Add config file for dungeon distribution

- Now you can specify probability of "spawn" of dungeon with specific
difficulty

assets/world/dungeon/difficulty_distribution.ron

@@ -0,0 +1,20 @@
+/// Distribution of different dungeon levels.
+///
+/// first number is dungeon level, integer
+/// second number is probability, any normal positive float (not a NaN, for example)
+///
+/// Values are relative to each other,
+/// lesser probability means there will be less dungeons.
+///
+/// General rules:
+/// 1) Probability should not be less or equal to zero
+/// 2) Keep it synced with number of dungeon levels
+/// 3) Keep these pairs sorted from lowest to highest tier
+([
+    (0, 1.0),
+    (1, 1.0),
+    (2, 1.0),
+    (3, 1.0),
+    (4, 1.0),
+    (5, 1.0),
+])

world/src/site/dungeon/mod.rs

@@ -9,7 +9,7 @@ use crate::{
 };
 
 use common::{
-    assets::AssetHandle,
+    assets::{self, AssetExt, AssetHandle},
     astar::Astar,
     comp::{self},
     generation::{ChunkSupplement, EntityInfo},
@@ -20,7 +20,7 @@ use common::{
 use core::{f32, hash::BuildHasherDefault};
 use fxhash::FxHasher64;
 use lazy_static::lazy_static;
-use rand::prelude::*;
+use rand::{prelude::*, seq::SliceRandom};
 use serde::Deserialize;
 use vek::*;
 
@@ -47,13 +47,34 @@ pub struct Colors {
 
 const ALT_OFFSET: i32 = -2;
 
+#[derive(Deserialize)]
+struct DungeonDistribution(Vec<(u32, f32)>);
+impl assets::Asset for DungeonDistribution {
+    type Loader = assets::RonLoader;
+
+    const EXTENSION: &'static str = "ron";
+}
+
+lazy_static! {
+    static ref DUNGEON_DISTRIBUTION: Vec<(u32, f32)> =
+        DungeonDistribution::load_expect("world.dungeon.difficulty_distribution")
+            .read()
+            .0
+            .clone();
+}
+
 impl Dungeon {
-    #[allow(clippy::let_and_return)] // TODO: Pending review in #587
     pub fn generate(wpos: Vec2<i32>, sim: Option<&WorldSim>, rng: &mut impl Rng) -> Self {
         let mut ctx = GenCtx { sim, rng };
-        let difficulty = ctx.rng.gen_range(0..6);
+
+        let difficulty = DUNGEON_DISTRIBUTION
+            .choose_weighted(&mut ctx.rng, |pair| pair.1)
+            .map(|(difficulty, _)| *difficulty)
+            .expect("this can never fail");
+
         let floors = 3 + difficulty / 2;
-        let this = Self {
+
+        Self {
             name: {
                 let name = NameGen::location(ctx.rng).generate();
                 match ctx.rng.gen_range(0..5) {
@@ -81,9 +102,7 @@ impl Dungeon {
                 })
                 .collect(),
             difficulty,
-        };
+        }
-
-        this
     }
 
     pub fn name(&self) -> &str { &self.name }