From 83e5d44f615dfe5113b8cb06dfad11c81a0748a2 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <code@jdkaplan.dev>
Date: Sun, 15 Dec 2024 17:15:24 -0800
Subject: [PATCH] 2024 Day 15 Part 2

---
 2024/src/day15.zig | 181 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 181 insertions(+)

diff --git a/2024/src/day15.zig b/2024/src/day15.zig
index abedb75..e583206 100644
--- a/2024/src/day15.zig
+++ b/2024/src/day15.zig
@@ -26,6 +26,7 @@ pub fn main() !void {
     const stdout = bw.writer();
 
     try stdout.print("{d}\n", .{try part1(grid, moves.items)});
+    try stdout.print("{d}\n", .{try part2(grid, moves.items)});
     try bw.flush();
 }
 
@@ -67,6 +68,88 @@ fn part1(start: Grid, moves: []const Direction) !i64 {
     return score;
 }
 
+fn part2(original: Grid, moves: []const Direction) !i64 {
+    var start: Pos = undefined;
+
+    var grid = v: {
+        var map = Map(Pos, u8).init(original.allocator);
+
+        var it = original.iterator();
+        while (it.next()) |entry| {
+            const pos = entry.key_ptr.*;
+            const char = entry.value_ptr.*;
+
+            const left, const right = .{
+                Pos.new(pos.r, 2 * pos.c),
+                Pos.new(pos.r, 2 * pos.c + 1),
+            };
+
+            if (char == '#') {
+                try map.put(left, '#');
+                try map.put(right, '#');
+            }
+            if (char == 'O') {
+                try map.put(left, '[');
+                try map.put(right, ']');
+            }
+            if (char == '.') {
+                try map.put(left, '.');
+                try map.put(right, '.');
+            }
+            if (char == '@') {
+                try map.put(left, '@');
+                try map.put(right, '.');
+                start = left;
+            }
+        }
+
+        break :v Grid{
+            .allocator = original.allocator,
+            .map = map,
+            .max = Pos.new(original.max.r, 2 * original.max.c + 1),
+        };
+    };
+    defer grid.deinit();
+
+    var pos = v: {
+        var it = grid.iterator();
+        while (it.next()) |entry| {
+            const pos = entry.key_ptr.*;
+            const char = entry.value_ptr.*;
+
+            if (char == '@') {
+                break :v pos;
+            }
+        }
+        unreachable;
+    };
+
+    for (moves) |dir| {
+        if (grid.can_move2(pos, dir)) {
+            var moved = Set(Pos).init(grid.allocator);
+            defer moved.deinit();
+
+            try grid.do_move2(pos, dir, &moved);
+            pos = pos.add(dir.delta());
+        }
+    }
+
+    var score: i64 = 0;
+    var it = grid.iterator();
+    while (it.next()) |entry| {
+        const box = entry.key_ptr.*;
+        const char = entry.value_ptr.*;
+
+        if (char != '[') {
+            continue;
+        }
+
+        score += 100 * box.r + box.c;
+    }
+
+    return score;
+}
+
 const Pos = struct {
     r: i32,
     c: i32,
@@ -201,6 +284,22 @@ const Grid = struct {
             0 <= pos.c and pos.c <= self.max.c;
     }
 
+    pub fn format(
+        self: Grid,
+        comptime _: []const u8,
+        _: std.fmt.FormatOptions,
+        writer: anytype,
+    ) !void {
+        var r: i32 = 0;
+        while (r <= self.max.r) : (r += 1) {
+            var c: i32 = 0;
+            while (c <= self.max.c) : (c += 1) {
+                try writer.print("{c}", .{self.get(Pos.new(r, c)).?});
+            }
+            try writer.print("\n", .{});
+        }
+    }
+
     fn move(self: *Grid, robot: Pos, dir: Direction) !Pos {
         var stack = List(Pos).init(self.allocator);
         defer stack.deinit();
@@ -223,4 +322,86 @@ const Grid = struct {
 
         return robot.add(delta);
     }
+
+    fn can_move2(self: *Grid, pos: Pos, dir: Direction) bool {
+        const delta = dir.delta();
+        const next = pos.add(delta);
+
+        switch (self.get(next).?) {
+            '#' => return false,
+            '.' => return true,
+            '@' => return self.can_move2(next, dir),
+
+            '[' => switch (dir) {
+                .n, .s => {
+                    const left = self.can_move2(next, dir);
+                    const right = self.can_move2(next.add(Direction.e.delta()), dir);
+                    return left and right;
+                },
+                .e => return self.can_move2(next.add(Direction.e.delta()), dir),
+                .w => return self.can_move2(next, dir),
+            },
+            ']' => switch (dir) {
+                .n, .s => {
+                    const left = self.can_move2(next, dir);
+                    const right = self.can_move2(next.add(Direction.w.delta()), dir);
+                    return left and right;
+                },
+                .w => return self.can_move2(next.add(Direction.w.delta()), dir),
+                .e => return self.can_move2(next, dir),
+            },
+            else => unreachable,
+        }
+    }
+
+    fn do_move2(self: *Grid, pos: Pos, dir: Direction, moved: *Set(Pos)) !void {
+        if (moved.contains(pos)) {
+            return;
+        }
+        try moved.put(pos);
+
+        const delta = dir.delta();
+        const next = pos.add(delta);
+
+        const content = self.get(pos).?;
+
+        switch (content) {
+            '#' => unreachable,
+            '.' => return,
+            '@' => try self.do_move2(next, dir, moved),
+
+            '[' => {
+                const pair = pos.add(Direction.e.delta());
+                switch (dir) {
+                    .n, .s => {
+                        try self.do_move2(next, dir, moved);
+                        try self.do_move2(pair, dir, moved);
+                    },
+                    .e => {
+                        try self.do_move2(pair, dir, moved);
+                        try self.do_move2(next, dir, moved);
+                    },
+                    .w => try self.do_move2(next, dir, moved),
+                }
+            },
+            ']' => {
+                const pair = pos.add(Direction.w.delta());
+                switch (dir) {
+                    .n, .s => {
+                        try self.do_move2(pair, dir, moved);
+                        try self.do_move2(next, dir, moved);
+                    },
+                    .w => {
+                        try self.do_move2(pair, dir, moved);
+                        try self.do_move2(next, dir, moved);
+                    },
+                    .e => try self.do_move2(next, dir, moved),
+                }
+            },
+            else => unreachable,
+        }
+
+        try self.map.put(next, content);
+        try self.map.put(pos, '.');
+    }
 };