Life: implementation

src/life.js

@@ -0,0 +1,46 @@
+const countNeighbours = grid => ([y, x]) => {
+  const sum = (a, b) => a + b
+
+  const area = (grid, y, x) => {
+    const [yf, xf] = [y, x].map(c => Math.max(0, c - 1))
+    const [yt, xt] = [y, x].map(c => c + 2)
+    return grid
+      .slice(yf, yt)
+      .map(row => row.slice(xf, xt))
+  }
+
+  const areaSum =
+    area(grid, y, x)
+      .map(row =>
+        row.reduce(sum, 0))
+      .reduce(sum, 0)
+
+  return [areaSum - grid[y][x], grid[y][x]]
+}
+
+/**
+  * Any live cell with fewer than two live neighbours dies, as if by underpopulation.
+  * Any live cell with more than three live neighbours dies, as if by overpopulation.
+  * Any live cell with two or three live neighbours lives on to the next generation.
+  * Any dead cell with exactly three live neighbours becomes a live cell, as if by reproduction.
+  */
+const deadOrAlive = ([neighbours, cell]) => {
+  if (cell && neighbours < 2) return 0
+  if (cell && neighbours > 3) return 0
+  if (cell && [2, 3].includes(neighbours)) return 1
+  if (!cell && neighbours === 3) return 1
+  else return 0
+}
+
+const lifeStep = grid =>
+  grid.map((row, y) =>
+    row
+      .map((_, x) => [y, x])
+      .map(countNeighbours(grid))
+      .map(deadOrAlive))
+
+module.exports = {
+  countNeighbours,
+  lifeStep,
+  deadOrAlive
+}