diff --git a/src/days/day09.zig b/src/days/day09.zig
index 5b8cfab..c167255 100644
--- a/src/days/day09.zig
+++ b/src/days/day09.zig
@@ -1,7 +1,242 @@
 const std = @import("std");
 
-pub const title = "Day 09";
+pub const title = "Day 09: Movie Theater";
 
-pub fn run(_: std.mem.Allocator) !void {
+pub fn run(allocator: std.mem.Allocator) !void {
+    const input = @embedFile("./input/day09.txt");
+    //const input =
+    //    \\7,1
+    //    \\11,1
+    //    \\11,7
+    //    \\9,7
+    //    \\9,5
+    //    \\2,5
+    //    \\2,3
+    //    \\7,3
+    //    ;
+
+    var lines = std.mem.tokenizeScalar(u8, input, '\n');
+
+    var tiles: std.ArrayList(Tile) = .empty;
+    defer tiles.deinit(allocator);
+
+    std.debug.print("parsing tiles...\n", .{});
+    var index: usize = 0;
+    while (lines.next()) |line| {
+        defer index += 1;
+        var coordinates = std.mem.tokenizeScalar(u8, line, ',');
+        const x = blk: {
+            const s = coordinates.next() orelse continue;
+            break :blk try std.fmt.parseUnsigned(u64, s, 10);
+        };
+        const y = blk: {
+            const s = coordinates.next() orelse continue;
+            break :blk try std.fmt.parseUnsigned(u64, s, 10);
+        };
+        try tiles.append(allocator, .{ .index = index, .pos = .{ x, y } });
+    }
+
+    std.debug.print("calculating edges...\n", .{});
+    var edges: std.ArrayList(Edge) = .empty;
+    defer edges.deinit(allocator);
+    for (tiles.items, 0..) |tile, i| {
+        const a = tile;
+        const b = if (i == tiles.items.len - 1) tiles.items[0] else tiles.items[i + 1];
+        try edges.append(allocator, Edge.fromTiles(a, b));
+    }
+
+    std.debug.print("finding largest rectangle...\n", .{});
+    var largest_area: usize = 0;
+    var largest_rectangle: Rectangle = undefined;
+    for (tiles.items, 0..) |first_tile, i| {
+        inner: for (tiles.items[i + 1..]) |second_tile| {
+            const rectangle = first_tile.rectangle(second_tile);
+
+            std.debug.print("checking rectangle {f}...\n", .{rectangle});
+
+            const has_inside = rectangle.width() >= 3 and rectangle.height() >= 3;
+            if (has_inside) {
+                const one: @Vector(2, u64) = @splat(1);
+                const top_left = rectangle.topLeft() + one;
+                const bottom_right = rectangle.bottomRight() - one;
+                edge_loop: for (edges.items) |edge| {
+                    if (edge.direction == .horizontal) {
+                        if (edge.a.pos[1] < top_left[1] or edge.a.pos[1] > bottom_right[1]) {
+                            continue :edge_loop;
+                        }
+
+                        const start, const end = if (edge.a.pos[0] < edge.b.pos[0] + 1) .{ edge.a.pos[0], edge.b.pos[0] } else .{ edge.b.pos[0], edge.a.pos[0] + 1 };
+                        for (start..end) |ex| {
+                            const edge_tile: @Vector(2, u64) = .{ ex, edge.a.pos[1] };
+                            if (top_left[0] <= edge_tile[0] and edge_tile[0] <= bottom_right[0]) {
+                                std.debug.print("rectangle {f} hits edge {f}\n", .{rectangle, edge});
+                                continue :inner;
+                            }
+                        }
+                    } else {
+                        if (edge.a.pos[0] < top_left[0] or edge.a.pos[0] > bottom_right[0]) {
+                            continue :edge_loop;
+                        }
+
+                        const start, const end = if (edge.a.pos[1] < edge.b.pos[1] + 1) .{ edge.a.pos[1], edge.b.pos[1] } else .{ edge.b.pos[1], edge.a.pos[1] + 1 };
+                        for (start..end) |ey| {
+                            const edge_tile: @Vector(2, u64) = .{ edge.a.pos[0], ey };
+                            if (top_left[1] <= edge_tile[1] and edge_tile[1] <= bottom_right[1]) {
+                                std.debug.print("rectangle {f} hits edge {f}\n", .{rectangle, edge});
+                                continue :inner;
+                            }
+                        }
+                    }
+                }
+            }
+
+            const area = rectangle.area();
+            if (area > largest_area) {
+                largest_area = area;
+                largest_rectangle = rectangle;
+            }
+        }
+    }
+
+    std.debug.print("rectangle = {f}\n", .{largest_rectangle});
+    std.debug.print("top_left = {any}, bottom_right = {any}\n", .{largest_rectangle.topLeft(), largest_rectangle.bottomRight()});
+
+    var buffer: [64]u8 = undefined;
+    var stdout_writer = std.fs.File.stdout().writer(&buffer);
+    const stdout = &stdout_writer.interface;
+
+    try stdout.print("{d}\n", .{largest_area});
+
+    try stdout.flush();
 }
 
+const Tile = struct {
+    index: usize,
+    pos: @Vector(2, u64),
+
+    const Self = @This();
+
+    pub fn format(self: Self, w: *std.io.Writer) std.io.Writer.Error!void {
+        try w.print("{d},{d}", .{self.pos[0], self.pos[1]});
+    }
+
+    pub fn rectangle(self: Self, other_corner: Self) Rectangle {
+        return Rectangle.fromTiles(self, other_corner);
+    }
+};
+
+const Edge = struct {
+    a: Tile,
+    b: Tile,
+
+    direction: Direction,
+
+    const Self = @This();
+
+    const Direction = enum {
+        horizontal,
+        vertical,
+    };
+
+    pub fn fromTiles(a: Tile, b: Tile) Self {
+        const direction: Direction = if (a.pos[0] == b.pos[0])
+            .vertical
+        else if (a.pos[1] == b.pos[1])
+            .horizontal
+        else
+            unreachable;
+
+        return .{
+            .a = a,
+            .b = b,
+            .direction = direction,
+        };
+    }
+
+    pub fn format(self: Self, w: *std.io.Writer) std.io.Writer.Error!void {
+        try w.print("{f} - {f}", .{self.a, self.b});
+    }
+};
+
+/// 0,0    1,0
+///  a ---- c
+///  |      |
+///  |      |
+///  d ---- b
+/// 0,1    1,1
+const Rectangle = struct {
+    a: Tile,
+    b: Tile,
+
+    c: @Vector(2, u64),
+    d: @Vector(2, u64),
+
+    const Self = @This();
+
+    pub fn fromTiles(a: Tile, b: Tile) Self {
+        const cx = if (a.pos[0] <= b.pos[0]) b.pos[0] else a.pos[0];
+        const cy = if (a.pos[1] >= b.pos[1]) a.pos[1] else b.pos[1];
+        const dx = if (a.pos[0] <= b.pos[0]) a.pos[0] else b.pos[0];
+        const dy = if (a.pos[1] >= b.pos[1]) b.pos[1] else a.pos[1];
+        return .{
+            .a = a,
+            .b = b,
+            .c = .{ cx, cy },
+            .d = .{ dx, dy },
+        };
+    }
+
+    pub fn format(self: Self, w: *std.io.Writer) std.io.Writer.Error!void {
+        try w.print("{d} to {d} ({f} - {d},{d} - {f} - {d},{d})", .{self.a.index, self.b.index, self.a, self.c[0], self.c[1], self.b, self.d[0], self.d[1]});
+    }
+
+    pub fn topLeft(self: Self) @Vector(2, u64) {
+        const verts = [_]@Vector(2, u64){ self.c, self.b.pos, self.d };
+        var top_left: @Vector(2, u64) = self.a.pos;
+        for (verts) |vert| {
+            const cond = vert <= top_left;
+            if (@reduce(.And, cond)) {
+                top_left = vert;
+            }
+        }
+        return top_left;
+    }
+
+    pub fn bottomRight(self: Self) @Vector(2, u64) {
+        const verts = [_]@Vector(2, u64){ self.d, self.a.pos, self.c };
+        var bottom_right: @Vector(2, u64) = self.b.pos;
+        for (verts) |vert| {
+            const cond = vert >= bottom_right;
+            if (@reduce(.And, cond)) {
+                bottom_right = vert;
+            }
+        }
+        return bottom_right;
+    }
+
+    pub fn width(self: Self) u64 {
+        const cond = self.a.pos > self.b.pos;
+        const high = @select(u64, cond, self.a.pos, self.b.pos);
+        const low = @select(u64, !cond, self.a.pos, self.b.pos);
+        const diff = high - low;
+        return diff[0] + 1;
+    }
+
+    pub fn height(self: Self) u64 {
+        const cond = self.a.pos > self.b.pos;
+        const high = @select(u64, cond, self.a.pos, self.b.pos);
+        const low = @select(u64, !cond, self.a.pos, self.b.pos);
+        const diff = high - low;
+        return diff[1] + 1;
+    }
+
+    pub fn area(self: Self) u64 {
+        const cond = self.a.pos > self.b.pos;
+        const high = @select(u64, cond, self.a.pos, self.b.pos);
+        const low = @select(u64, !cond, self.a.pos, self.b.pos);
+        const diff = high - low;
+        const one: @Vector(2, u64) = @splat(1);
+        return @reduce(.Mul, diff + one);
+    }
+};
+
diff --git a/src/days/input/day09.txt b/src/days/input/day09.txt
new file mode 100755
index 0000000..44cac4b
--- /dev/null
+++ b/src/days/input/day09.txt
@@ -0,0 +1,496 @@
+98241,50245
+98241,51464
+98122,51464
+98122,52660
+97649,52660
+97649,53867
+97585,53867
+97585,55104
+97774,55104
+97774,56255
+97196,56255
+97196,57546
+97636,57546
+97636,58780
+97591,58780
+97591,59958
+97238,59958
+97238,61063
+96596,61063
+96596,62184
+96091,62184
+96091,63423
+96024,63423
+96024,64739
+96152,64739
+96152,65592
+94869,65592
+94869,66846
+94784,66846
+94784,67925
+94219,67925
+94219,69237
+94206,69237
+94206,70195
+93369,70195
+93369,71228
+92725,71228
+92725,72421
+92396,72421
+92396,73353
+91573,73353
+91573,74531
+91190,74531
+91190,75402
+90295,75402
+90295,76446
+89686,76446
+89686,77385
+88921,77385
+88921,78413
+88284,78413
+88284,79671
+87931,79671
+87931,80568
+87102,80568
+87102,81220
+85994,81220
+85994,82478
+85581,82478
+85581,83480
+84863,83480
+84863,83949
+83605,83949
+83605,84603
+82559,84603
+82559,85593
+81829,85593
+81829,86686
+81171,86686
+81171,87253
+80061,87253
+80061,87969
+79082,87969
+79082,88669
+78093,88669
+78093,89502
+77196,89502
+77196,90250
+76233,90250
+76233,91130
+75347,91130
+75347,91250
+74000,91250
+74000,92327
+73212,92327
+73212,92894
+72128,92894
+72128,93270
+70952,93270
+70952,93370
+69660,93370
+69660,94478
+68819,94478
+68819,94708
+67595,94708
+67595,95183
+66472,95183
+66472,95207
+65195,95207
+65195,96147
+64221,96147
+64221,96615
+63081,96615
+63081,96461
+61777,96461
+61777,97201
+60700,97201
+60700,97015
+59412,97015
+59412,97740
+58305,97740
+58305,97562
+57038,97562
+57038,97176
+55764,97176
+55764,97302
+54566,97302
+54566,97656
+53385,97656
+53385,98146
+52195,98146
+52195,97889
+50969,97889
+50969,97871
+49756,97871
+49756,97859
+48543,97859
+48543,97449
+47351,97449
+47351,97738
+46119,97738
+46119,97642
+44909,97642
+44909,97226
+43740,97226
+43740,97219
+42519,97219
+42519,96788
+41367,96788
+41367,97309
+40026,97309
+40026,96882
+38868,96882
+38868,96221
+37780,96221
+37780,95804
+36640,95804
+36640,96136
+35265,96136
+35265,95792
+34086,95792
+34086,94892
+33112,94892
+33112,94733
+31866,94733
+31866,94172
+30777,94172
+30777,93317
+29828,93317
+29828,92857
+28705,92857
+28705,92156
+27705,92156
+27705,91484
+26696,91484
+26696,91543
+25258,91543
+25258,90924
+24200,90924
+24200,90213
+23201,90213
+23201,89043
+22528,89043
+22528,88927
+21108,88927
+21108,88089
+20204,88089
+20204,87315
+19255,87315
+19255,86250
+18557,86250
+18557,85773
+17347,85773
+17347,84679
+16696,84679
+16696,83809
+15843,83809
+15843,82679
+15268,82679
+15268,82055
+14153,82055
+14153,81303
+13157,81303
+13157,80213
+12558,80213
+12558,79102
+12004,79102
+12004,78475
+10804,78475
+10804,76971
+10824,76971
+10824,76102
+9952,76102
+9952,75038
+9369,75038
+9369,74238
+8339,74238
+8339,72905
+8231,72905
+8231,71987
+7378,71987
+7378,70721
+7205,70721
+7205,69997
+5885,69997
+5885,68793
+5582,68793
+5582,67760
+4871,67760
+4871,66522
+4679,66522
+4679,65214
+4737,65214
+4737,64207
+3895,64207
+3895,62932
+3915,62932
+3915,61848
+3259,61848
+3259,60571
+3366,60571
+3366,59436
+2864,59436
+2864,58155
+3122,58155
+3122,57084
+2124,57084
+2124,55838
+2213,55838
+2213,54652
+1807,54652
+1807,53424
+1794,53424
+1794,52197
+1810,52197
+1810,50973
+1890,50973
+1890,50228
+94822,50228
+94822,48525
+1557,48525
+1557,47341
+2383,47341
+2383,46107
+2110,46107
+2110,44867
+1961,44867
+1961,43734
+2727,43734
+2727,42467
+2452,42467
+2452,41283
+2754,41283
+2754,40077
+2933,40077
+2933,38872
+3134,38872
+3134,37713
+3522,37713
+3522,36439
+3506,36439
+3506,35262
+3852,35262
+3852,34252
+4684,34252
+4684,33262
+5505,33262
+5505,32101
+5846,32101
+5846,31022
+6392,31022
+6392,29688
+6381,29688
+6381,28480
+6689,28480
+6689,27621
+7685,27621
+7685,26247
+7715,26247
+7715,25643
+9102,25643
+9102,24398
+9389,24398
+9389,23514
+10255,23514
+10255,22573
+11021,22573
+11021,21166
+11150,21166
+11150,20538
+12337,20538
+12337,19792
+13336,19792
+13336,18451
+13627,18451
+13627,17941
+14878,17941
+14878,16560
+15179,16560
+15179,15977
+16322,15977
+16322,14917
+16989,14917
+16989,14500
+18254,14500
+18254,13050
+18605,13050
+18605,12768
+19956,12768
+19956,11585
+20576,11585
+20576,11164
+21786,11164
+21786,10359
+22708,10359
+22708,9858
+23836,9858
+23836,8777
+24596,8777
+24596,8258
+25713,8258
+25713,7978
+26955,7978
+26955,7540
+28095,7540
+28095,6507
+28940,6507
+28940,6682
+30363,6682
+30363,6251
+31489,6251
+31489,5782
+32598,5782
+32598,4548
+33429,4548
+33429,4750
+34790,4750
+34790,4015
+35829,4015
+35829,4049
+37104,4049
+37104,3690
+38261,3690
+38261,2708
+39279,2708
+39279,3314
+40653,3314
+40653,2568
+41748,2568
+41748,2300
+42941,2300
+42941,2002
+44135,2002
+44135,2479
+45412,2479
+45412,1845
+46579,1845
+46579,2346
+47826,2346
+47826,2049
+49029,2049
+49029,2462
+50242,2462
+50242,2326
+51451,2326
+51451,1711
+52695,1711
+52695,1956
+53904,1956
+53904,1808
+55148,1808
+55148,2309
+56320,2309
+56320,3006
+57444,3006
+57444,3165
+58640,3165
+58640,3456
+59812,3456
+59812,3478
+61045,3478
+61045,3412
+62316,3412
+62316,4049
+63401,4049
+63401,4368
+64572,4368
+64572,4266
+65892,4266
+65892,5403
+66775,5403
+66775,5880
+67884,5880
+67884,6132
+69090,6132
+69090,6792
+70119,6792
+70119,6784
+71472,6784
+71472,7784
+72326,7784
+72326,8026
+73577,8026
+73577,8876
+74490,8876
+74490,8978
+75860,8978
+75860,9930
+76701,9930
+76701,10352
+77896,10352
+77896,11232
+78772,11232
+78772,11817
+79867,11817
+79867,12626
+80792,12626
+80792,14062
+81171,14062
+81171,14424
+82471,14424
+82471,15055
+83557,15055
+83557,16158
+84188,16158
+84188,16772
+85314,16772
+85314,18188
+85573,18188
+85573,18598
+86957,18598
+86957,19899
+87301,19899
+87301,20942
+87937,20942
+87937,21902
+88675,21902
+88675,22692
+89662,22692
+89662,23972
+89932,23972
+89932,24741
+90987,24741
+90987,25832
+91538,25832
+91538,27127
+91708,27127
+91708,28094
+92461,28094
+92461,29086
+93191,29086
+93191,30203
+93671,30203
+93671,31293
+94211,31293
+94211,32525
+94402,32525
+94402,33715
+94665,33715
+94665,34739
+95400,34739
+95400,35775
+96158,35775
+96158,36914
+96627,36914
+96627,38153
+96733,38153
+96733,39465
+96473,39465
+96473,40509
+97404,40509
+97404,41858
+96797,41858
+96797,42955
+97601,42955
+97601,44214
+97352,44214
+97352,45389
+97756,45389
+97756,46601
+97847,46601
+97847,47809
+98021,47809
+98021,49037
+97534,49037
+97534,50245