3 files changed, 56 insertions, 9 deletions
diff --git a/src/LivingVillage.Kernel/LivingVillage.Kernel.fsproj b/src/LivingVillage.Kernel/LivingVillage.Kernel.fsproj
index a8ed312..8a4fad1 100644
--- a/src/LivingVillage.Kernel/LivingVillage.Kernel.fsproj
+++ b/src/LivingVillage.Kernel/LivingVillage.Kernel.fsproj
@@ -5,6 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <Compile Include="Rng.fs" />
     <Compile Include="Sim.fs" />
   </ItemGroup>
 
diff --git a/src/LivingVillage.Kernel/Rng.fs b/src/LivingVillage.Kernel/Rng.fs
new file mode 100644
index 0000000..9564b56
--- /dev/null
+++ b/src/LivingVillage.Kernel/Rng.fs
@@ -0,0 +1,24 @@
+namespace LivingVillage.Kernel
+
+[<Struct>]
+type RngState =
+    { State: uint64 }
+
+module Rng =
+
+    let goldenGamma = 0x9E3779B97F4A7C15UL
+
+    let ofSeed (seed: uint64) : RngState = { State = seed }
+
+    let nextUInt64 (rng: RngState) : uint64 * RngState =
+        let s = rng.State + goldenGamma
+        let mutable z = s
+        z <- (z ^^^ (z >>> 30)) * 0xBF58476D1CE4E5B9UL
+        z <- (z ^^^ (z >>> 27)) * 0x94D049BB133111EBUL
+        let out = z ^^^ (z >>> 31)
+        out, { State = s }
+
+    let nextFloat32 (rng: RngState) : float32 * RngState =
+        let z, rng' = nextUInt64 rng
+        let bits = uint32 (z >>> 40)
+        float32 bits / 16777216.0f, rng'
diff --git a/src/LivingVillage.Kernel/Sim.fs b/src/LivingVillage.Kernel/Sim.fs
index d1b2e57..0e3683b 100644
--- a/src/LivingVillage.Kernel/Sim.fs
+++ b/src/LivingVillage.Kernel/Sim.fs
@@ -2,10 +2,12 @@ namespace LivingVillage.Kernel
 
 module Sim =
 
+    [<Struct>]
     type Vec2 =
         { X: float32
           Y: float32 }
 
+    [<Struct>]
     type Avatar =
         { Pos: Vec2 }
 
@@ -14,33 +16,53 @@ module Sim =
           MoveY: float32 }
 
     type TimeStep =
-        { Dt: float32
-          Input: Input }
+        { Input: Input }
 
+    [<Struct>]
+    type NoHost =
+        { Reserved: uint64 }
+
+    [<Struct>]
     type World =
         { Tick: int64
-          Avatar: Avatar }
+          Time: float
+          Rng: RngState
+          Avatar: Avatar
+          NoHost: NoHost }
+
+    let ticksPerSecond = 60L
+    let secondsPerDay = 86400L
+    let ticksPerDay = secondsPerDay * ticksPerSecond
+    let dtSeconds = 1.0 / float ticksPerSecond
+    let dtSecondsF = 1.0f / float32 ticksPerSecond
 
     let mapWidthTiles = 64
     let mapHeightTiles = 48
     let tilePixels = 32
     let avatarSpeed = 160.0f
 
-    let initialWorld () : World =
+    let initialWorld (seed: uint64) : World =
         let centerX = float32 (mapWidthTiles * tilePixels / 2 - tilePixels / 2)
         let centerY = float32 (mapHeightTiles * tilePixels / 2 - tilePixels / 2)
         { Tick = 0L
-          Avatar = { Pos = { X = centerX; Y = centerY } } }
+          Time = 0.0
+          Rng = Rng.ofSeed seed
+          Avatar = { Pos = { X = centerX; Y = centerY } }
+          NoHost = { Reserved = 0UL } }
 
     let clamp v lo hi = if v < lo then lo elif v > hi then hi else v
 
     let step (ts: TimeStep) (world: World) : World =
+        let tick = world.Tick + 1L
         let maxX = float32 (mapWidthTiles * tilePixels - tilePixels)
         let maxY = float32 (mapHeightTiles * tilePixels - tilePixels)
-        let dx = ts.Input.MoveX * avatarSpeed * ts.Dt
-        let dy = ts.Input.MoveY * avatarSpeed * ts.Dt
-        { Tick = world.Tick + 1L
+        let dx = ts.Input.MoveX * avatarSpeed * dtSecondsF
+        let dy = ts.Input.MoveY * avatarSpeed * dtSecondsF
+        { Tick = tick
+          Time = float tick * dtSeconds
+          Rng = world.Rng
           Avatar =
             { Pos =
                 { X = clamp (world.Avatar.Pos.X + dx) 0.0f maxX
-                  Y = clamp (world.Avatar.Pos.Y + dy) 0.0f maxY } } }
+                  Y = clamp (world.Avatar.Pos.Y + dy) 0.0f maxY } }
+          NoHost = world.NoHost }