From 51e1252fb962fd53b8e856fac62d34ec092db226 Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom@tomsmeding.com>
Date: Sun, 18 Feb 2024 13:58:20 +0100
Subject: Angled bounce-back from pad

---
 game.js | 89 +++++++++++++++++++++++++++++++++++++++++++----------------------
 1 file changed, 59 insertions(+), 30 deletions(-)

diff --git a/game.js b/game.js
index 08baa51..e76bee7 100644
--- a/game.js
+++ b/game.js
@@ -18,10 +18,11 @@ function redraw() {
 	redrawFast();
 }
 
-function calcBallX() {return bdltX + bdW * (flip ? 1 - ballX : ballX);}
-function calcBallY() {return bdltY + bdH * ballY;}
-function calcPadY() {return bdltY + bdH * ((flip ? pad2Pos : padPos) - padHeight / 2);}
-function calcPad2Y() {return bdltY + bdH * ((flip ? padPos : pad2Pos) - padHeight / 2);}
+function calcFlippedX(x) {return bdltX + bdW * (flip ? 1 - x : x);}
+function calcY(y) {return bdltY + bdH * y;}
+
+function calcLeftPadY() {return calcY((flip ? pad2Pos : padPos) - padHeight / 2);}
+function calcRightPadY() {return calcY((flip ? padPos : pad2Pos) - padHeight / 2);}
 
 function redrawFast() {
 	ctx.strokeStyle = "#ffffff";
@@ -37,9 +38,9 @@ function redrawFast() {
 	if (playing) {
 		ctx.fillStyle = "#ffffff";
 		ctx.beginPath();
-		ctx.arc(calcBallX(), calcBallY(), ballRadius * bdW, 0, 2 * Math.PI);
-		ctx.rect(bdltX + bdW * padX, calcPadY(), bdW * padWidth, bdH * padHeight);
-		ctx.rect(bdltX + bdW * (1 - padX - padWidth), calcPad2Y(), bdW * padWidth, bdH * padHeight);
+		ctx.arc(calcFlippedX(ballX), calcY(ballY), ballRadius * bdW, 0, 2 * Math.PI);
+		ctx.rect(bdltX + bdW * padX, calcLeftPadY(), bdW * padWidth, bdH * padHeight);
+		ctx.rect(bdltX + bdW * (1 - padX - padWidth), calcRightPadY(), bdW * padWidth, bdH * padHeight);
 		ctx.fill();
 	}
 }
@@ -48,13 +49,13 @@ function undrawElements() {
 	ctx.fillStyle = "#000000";
 	ctx.beginPath();
 	ctx.rect(
-		calcBallX() - ballRadius * bdW - 2, calcBallY() - ballRadius * bdW - 2,
+		calcFlippedX(ballX) - ballRadius * bdW - 2, calcY(ballY) - ballRadius * bdW - 2,
 		2 * ballRadius * bdW + 4, 2 * ballRadius * bdW + 4);
 	ctx.rect(
-		bdltX + bdW * padX - 2, calcPadY() - 2,
+		bdltX + bdW * padX - 2, calcLeftPadY() - 2,
 		bdW * padWidth + 4, bdH * padHeight + 4);
 	ctx.rect(
-		bdltX + bdW * (1 - padX - padWidth) - 2, calcPad2Y() - 2,
+		bdltX + bdW * (1 - padX - padWidth) - 2, calcRightPadY() - 2,
 		bdW * padWidth + 4, bdH * padHeight + 4);
 	ctx.fill();
 }
@@ -231,6 +232,16 @@ function initPhysics() {
 	padVel = pad2Vel = 0;
 }
 
+// -1 <= frac <= 1
+function padHitAdjustCurve(frac) {
+	// ✨
+	const a = 0.8, b = 0.12;
+	const alpha = b / a**3;
+	const beta = (1 - alpha) / (1 - a)**2;
+	const x = Math.abs(frac);
+	return Math.sign(frac) * (alpha * x**3 + (x > a) * beta * (x - a)**2);
+}
+
 // Returns {bounce, otherBounce, ourSide : Bool}
 function advancePhysics(deltaT) {
 	const ret = {bounce: false, otherBounce: false, ourSide: false};
@@ -240,33 +251,51 @@ function advancePhysics(deltaT) {
 	const newpadPos = padPos + padVel * deltaT;
 	const newpad2Pos = pad2Pos + pad2Vel * deltaT;
 
-	let t;
 	if (newballY <= ballRadius) {
-		t = (ballRadius - ballY) / ballVY;
+		const t = (ballRadius - ballY) / ballVY;
+		newballY = ballY + ballVY * t + (-ballVY) * (deltaT - t);
+		ballVY = -ballVY;
 	} else if (newballY >= 1 - ballRadius) {
-		t = (1 - ballRadius - ballY) / ballVY;
-	} else {
-		t = -1;
-	}
-	if (t >= 0) {
-		newballY = ballY + ballVY * (2 * t - deltaT);
+		const t = (1 - ballRadius - ballY) / ballVY;
+		newballY = ballY + ballVY * t + (-ballVY) * (deltaT - t);
 		ballVY = -ballVY;
 	}
 
-	if (newballX <= padX + padWidth + ballRadius &&
-				Math.abs(newballY - padPos) < padHeight / 2 + ballRadius) {
-		t = (padX + padWidth + ballRadius - ballX) / ballVX;
+	function bounceLeft(x, y, vx, vy, pady) {
+		if (x + vx * deltaT > padX + padWidth + ballRadius) return null;
+
+		const t = (padX + padWidth + ballRadius - x) / vx;
+		if (Math.abs((y + t * vy) - pady) >= padHeight / 2 + ballRadius) return null;
+
+		const frac = ((y + t * vy) - pady) / (padHeight / 2 + ballRadius);
+		const angAdj = Math.PI / 4 * padHitAdjustCurve(frac);
+		const curAng = Math.atan2(vy, -vx);  // normal outgoing angle
+		const newAng = Math.max(-0.45 * Math.PI, Math.min(0.45 * Math.PI, curAng + angAdj));
+		const speed = Math.hypot(vx, vy);
+		const newvx = speed * Math.cos(newAng);
+		const newvy = speed * Math.sin(newAng);
+		const newx = x + vx * t + newvx * (deltaT - t);
+		const newy = y + vy * t + newvy * (deltaT - t);
+
+		return [newx, newy, newvx, newvy];
+	}
+
+	let bounce = bounceLeft(ballX, ballY, ballVX, ballVY, padPos);
+	if (bounce != null) {
+		newballX = bounce[0];
+		newballY = bounce[1];
+		ballVX = bounce[2];
+		ballVY = bounce[3];
 		ret.bounce = true;
-	} else if (newballX >= 1 - (padX + padWidth + ballRadius) &&
-				Math.abs(newballY - pad2Pos) < padHeight / 2 + ballRadius) {
-		t = (1 - (padX + padWidth + ballRadius) - ballX) / ballVX;
-		ret.otherBounce = true;
-	} else {
-		t = -1;
 	}
-	if (t >= 0) {
-		newballX = ballX + ballVX * (2 * t - deltaT);
-		ballVX = -ballVX;
+
+	bounce = bounceLeft(1 - ballX, ballY, -ballVX, ballVY, pad2Pos);
+	if (bounce != null) {
+		newballX = 1 - bounce[0];
+		newballY = bounce[1];
+		ballVX = -bounce[2];
+		ballVY = bounce[3];
+		ret.otherBounce = true;
 	}
 
 	// If the ball has come to our side, send a ballvec
-- 
cgit v1.2.3-54-g00ecf