Conway's Game of Life

The goal of this challenge is to implement the logic used in Conway's Game of Life. Wikipedia will give a better understanding of what it is and how it works.

Rules

• For a space that's "populated":
  • Each cell with 0 or 1 neighbours dies, as if by solitude.
  • Each cell with 2 or 3 neighbours survives.
  • Each cell with 4 or more neighbours dies, as if by overpopulation.
• For a space that's "empty" or "unpopulated":
  • Each cell with 3 neighbours becomes populated.

Parameters

board: a 2-dimensional list of values 0 to 1.

• 0 means that the cell is empty.
• 1 means the cell is populated.

Return Value

A string containing the board's state after the game logic has been applied once.

On character: I
Off character: _

Notes

• The string should be divided by newlines \n to signal the end of each row.
• A cell's "neighbours" are the eight cells that are vertically, horizontally and diagonally adjacent to it.