grid-math.js

#!/usr/bin/env node

const assert = require('assert');

function fieldNumberToLetter(number) {
  return String.fromCharCode(number + 65);
}

function squareToNeighbors(square) {
  square = square.toUpperCase();

  const neighborMatrix = [
    [-1, +1], // NW
    [0,  +1], // N
    [+1, +1], // NE

    [-1,  0], // W
    [0,   0], // Self
    [+1,  0], // E

    [-1, -1], // SW
    [0,  -1], // S
    [+1, -1], // SE
  ];

  // RL90
  // ^^---- RL is the "field"
  //   ^^---- 90 is the "square"

  const fieldX = square.charCodeAt(0) - 65;
  const fieldY = square.charCodeAt(1) - 65;

  const squareX = square.charCodeAt(2) - 48;
  const squareY = square.charCodeAt(3) - 48;

  const neighbors = [];

  for (const [dx, dy] of neighborMatrix) {
    let neighborFieldX = fieldX;
    let neighborFieldY = fieldY;

    let neighborSquareX = squareX + dx;
    let neighborSquareY = squareY + dy;

    if (neighborSquareX < 0) {
      neighborSquareX += 10;
      neighborFieldX -= 1;
    } else if (neighborSquareX > 9) {
      neighborSquareX -= 10;
      neighborFieldX += 1;
    }

    if (neighborSquareY < 0) {
      neighborSquareY += 10;
      neighborFieldY -= 1;
    } else if (neighborSquareY > 9) {
      neighborSquareY -= 10;
      neighborFieldY += 1;
    }

    if (neighborFieldX < 0) {
      neighborFieldX += 18;
    } else if (neighborFieldX > 17) {
      neighborFieldX -= 18;
    }

    if (neighborFieldY < 0 || neighborFieldY > 17) {
      continue;
    }

    const neighbor = `${fieldNumberToLetter(neighborFieldX)}${fieldNumberToLetter(neighborFieldY)}${neighborSquareX}${neighborSquareY}`;

    neighbors.push(neighbor);
  }

  return neighbors;
}

function assertCorrect(neighbors, correct) {
  for (let i = 0; i < neighbors.length; i++) {
    assert.equal(neighbors[i], correct[i]);
  }
}

console.log('RL90');
assertCorrect(
  squareToNeighbors('RL90'),
  ['RL81', 'RL91', 'AL01', 'RL80', 'RL90', 'AL00', 'RK89', 'RK99', 'AK09']);

console.log('RL00');
assertCorrect(
  squareToNeighbors('RL00'),
  ['QL91', 'RL01', 'RL11', 'QL90', 'RL00', 'RL10', 'QK99', 'RK09', 'RK19']);

console.log('AL00');
assertCorrect(
  squareToNeighbors('AL00'),
  ['RL91', 'AL01', 'AL11', 'RL90', 'AL00', 'AL10', 'RK99', 'AK09', 'AK19']);

console.log(squareToNeighbors('BR99'));

console.log(squareToNeighbors('BA00'));