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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();
var output = try solve(example, allocator);
try std.testing.expect(std.mem.eql(u8, output, "MCD"));
const result = try solve(input, allocator);
try std.io.getStdOut().writer().print("{s}\n", .{result});
}
fn solve(puzzle: []const u8, allocator: std.mem.Allocator) ![]u8 {
var it = std.mem.tokenize(u8, puzzle, "\n");
var line = it.next() orelse unreachable;
// Initializations from the first line
const size = (line.len + 1) / 4;
var output = try allocator.alloc(u8, size);
var stacks = try allocator.alloc(std.ArrayList(u8), size);
for (stacks) |*stack| {
stack.* = std.ArrayList(u8).init(allocator);
}
// parse stacks
while (true) {
var i: usize = 0;
while (i < size) : (i += 1) {
const v = line[i * 4 + 1];
if (v != ' ') {
try stacks[i].insert(0, v);
}
}
line = it.next() orelse unreachable;
if (line[0] != '[') { // we got the line with numbers
break;
}
}
// process rules
while (it.next()) |l| {
var elts = std.mem.split(u8, l, " ");
_ = elts.next() orelse unreachable; // the move word
var n = try std.fmt.parseInt(u8, elts.next() orelse unreachable, 10);
_ = elts.next() orelse unreachable; // the from word
const s = try std.fmt.parseInt(u8, elts.next() orelse unreachable, 10) - 1;
_ = elts.next() orelse unreachable; // the to word
const d = try std.fmt.parseInt(u8, elts.next() orelse unreachable, 10) - 1;
const nl = stacks[s].items.len - n;
try stacks[d].appendSlice(stacks[s].items[nl..]);
stacks[s].items.len = nl;
}
// generate output
var i: usize = 0;
while (i < size) : (i += 1) {
output[i] = stacks[i].pop();
}
return output;
}
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