Generalized branch-and-bound implementation

src/day19.rs

@@ -1,7 +1,7 @@
-use std::collections::HashSet;
-
 use regex::Regex;
 
+use crate::util::{self, BnBState};
+
 #[derive(Debug, Clone)]
 pub struct Blueprint {
     pub ore_ore_cost: usize,
@@ -77,29 +77,6 @@ impl State {
         }
     }
 
-    pub fn finished(&self) -> bool {
-        self.time_remaining == 0
-    }
-
-    pub fn possible_actions(&self, blueprint: &Blueprint) -> Vec<State> {
-        assert!(!self.finished());
-
-        let mut result = Vec::new();
-
-        if self.time_remaining > 1 {
-            self.produce_ore_next(blueprint).map(|a| result.push(a));
-            self.produce_clay_next(blueprint).map(|a| result.push(a));
-            self.produce_obsidian_next(blueprint)
-                .map(|a| result.push(a));
-            self.produce_geode_next(blueprint).map(|a| result.push(a));
-        }
-        let mut do_nothing = self.clone();
-        do_nothing.run_steps(self.time_remaining);
-        result.push(do_nothing);
-
-        result
-    }
-
     fn produce<F: Fn(&mut State)>(&self, time_for_ore_prod: isize, produce: F) -> Option<State> {
         let time_until_robot_ready = (time_for_ore_prod.max(0) as usize) + 1;
         // For this to make sense, we also need at least one minute
@@ -180,16 +157,6 @@ impl State {
         })
     }
 
-    pub fn upper_bound(&self) -> usize {
-        // Build one geode robot each remaining turn
-        // \sum_{k=1}^n {k - 1} = \sum_{k=0}^{n-1} {k} = 1/2 (n-1) n
-        self.lower_bound() + ((self.time_remaining - 1) * self.time_remaining) / 2
-    }
-
-    pub fn lower_bound(&self) -> usize {
-        self.geodes + self.geode_robots * self.time_remaining
-    }
-
     fn run_steps(&mut self, n: usize) {
         assert!(self.time_remaining >= n);
 
@@ -201,38 +168,43 @@ impl State {
     }
 }
 
-pub fn max_geodes(minutes: usize, blueprint: &Blueprint) -> usize {
-    let initial = State::new(minutes);
-    let initial_upper = initial.upper_bound();
-
-    let mut next = Vec::new();
-    next.push((initial.clone(), initial_upper));
-    let mut visited = HashSet::new();
-    visited.insert(initial);
-
-    let mut lower_bound = 0;
-    while let Some(n) = next.pop() {
-        if n.1 < lower_bound {
-            // Between pushing this state and popping it, we've found a better lower bound
-            continue;
-        }
-
-        let state = n.0;
-        for action in state.possible_actions(blueprint) {
-            if action.finished() {
-                let action_lower = action.lower_bound();
-                if action_lower > lower_bound {
-                    lower_bound = action_lower;
-                }
-            } else {
-                let action_upper = action.upper_bound();
-                if action_upper > lower_bound && !visited.contains(&action) {
-                    next.push((action.clone(), action_upper));
-                    visited.insert(action);
-                }
-            }
-        }
+impl util::BnBState<Blueprint> for State {
+    fn finished(&self) -> bool {
+        self.time_remaining == 0
     }
 
-    lower_bound
+    fn possible_actions(&self, blueprint: &Blueprint) -> Vec<State> {
+        assert!(!self.finished());
+
+        let mut result = Vec::new();
+
+        if self.time_remaining > 1 {
+            self.produce_ore_next(blueprint).map(|a| result.push(a));
+            self.produce_clay_next(blueprint).map(|a| result.push(a));
+            self.produce_obsidian_next(blueprint)
+                .map(|a| result.push(a));
+            self.produce_geode_next(blueprint).map(|a| result.push(a));
+        }
+        let mut do_nothing = self.clone();
+        do_nothing.run_steps(self.time_remaining);
+        result.push(do_nothing);
+
+        result
+    }
+
+    fn upper_bound(&self, b: &Blueprint) -> usize {
+        // Build one geode robot each remaining turn
+        // \sum_{k=1}^n {k - 1} = \sum_{k=0}^{n-1} {k} = 1/2 (n-1) n
+        self.lower_bound(b) + ((self.time_remaining - 1) * self.time_remaining) / 2
+    }
+
+    fn lower_bound(&self, _: &Blueprint) -> usize {
+        self.geodes + self.geode_robots * self.time_remaining
+    }
+}
+
+pub fn max_geodes(minutes: usize, blueprint: &Blueprint) -> usize {
+    let initial = State::new(minutes);
+
+    util::maximize(&initial, blueprint).lower_bound(blueprint)
 }

src/util.rs

@@ -1,5 +1,7 @@
+use std::collections::HashSet;
 use std::env;
 use std::fs;
+use std::hash::Hash;
 
 pub fn parse_input() -> String {
     let args: Vec<String> = env::args().collect();
@@ -73,3 +75,50 @@ impl Coordinate for SignedCoord {
         (self.0 as usize, self.1 as usize)
     }
 }
+
+pub trait BnBState<T> {
+    fn finished(&self) -> bool;
+    fn lower_bound(&self, extra: &T) -> usize;
+    fn upper_bound(&self, extra: &T) -> usize;
+    fn possible_actions(&self, extra: &T) -> Vec<Self>
+    where
+        Self: Sized;
+}
+
+pub fn maximize<E, S>(initial_state: &S, extra: &E) -> S
+where
+    S: BnBState<E> + Clone + Hash + Eq,
+{
+    let mut lower_bound = initial_state.lower_bound(extra);
+    let mut best = initial_state.clone();
+
+    let mut next = vec![(initial_state.clone(), initial_state.upper_bound(extra))];
+    let mut visited = HashSet::new();
+    visited.insert(initial_state.clone());
+
+    while let Some(n) = next.pop() {
+        if n.1 < lower_bound {
+            // Between pushing this state and popping it, we've found a better solution
+            continue;
+        }
+
+        let state = n.0;
+        for action in state.possible_actions(extra) {
+            if action.finished() {
+                let action_lower = action.lower_bound(extra);
+                if action_lower > lower_bound {
+                    lower_bound = action_lower;
+                    best = action;
+                }
+            } else {
+                let action_upper = action.upper_bound(extra);
+                if action_upper > lower_bound && !visited.contains(&action) {
+                    next.push((action.clone(), action_upper));
+                    visited.insert(action);
+                }
+            }
+        }
+    }
+
+    best
+}