diff --git a/src/agent.rs b/src/agent.rs
new file mode 100644
index 0000000..58ecea0
--- /dev/null
+++ b/src/agent.rs
@@ -0,0 +1,30 @@
+pub type Distance = isize;
+
+pub struct Position {
+  pub x: Distance,
+  pub y: Distance,
+}
+
+pub struct Direction {
+  pub x: Distance,
+  pub y: Distance,
+}
+
+pub type AgentId = usize;
+
+pub struct WorldView {
+  pub self_tagged: bool,
+  pub bounds_distance: (Distance, Distance, Distance, Distance),
+  pub other_agents: Vec<(Direction, AgentId)>,
+}
+
+pub enum Move {
+  Noop, // Why not an Option?
+  TryTag(AgentId),
+  TryMove(Direction),
+}
+
+pub trait Agent {
+  fn next_move(&self, view: WorldView) -> Move;
+  fn get_tagged(&mut self, by: AgentId);
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..1ad7f26
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,2 @@
+pub mod agent;
+pub mod world;
diff --git a/src/world.rs b/src/world.rs
new file mode 100644
index 0000000..9c9dda4
--- /dev/null
+++ b/src/world.rs
@@ -0,0 +1,62 @@
+use crate::agent::{Agent, AgentId, Direction, Move, Position, WorldView};
+use std::collections::HashMap;
+
+pub struct ActualWorld {
+  size: Position,
+  pub agent_positions: HashMap<AgentId, Position>,
+  pub agents: HashMap<AgentId, Box<dyn Agent>>,
+  pub tagged: AgentId,
+  pub tagged_by: Option<AgentId>,
+}
+
+impl ActualWorld {
+  pub fn do_step(&mut self) {
+    let moves: Vec<_> = self
+      .agents
+      .iter()
+      .map(|(id, agent)| {
+        let pos = self.agent_positions.get(id).unwrap();
+        (
+          *id,
+          agent.next_move(WorldView {
+            self_tagged: *id == self.tagged,
+            bounds_distance: (pos.y, self.size.x - pos.x, self.size.y - pos.y, pos.x),
+            other_agents: self
+              .agent_positions
+              .iter()
+              .map(|(id, other_pos)| {
+                (
+                  Direction {
+                    x: other_pos.x - pos.x,
+                    y: other_pos.y - pos.y,
+                  },
+                  *id,
+                )
+              })
+              .collect(),
+          }),
+        )
+      })
+      .collect();
+    for (id, mv) in moves {
+      match mv {
+        Move::Noop => {}
+        Move::TryTag(other_id) => {
+          // FIXME check distance
+          // FIXME check tagged_by
+          self.agents.get_mut(&other_id).unwrap().get_tagged(other_id);
+          // FIXME update tagged
+          // FIXME update tagged_by
+        }
+        Move::TryMove(dir) => {
+          // FIXME check out of bounds
+          // FIXME matches!
+          self.agent_positions.entry(id).and_modify(|e| {
+            e.x += dir.x;
+            e.y += dir.y;
+          });
+        }
+      }
+    }
+  }
+}