = line[index.*];
+        switch (char) {
+            '0'...'9' => {
+                result = result * 10 + (char - '0');
+            },
+            else => {
+                break;
+            },
+        }
+    }
+
+    return if (is_negative) (0 - result) else result;
+}
+
+const Hailstone = struct {
+    initial_x: i64,
+    initial_y: i64,
+    initial_z: i64,
+    velocity_x: i16,
+    velocity_y: i16,
+    velocity_z: i16,
+};
+
+const RationalNumber = struct {
+    numerator: i128,
+    denominator: i64,
+
+    fn init(numerator: i128, denominator: i64) RationalNumber {
+        std.debug.assert(denominator != 0);
+
+        if (denominator < 0) {
+            return .{
+                .numerator = -numerator,
+                .denominator = -denominator,
+            };
+        }
+
+        return .{
+            .numerator = numerator,
+            .denominator = denominator,
+        };
+    }
+
+    fn isNonNegative(self: *const RationalNumber) bool {
+        return self.numerator >= 0;
+    }
+
+    fn isIn(self: *const RationalNumber, first: i64, last: i64) bool {
+        return first * @as(i128, self.denominator) <= self.numerator and self.numerator <= last * @as(i128, self.denominator);
+    }
+};
+
+const ProjectionIntersection = struct {
+    x: RationalNumber,
+    y: RationalNumber,
+    time_first: RationalNumber,
+    time_second: RationalNumber,
+};
+
+const TestArea = struct {
+    x_first: i64,
+    x_last: i64,
+    y_first: i64,
+    y_last: i64,
+};
+
+fn getProjectionIntersection(a: Hailstone, b: Hailstone) ?ProjectionIntersection {
+    if (@as(i32, a.velocity_x) * @as(i32, b.velocity_y) == @as(i32, a.velocity_y) * @as(i32, b.velocity_x)) {
+        // projections of paths are parallel
+
+        if (@as(i128, a.initial_x - b.initial_x) * a.velocity_y == @as(i128, a.initial_y - b.initial_y) * a.velocity_x) {
+            // both paths are on the same line, the puzzle does not document this behavior
+            unreachable;
+        } else {
+            // paths are on different parallel lines, and never intersect
+            return null;
+        }
+    }
+
+    // set of linearly independent equations:
+    // t1 * a.velocity_x - t2 * b.velocity_x = b.initial_x - a.initial_x
+    // t1 * a.velocity_y - t2 * b.velocity_y = b.initial_y - a.initial_y
+
+    const determinant = -@as(i64, a.velocity_x) * b.velocity_y + @as(i64, a.velocity_y) * b.velocity_x;
+    const dt1 = -@as(i128, b.initial_x - a.initial_x) * b.velocity_y + @as(i128, b.initial_y - a.initial_y) * b.velocity_x;
+    const dt2 = a.velocity_x * @as(i128, b.initial_y - a.initial_y) - a.velocity_y * @as(i128, b.initial_x - a.initial_x);
+
+    const t1 = RationalNumber.init(dt1, determinant);
+    const t2 = RationalNumber.init(dt2, determinant);
+    const ax = RationalNumber.init(@as(i128, a.initial_x) * determinant + a.velocity_x * dt1, determinant);
+    const ay = RationalNumber.init(@as(i128, a.initial_y) * determinant + a.velocity_y * dt1, determinant);
+    const bx = RationalNumber.init(@as(i128, b.initial_x) * determinant + b.velocity_x * dt2, determinant);
+    const by = RationalNumber.init(@as(i128, b.initial_y) * determinant + b.velocity_y * dt2, determinant);
+    std.debug.assert(ax.numerator == bx.numerator);
+    std.debug.assert(ay.numerator == by.numerator);
+
+    return .{
+        .x = ax,
+        .y = ay,
+        .time_first = t1,
+        .time_second = t2,
+    };
+}
+
+fn solveForHailstones(hailstones: []const Hailstone, test_area: TestArea) usize {
+    var result: usize = 0;
+    for (0..hailstones.len) |i| {
+        for (0..i) |j| {
+            if (getProjectionIntersection(hailstones[i], hailstones[j])) |intersection| {
+                if (intersection.time_first.isNonNegative() and intersection.time_second.isNonNegative() and intersection.x.isIn(test_area.x_first, test_area.x_last) and intersection.y.isIn(test_area.y_first, test_area.y_last)) {
+                    //std.debug.print("found intersection for {d} and {d}\n", .{ i, j });
+                    result += 1;
+                }
+            }
+        }
+    }
+    return result;
+}
+
+fn parseFirstLine(line: []const u8) TestArea {
+    var i: usize = 0;
+    var x_first = readNumber(i64, line, &i);
+
+    i += 1;
+    var x_last = readNumber(i64, line, &i);
+
+    i += 1;
+    var y_first = readNumber(i64, line, &i);
+
+    i += 1;
+    var y_last = readNumber(i64, line, &i);
+
+    std.debug.assert(i >= line.len);
+
+    return .{
+        .x_first = x_first,
+        .x_last = x_last,
+        .y_first = y_first,
+        .y_last = y_last,
+    };
+}
+
+fn parseLine(line: []const u8) Hailstone {
+    var i: usize = 0;
+    var initial_x = readNumber(i64, line, &i);
+
+    i += 1;
+    var initial_y = readNumber(i64, line, &i);
+
+    i += 1;
+    var initial_z = readNumber(i64, line, &i);
+
+    i += 2;
+    var velocity_x = readNumber(i16, line, &i);
+
+    i += 1;
+    var velocity_y = readNumber(i16, line, &i);
+
+    i += 1;
+    var velocity_z = readNumber(i16, line, &i);
+
+    std.debug.assert(i >= line.len);
+
+    return .{
+        .initial_x = initial_x,
+        .initial_y = initial_y,
+        .initial_z = initial_z,
+        .velocity_x = velocity_x,
+        .velocity_y = velocity_y,
+        .velocity_z = velocity_z,
+    };
+}
+
+fn solveLines(lines: []const []const u8) usize {
+    var hailstones_list = StackList(Hailstone, usize, 400).init();
+
+    const test_area = parseFirstLine(lines[0]);
+
+    for (lines[1..]) |line| {
+        hailstones_list.add(parseLine(line));
+    }
+
+    return solveForHailstones(hailstones_list.getSlice(), test_area);
+}
+
+pub fn solveAll(reader: anytype) !usize {
+    var result: usize = 0;
+    while (true) {
+        var allocator_buffer: [50000]u8 = undefined;
+        var fba = std.heap.FixedBufferAllocator.init(&allocator_buffer);
+        var allocator = fba.allocator();
+
+        var lines = StackList([]u8, usize, 1500).init();
+        var empty_line_reached = false;
+
+        var line_buffer: [1000]u8 = undefined;
+        while (try reader.readUntilDelimiterOrEof(&line_buffer, '\n')) |line| {
+            if (line.len == 0) {
+                empty_line_reached = true;
+                break;
+            }
+            lines.add(try allocator.dupe(u8, line));
+        }
+
+        result += solveLines(lines.getSlice());
+
+        if (!empty_line_reached) {
+            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();
+    const result = try solveAll(&reader);
+    try stdout.print("{d}\n", .{result});
+}