Advent of Code 2020 day 24

import 'dart:math';
import 'package:petitparser/petitparser.dart';
import 'advent2020.dart';
import 'package:advent2020/eulib.dart' as eulib;

void main() {
  day24a();
  day24b();
}

void day24a() {
  var p = buildParser();
  flipped = <Point, bool>{};
  var lines = readLinesFromFile('data/day24_data.txt');
  for (var l in lines) {
    CURR = Point(0, 0);
    p.parse(l);
    flipped[CURR] = !(flipped[CURR] ?? false);
  }
  assert(flipped.values.where((e) => e).length == 254);
}

void day24b() {
  var p = buildParser();
  flipped = <Point, bool>{};
  var lines = readLinesFromFile('data/day24_data.txt');
  for (var l in lines) {
    CURR = Point(0, 0);
    p.parse(l);
    flipped[CURR] = !(flipped[CURR] ?? false);
  }
  // Actual test.
  for (var _ in eulib.range(0, 100)) {
    flipped = life(flipped);
  }
  assert(flipped.values.where((e) => e).length == 3697);
}

// x y = bottom left.
/*
Two overlapping grids

      (1,3)---|---(3,3)------
        |     |     |
(0,2)-------(2,2)-------(4,2)--
        |     |     |
      (1,1)---|---(3,1)-------
*/

var dirs = {
  'ne': Point(1, 1),
  'se': Point(1, -1),
  'sw': Point(-1, -1),
  'nw': Point(-1, 1),
  'e': Point(2, 0),
  'w': Point(-2, 0),
};
Point CURR;
void move(String dir) => CURR += dirs[dir];

Parser buildParser() {
  var ne = (char('n') & char('e')).map((arr) => move('ne'));
  var se = (char('s') & char('e')).map((arr) => move('se'));
  var sw = (char('s') & char('w')).map((arr) => move('sw'));
  var nw = (char('n') & char('w')).map((arr) => move('nw'));
  var e = (char('e')).map((arr) => move('e'));
  var w = (char('w')).map((arr) => move('w'));
  var exp = (ne | se | sw | nw | e | w).plus();

  var b = ExpressionBuilder();
  b.group().primitive(exp);
  var rp = b.build().end();
  return rp;
}

Map<Point, bool> flipped;

Map<Point, bool> life(Map<Point, bool> current) {
  // Expand borders to include all needed whites.
  for (var p in current.keys.toList()) {
    for (var d in dirs.values) {
      current.putIfAbsent(p + d, () => false);
    }
  }
  // Copy current state into future state.
  var next = Map<Point, bool>.fromEntries(current.entries);
  for (var p in current.keys) {
    // Count flipped (=black) neighbours
    var n = dirs.values.where((e) => current[p + e] ?? false).length;
    // Are we on a black tile?
    if (current[p]) {
      if (n == 0 || n > 2) {
        next[p] = false;
      }
      // White.
    } else {
      if (n == 2) {
        next[p] = true;
      }
    }
  }
  return next;
}