Implement selected piece highlighting

Issue #3

frontend/main.js

@@ -31,6 +31,7 @@ class Chess {
     this.canvas = new visuals.Canvas();
     this.configVis = new visuals.ConfigVis(this.backend.getConfig());
     this.configVis.draw(this.canvas);
+    this.activeSquares = new visuals.ActiveSquares();
     this.moveSource = null;
     this.canMoveTo = [];
     document.onclick = (ev) => this.click(ev);
@@ -44,12 +45,14 @@ class Chess {
     if (source === null) return;
     let validMoves = this.backend.getAvailableMoves(source);
     if (validMoves === null) return;
+    this.activeSquares.selectSquare(this.canvas, source);
     this.moveSource = source;
   }
   finalizeMove(target) {
     if (target !== null) {
       this.backend.makeMove(this.moveSource, target);
     }
+    this.activeSquares.unselectSquare(this.canvas);
     this.syncBackend();
     this.moveSource = null;
   }

frontend/visuals.js

@@ -86,6 +86,23 @@ export function rowColToBoardIndex(row, col) {
   return `${colLetter}${rowDigit}`;
 }
 
+export class ActiveSquares {
+  constructor() {
+    this.selectedSquare = null;
+  }
+  selectSquare(canvas, position) {
+    let [r, c] = boardIndexToRowCol(position);
+    canvas.drawSquareColor("#c0c0ff", r, c);
+    this.selectedSquare = position;
+  }
+  unselectSquare(canvas) {
+    if (this.selectedSquare === null) return;
+    let [r, c] = boardIndexToRowCol(this.selectedSquare);
+    canvas.drawSquareColor(canvas.decideColor(r, c), r, c);
+    this.selectedSquare = null;
+  }
+}
+
 class PieceVis {
   constructor(piece) {
     this.piece = piece;