day 4

day4/day4_part2.zig

@@ -0,0 +1,78 @@
+const std = @import("std");
+const input = @embedFile("input");
+
+const Coord = struct {
+    x: i32,
+    y: i32,
+};
+
+const Direction = struct {
+    x: i64,
+    y: i64,
+
+    const Left = Direction{ .x = -1, .y = 0 };
+    const Right = Direction{ .x = 1, .y = 0 };
+    const Up = Direction{ .x = 0, .y = -1 };
+    const Down = Direction{ .x = 0, .y = 1 };
+    const UpLeft = Direction{ .x = -1, .y = -1 };
+    const UpRight = Direction{ .x = 1, .y = -1 };
+    const DownLeft = Direction{ .x = -1, .y = 1 };
+    const DownRight = Direction{ .x = 1, .y = 1 };
+};
+
+fn diagonal_mas_exists(grid: [][]const u8, start_x: i64, start_y: i64, direction: Direction) bool {
+    const mas1 = "MAS";
+    const mas2 = "SAM";
+
+    var bmas1 = true;
+    var bmas2 = true;
+
+    var x = start_x;
+    var y = start_y;
+    for (mas1) |c| {
+        if (x < 0 or y < 0 or y >= grid.len or x >= grid[@intCast(y)].len or grid[@intCast(y)][@intCast(x)] != c) {
+            bmas1 = false;
+            break;
+        }
+        x += direction.x;
+        y += direction.y;
+    }
+    x = start_x;
+    y = start_y;
+    for (mas2) |c| {
+        if (x < 0 or y < 0 or y >= grid.len or x >= grid[@intCast(y)].len or grid[@intCast(y)][@intCast(x)] != c) {
+            bmas2 = false;
+            break;
+        }
+        x += direction.x;
+        y += direction.y;
+    }
+
+    return bmas1 or bmas2;
+}
+
+pub fn main() !void {
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    const ally = gpa.allocator();
+    defer _ = gpa.deinit();
+
+    var lines = std.mem.tokenize(u8, input, "\r\n");
+
+    var grid = std.ArrayList([]const u8).init(ally);
+    defer grid.deinit();
+
+    while (lines.next()) |line| {
+        if (line.len > 0) {
+            try grid.append(line);
+        }
+    }
+
+    var xmas: u16 = 0;
+    for (grid.items, 0..) |row, y| {
+        for (row, 0..) |_, x| {
+            if (diagonal_mas_exists(grid.items, @as(i64, @intCast(x)) - 1, @as(i64, @intCast(y)) - 1, Direction.DownRight)
+            and diagonal_mas_exists(grid.items, @as(i64, @intCast(x)) - 1, @as(i64, @intCast(y)) + 1, Direction.UpRight)) xmas += 1;
+        }
+    }
+    std.debug.print("XMAS count: {}\n", .{xmas});
+}