Plan

General:
Light blue for available squares to move to
red for available capture
light up selected piece dark green
hovered piece light green

Pawn:
on click:
if no piece in front: light up square in front
if first move and no piece within 2 spaces: light up 2 squares in front
if enemy piece diagonally forward: light up enemy square
if on other side of board: turn into queen
functions:
{
passantvulnerable: if enemy pawn 1 square to the side: if said pawn selected: light up diagonal square behind team pawn
on moved 2 spaces: set passantvulnerable = 1
on next turn: set passantvulnerable = 0
}

Rook:
on click
light up squares in all cardinal directions until run into another piece or edge of board
light up enemy piece if in the way, then stop
functions:
{
castle: if castle initiated move to the other side of king (2 or 3 spaces depending on side)
}

Bishop:
light up squares in all diagonal directions until run into another piece or edge of board
light up enemy piece in the way, then stop

Queen
see Rook + Bishop

Knight
Light up all squares an "L" shape away from piece unless ally piece is occupying the square or it is off the board
Ex. if on square 53
Available moves: 61, 72, 74, 65, 45, 34, 32, 41
x: +2/+1/-1/-2, y: +2/+1/-1/-2 TOTAL absolute value = 3

King
Light up all squares 1 square away directionally or diagonally, UNLESS
1. ally piece occupyes the square
2. square is off the board
3. enemy piece has the ability to move to new square next move
If king is currently being threatened by another piece, and if king has no available moves using the above ruleset, and if no ally piece can block the attack(s), and if no ally piece can capture the attacking piece(s): other team wins
Functions:
{
Castle: If King moves = 0 and CastlingRook moves = 0 and no pieces between king and CastlingRook and no piece threatening new king spot or new rook spot: castle(left/right) = 1        once a condition is no longer met: castle(left/right) = 0
}