From 87065dc4abe2e987da18e6735b00ef88a940372a Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Tue, 20 Aug 2024 19:33:43 -0700
Subject: Basic enemy AI.

---
 Demos/Pong/Pong.hs | 29 ++++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/Demos/Pong/Pong.hs b/Demos/Pong/Pong.hs
index e834c5d..b05bb24 100644
--- a/Demos/Pong/Pong.hs
+++ b/Demos/Pong/Pong.hs
@@ -26,11 +26,12 @@ import           Data.Monoid         (mconcat)
 
 padSize             = vec2 0.07 0.02
 ballSize            = 0.012 :: Float
-ballSpeed           = 0.6 :: Float
+ballSpeed           = 0.7 :: Float
 initialBallVelocity = vec2 1 1
 maxBounceAngle      = (65::Float) * (pi::Float)/(180::Float)
 playerSpeed         = 1.0 :: Float
-enemySpeed          = 3.0 :: Float
+enemySpeed          = 7.0 :: Float
+enemyMomentum       = 0.1 :: Float
 initialEnemyPos     = vec2 0.5 0.9
 initialPlayerPos    = vec2 0.5 0.1
 initialBallPos      = vec2 0.5 0.5
@@ -146,17 +147,19 @@ moveBall = step $ \_ dt _ _ (vel, ball) -> (translate (vel * dt) ball, moveBall)
 
 -- Enemy stepper
 
-stepEnemy = movePad
-
-movePad :: Step s e GameObject GameObject
-movePad = step $ \elapsed _ _ _ pad ->
-  let enemyY = 0.9
-      p = vec2 px enemyY
-      px =
-        (sin (enemySpeed * elapsed) * (0.5::Float) + (0.5::Float))
-          * ((1::Float) - (2::Float) * x padSize)
-          + x padSize
-   in (setPosition p pad, movePad)
+stepEnemy = movePad 0 .> clamp
+
+movePad :: Float -> Step [GameObject] e GameObject GameObject
+movePad previousMomentum = step $ \_ dt gos _ pad ->
+  let ball        = head gos
+      offset      = (x . position $ ball) - (x . position $ pad)
+      chaseVector = enemySpeed * offset
+      momentum    = previousMomentum + enemyMomentum * chaseVector
+      vx          = chaseVector + momentum
+   in (translate (vec2 (vx * dt) 0) pad, movePad momentum)
+
+sign :: Float -> Float
+sign x = if x >= 0 then 1 else -1
 
 -- Player stepper
 
-- 
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