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const std = @import("std");
const example = @embedFile("example");
const input = @embedFile("input");
pub fn main() anyerror!void {
var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
defer arena.deinit();
const allocator = arena.allocator();
try std.testing.expectEqual(try solve(example, allocator), 29);
const result = try solve(input, allocator);
try std.io.getStdOut().writer().print("{}\n", .{result});
}
const Node = struct {
x: u8,
y: u8,
cost: ?u64,
value: u8,
fn evalAndAdd(e: *Node, n: *Node, nq: *std.PriorityQueue(*Node, void, lesserThan)) !void {
if (e.cost == null and n.value <= e.value + 1) {
e.cost = n.cost.? + 1;
try nq.add(e);
}
}
};
fn lesserThan(context: void, a: *Node, b: *Node) std.math.Order {
_ = context;
return std.math.order(a.cost.?, b.cost.?);
}
fn solve(puzzle: []const u8, allocator: std.mem.Allocator) !u64 {
var it = std.mem.tokenize(u8, puzzle, "\n");
var grid = std.ArrayList(std.ArrayList(Node)).init(allocator);
var Sx: u8 = undefined;
var Sy: u8 = undefined;
var Ex: u8 = undefined;
var Ey: u8 = undefined;
// process input
var y: u8 = 0;
var line = it.next() orelse unreachable;
const width = line.len;
while (true) : (line = it.next() orelse break) {
var l = try grid.addOne();
l.* = try std.ArrayList(Node).initCapacity(allocator, width);
var x: u8 = 0;
while (x < line.len) : (x += 1) {
var n = l.*.addOneAssumeCapacity();
n.*.x = x;
n.*.y = y;
n.*.cost = null;
n.*.value = switch (line[x]) {
'S' => blk: {
Sx = x;
Sy = y;
break :blk 'a';
},
'E' => blk: {
Ex = x;
Ey = y;
n.*.cost = 0;
break :blk 'z';
},
else => line[x],
};
}
y += 1;
}
const height = grid.items.len;
// find shortest path
var nq = std.PriorityQueue(*Node, void, lesserThan).init(allocator, {});
try nq.add(&grid.items[Ey].items[Ex]);
while (true) {
var n = nq.remove();
if (n.value == 'a') {
return n.cost.?;
}
if (n.x > 0) {
try grid.items[n.y].items[n.x - 1].evalAndAdd(n, &nq);
}
if (n.x < width - 1) {
try grid.items[n.y].items[n.x + 1].evalAndAdd(n, &nq);
}
if (n.y > 0) {
try grid.items[n.y - 1].items[n.x].evalAndAdd(n, &nq);
}
if (n.y < height - 1) {
try grid.items[n.y + 1].items[n.x].evalAndAdd(n, &nq);
}
}
}
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