From e5c68b0c3761de9d10b8a54e0ee7fd65bdce671b Mon Sep 17 00:00:00 2001
From: Tangent Wantwight <tangent128@gmail.com>
Date: Mon, 18 May 2020 19:32:56 -0400
Subject: [PATCH] Fix prediction from null state to permit local prediction
 before server starts sending input.

---
 src/ecs/Lockstep.ts        |  4 ++--
 src/game/GameComponents.ts | 11 +++++++++--
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/src/ecs/Lockstep.ts b/src/ecs/Lockstep.ts
index e1e78dd..ae0f950 100644
--- a/src/ecs/Lockstep.ts
+++ b/src/ecs/Lockstep.ts
@@ -3,7 +3,7 @@ export const INPUT_FREQUENCY = 33; // roughly 30fps
 
 export interface LockstepProcessor<LocalInput, GlobalInput, State> {
     compareInput(a: GlobalInput, b: GlobalInput): boolean;
-    predictInput(prev: GlobalInput | null, localPlayer: number, localInput: LocalInput): GlobalInput;
+    predictInput(prev: GlobalInput | undefined, localPlayer: number, localInput: LocalInput): GlobalInput;
     cloneState(source: State): State;
     advanceState(state: State, input: GlobalInput): void;
 }
@@ -45,7 +45,7 @@ export class LockstepState<LocalInput, GlobalInput, State> {
         // ensure that we don't overwrite the canon input with local input somehow
         // (probably only possible in situations where game is unplayable anyways? but still for sanity.)
         if(this.inputIndex > this.canonIndex) {
-            this.inputLog[this.inputIndex] = this.engine.predictInput(this.inputLog[this.inputIndex - 1] ?? null, player, input);
+            this.inputLog[this.inputIndex] = this.engine.predictInput(this.inputLog[this.inputIndex - 1] ?? undefined, player, input);
         }
     }
 
diff --git a/src/game/GameComponents.ts b/src/game/GameComponents.ts
index d82d2f7..64a1091 100644
--- a/src/game/GameComponents.ts
+++ b/src/game/GameComponents.ts
@@ -181,8 +181,15 @@ export class Engine implements LockstepProcessor<KeyName[], KeyName[][], Data> {
         return new Data(old);
     }
 
-    predictInput(prev: KeyName[][] | null, localPlayer: number, localInput: KeyName[]): KeyName[][] {
-        return prev?.map((prevInput, player) => (player == localPlayer) ? localInput : prevInput) ?? [];
+    predictInput(prev: KeyName[][] = [], localPlayer: number, localInput: KeyName[]): KeyName[][] {
+        const prediction = prev.slice();
+        for(let i = 0; i <= localPlayer; i++) {
+            if(!prediction[i]) {
+                prediction[i] = [];
+            }
+        }
+        prediction[localPlayer] = localInput;
+        return prediction;
     }
 
     compareInput(a: KeyName[][], b: KeyName[][]): boolean {