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const std = @import("std");
const Pos = struct { x: usize, y: usize };
const Dir = enum { left, right, up, down };
fn step(pos: Pos, dir: Dir) Pos {
return switch (dir) {
.left => .{ .x = pos.x -% 1, .y = pos.y },
.right => .{ .x = pos.x + 1, .y = pos.y },
.up => .{ .x = pos.x, .y = pos.y -% 1 },
.down => .{ .x = pos.x, .y = pos.y + 1 },
};
}
fn invalid(pos: Pos, input: Input) bool {
return pos.x >= input.width or pos.y >= input.height;
}
const Input = struct {
plants: []u8,
width: usize,
height: usize,
};
t:
\\ -
\\ RRRR|
\\|RRRR
\\ RRR|
\\ R -
\\ -
,
const test_input =
\\RRRRIICCFF
\\RRRRIICCCF
\\VVRRRCCFFF
\\VVRCCCJFFF
\\VVVVCJJCFE
\\VVIVCCJJEE
\\VVIIICJJEE
\\MIIIIIJJEE
\\MIIISIJEEE
\\MMMISSJEEE
\\
;
pub fn parse(allocator: std.mem.Allocator, data: []const u8) !Input {
var it = std.mem.split(u8, data, "\n");
var plants = std.ArrayList(u8).init(allocator);
var width: usize = undefined;
var height: usize = 0;
while (it.next()) |line| {
if (line.len == 0) break;
width = line.len;
height += 1;
try plants.appendSlice(line);
}
return .{ .plants = plants.items, .width = width, .height = height };
}
test "part1" {
var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
defer arena.deinit();
const output = try part1(arena.allocator(), try parse(arena.allocator(), test_input));
try std.testing.expectEqual(1930, output);
}
pub fn part1(allocator: std.mem.Allocator, input: Input) !u32 {
var visited = try allocator.alloc(bool, input.plants.len);
var stack = std.ArrayList(Pos).init(allocator);
var cost: u32 = 0;
for (0..input.height) |start_y| for (0..input.width) |start_x| {
if (visited[start_y * input.width + start_x]) continue;
stack.clearRetainingCapacity();
try stack.append(.{ .x = start_x, .y = start_y });
visited[start_y * input.width + start_x] = true;
const plant = input.plants[start_y * input.width + start_x];
var area: u32 = 1;
var perimeter: u32 = 0;
while (stack.popOrNull()) |pos| {
// std.debug.print("considering {}, {}\n", .{ pos.x, pos.y });
for ([4]Pos{
.{ .x = pos.x -% 1, .y = pos.y },
.{ .x = pos.x + 1, .y = pos.y },
.{ .x = pos.x, .y = pos.y -% 1 },
.{ .x = pos.x, .y = pos.y + 1 },
}) |n| {
if (n.x >= input.width or n.y >= input.height or input.plants[n.y * input.width + n.x] != plant) {
// std.debug.print("1 perimeter from {}, {} towards {}, {}\n", .{ pos.x, pos.y, @as(i64, @bitCast(n.x)), @as(i64, @bitCast(n.y)) });
perimeter += 1;
continue;
}
if (visited[n.y * input.width + n.x]) continue;
try stack.append(n);
visited[n.y * input.width + n.x] = true;
area += 1;
}
}
// std.debug.print("A region of {c} plants with price {} * {}\n", .{ plant, area, perimeter });
cost += area * perimeter;
};
return cost;
}
test "part2" {
var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
defer arena.deinit();
const output = try part2(arena.allocator(), try parse(arena.allocator(), test_input));
std.debug.print("got {}\n", .{output});
try std.testing.expectEqual(1206, output);
}
pub fn part2(allocator: std.mem.Allocator, input: Input) !u32 {
var visited = try allocator.alloc(bool, input.plants.len);
var stack = std.ArrayList(Pos).init(allocator);
var cost: u32 = 0;
for (0..input.height) |start_y| for (0..input.width) |start_x| {
if (visited[start_y * input.width + start_x]) continue;
stack.clearRetainingCapacity();
try stack.append(.{ .x = start_x, .y = start_y });
visited[start_y * input.width + start_x] = true;
const plant = input.plants[start_y * input.width + start_x];
var area: u32 = 1;
var sides: u32 = 0;
while (stack.popOrNull()) |pos| {
// std.debug.print("considering {}, {}\n", .{ pos.x, pos.y });
for (std.meta.tags(Dir)) |dir| {
const n = step(pos, dir);
if (invalid(n, input) or input.plants[n.y * input.width + n.x] != plant) {
const side: Dir = switch (dir) {
.left => .down,
.down => .right,
.right => .up,
.up => .left,
};
const s = step(pos, side);
const is_leftmost = invalid(s, input) or
input.plants[s.y * input.width + s.x] != plant or
blk: {
const x = step(s, dir);
break :blk !invalid(x, input) and input.plants[x.y * input.width + x.x] == plant;
};
if (is_leftmost) {
// std.debug.print("1 side from {}, {} towards {}, {}\n", .{ pos.x, pos.y, @as(i64, @bitCast(n.x)), @as(i64, @bitCast(n.y)) });
sides += 1;
}
continue;
}
if (visited[n.y * input.width + n.x]) continue;
try stack.append(n);
visited[n.y * input.width + n.x] = true;
area += 1;
}
}
// std.debug.print("A region of {c} plants with price {} * {}\n", .{ plant, area, sides });
cost += area * sides;
};
return cost;
}
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