Move some functionality into the new Engine mod

This will help with separating the "thinking" logic from the move rules
and stuff.

rs/src/ui.rs

@@ -1,5 +1,6 @@
 use crate::board;
 use crate::board::GridAxis;
+use crate::engine;
 use std::io::BufRead;
 
 impl std::fmt::Display for board::Color {
@@ -140,20 +141,18 @@ impl board::Board {
 }
 
 pub struct Ui {
-    board: board::Board,
+    engine: engine::Engine,
 }
 
 impl Ui {
     pub fn new() -> Self {
-        let mut board = board::Board::new();
-        board.reset();
         Ui {
-            board,
+            engine: engine::Engine::new(),
         }
     }
     fn get_state(&self, args: &[&str]) -> Result<String, String> {
         if let [] = args {
-            let result = format!("{}", self.board);
+            let result = format!("{}", self.engine.get_state());
             Ok(result)
         } else {
             Err("get_state takes no args".to_owned())
@@ -161,9 +160,12 @@ impl Ui {
     }
     fn set_state(&mut self, args: &[&str]) -> Result<String, String> {
         if let [state_str] = args {
-            self.board = board::Board::parse(state_str)
-                .map_err(|_| format!("Error parsing state {}", state_str))?;
-            let result = format!("{}", self.board);
+            self.engine.set_state(
+                board::Board::parse(state_str).map_err(|_| {
+                    format!("Error parsing state {}", state_str)
+                })?,
+            );
+            let result = format!("{}", self.engine.get_state());
             Ok(result)
         } else {
             Err("set_state takes 1 arg".to_owned())
@@ -175,12 +177,12 @@ impl Ui {
                 board::Position::parse(position_str).map_err(|_| {
                     format!("Error parsing position {}", position_str)
                 })?;
-            match self.board.get_legal_moves(&position) {
-                None => {
+            match self.engine.get_legal_moves(&position) {
+                Err(_) => {
                     let error = format!("No moves possible from {}", position);
                     Err(error)
                 }
-                Some(positions) => Ok(positions
+                Ok(positions) => Ok(positions
                     .iter()
                     .map(|pos| format!("{}", pos))
                     .collect()),
@@ -195,12 +197,12 @@ impl Ui {
                 .map_err(|_| format!("Error parsing source {}", source_str))?;
             let target = board::Position::parse(target_str)
                 .map_err(|_| format!("Error parsing target {}", target_str))?;
-            match self.board.make_move(&source, target) {
+            match self.engine.make_move(&source, target) {
                 Err(()) => Err(format!(
                     "Move not possible {}-{}",
                     source_str, target_str
                 )),
-                Ok(()) => Ok(self.board.to_string()),
+                Ok(()) => Ok(format!("{}", self.engine.get_state())),
             }
         } else {
             Err("make_move takes 2 args".to_owned())