From 219f8d74a0259efc4c2b3538edb0d749902b69ae Mon Sep 17 00:00:00 2001
From: Antonio Martinez <aj@worldsbe.st>
Date: Thu, 22 May 2025 16:35:53 -0400
Subject: [PATCH] Bot grip experiment

---
 src/d_player.h | 2 ++
 src/k_bot.cpp  | 8 ++++++--
 src/k_hud.cpp  | 2 ++
 src/k_kart.c   | 9 +++++++--
 src/p_user.c   | 3 +++
 5 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/src/d_player.h b/src/d_player.h
index d22f6f734..f056ca978 100644
--- a/src/d_player.h
+++ b/src/d_player.h
@@ -420,6 +420,8 @@ struct botvars_t
 
 	UINT8 roulettePriority; // What items to go for on the roulette
 	tic_t rouletteTimeout; // If it takes too long to decide, try lowering priority until we find something valid.
+
+	angle_t predictionError; // How bad is our momentum angle relative to where we're trying to go?
 };
 
 // player_t struct for round-specific condition tracking
diff --git a/src/k_bot.cpp b/src/k_bot.cpp
index 041a0be38..bc086c649 100644
--- a/src/k_bot.cpp
+++ b/src/k_bot.cpp
@@ -1427,10 +1427,11 @@ static INT32 K_HandleBotTrack(const player_t *player, ticcmd_t *cmd, botpredicti
 	I_Assert(predict != nullptr);
 
 	destangle = K_BotSmoothLanding(player, destangle);
-
 	moveangle = player->mo->angle + K_GetUnderwaterTurnAdjust(player);
 	anglediff = AngleDeltaSigned(moveangle, destangle);
 
+	cmd->angle = std::min(destangle - moveangle, moveangle - destangle) >> TICCMD_REDUCE;
+
 	if (anglediff < 0)
 	{
 		turnsign = 1;
@@ -1712,7 +1713,7 @@ static void K_BuildBotPodiumTiccmd(const player_t *player, ticcmd_t *cmd)
 
 		Build ticcmd for bots with a style of BOT_STYLE_NORMAL
 --------------------------------------------------*/
-static void K_BuildBotTiccmdNormal(const player_t *player, ticcmd_t *cmd)
+static void K_BuildBotTiccmdNormal(player_t *player, ticcmd_t *cmd)
 {
 	precise_t t = 0;
 
@@ -1724,6 +1725,9 @@ static void K_BuildBotTiccmdNormal(const player_t *player, ticcmd_t *cmd)
 	UINT8 spindash = 0;
 	INT32 turnamt = 0;
 
+	cmd->angle = 0; // For bots, this is used to transmit prediction error to gamelogic.
+	// Will be overwritten by K_HandleBotTrack if we have a destination.
+
 	if (!(gametyperules & GTR_BOTS) // No bot behaviors
 		|| K_GetNumWaypoints() == 0 // No waypoints
 		|| leveltime <= introtime // During intro camera
diff --git a/src/k_hud.cpp b/src/k_hud.cpp
index 616861b37..1aae11443 100644
--- a/src/k_hud.cpp
+++ b/src/k_hud.cpp
@@ -6689,6 +6689,8 @@ static void K_DrawBotDebugger(void)
 
 	V_DrawSmallString(8, 66, 0, va("Complexity: %d", K_GetTrackComplexity()));
 	V_DrawSmallString(8, 70, 0, va("Bot modifier: %.2f", FixedToFloat(K_BotMapModifier())));
+
+	V_DrawSmallString(8, 76, 0, va("Prediction error: %d", bot->botvars.predictionError));
 }
 
 static void K_DrawGPRankDebugger(void)
diff --git a/src/k_kart.c b/src/k_kart.c
index 0371c1134..9c0264568 100644
--- a/src/k_kart.c
+++ b/src/k_kart.c
@@ -13022,14 +13022,19 @@ fixed_t K_PlayerBaseFriction(const player_t *player, fixed_t original)
 
 			// A bit extra friction to help them without drifting.
 			// Remove this line once they can drift.
-			frict -= extraFriction;
+			// frict -= extraFriction;
+
+			angle_t MAXERROR = 45*ANG1;
+			fixed_t errorfrict = Easing_Linear(min(FRACUNIT, FixedDiv(player->botvars.predictionError, MAXERROR)), 0, FRACUNIT>>2);
+
+			frict -= errorfrict;
 
 			// Bots gain more traction as they rubberband.
 			const fixed_t traction_value = FixedMul(player->botvars.rubberband, max(FRACUNIT, K_BotMapModifier()));
 			if (traction_value > FRACUNIT)
 			{
 				const fixed_t traction_mul = traction_value - FRACUNIT;
-				frict -= FixedMul(extraFriction, traction_mul);
+				// frict -= FixedMul(extraFriction, traction_mul);
 			}
 		}
 	}
diff --git a/src/p_user.c b/src/p_user.c
index 9e811dc01..c487f8ea3 100644
--- a/src/p_user.c
+++ b/src/p_user.c
@@ -2321,6 +2321,9 @@ static void P_UpdatePlayerAngle(player_t *player)
 	{
 		// You're a bot. Go where you're supposed to go
 		player->steering = targetsteering;
+		// But the "angle" field of this ticcmd stores your prediction error,
+		// which we use to apply friction. Transfer it!
+		player->botvars.predictionError = player->cmd.angle << TICCMD_REDUCE;
 	}
 	else if ((!(player->cmd.flags & TICCMD_RECEIVED)) && (!!(player->oldcmd.flags && TICCMD_RECEIVED)))
 	{