Tell native mods when the server's difficulty changes

MinecraftServer.setDifficulty is the one door /difficulty, a dedicated
server's management and a test environment's difficulty all go through, and
nothing told a mod about it. native_difficulty_changed(from, to) now runs
there, after the change, with the two difficulty names, and only when the
difficulty is another one.

crates/horizon-client/src/game/commands.rs

@@ -2958,6 +2958,7 @@ impl Game<'_> {
     }
     /// `MinecraftServer.setDifficulty(difficulty, true)`.
     pub(super) fn set_difficulty_forced(&mut self, i: u8) {
+        let from = self.ui.difficulty;
         self.ui.difficulty = i;
         // The level data holds the world's difficulty: it survives the
         // reopen, so it follows the command.
@@ -2976,6 +2977,10 @@ impl Game<'_> {
                     locked: self.level.difficulty_locked,
                 }));
         }
+        if from != i {
+            let (from, to) = difficulty_names(from, i);
+            self.native_difficulty_changed(from, to);
+        }
     }
     fn cmd_spawnpoint(&mut self, ctx: &Context) -> R<i32> {
         let targets = if ctx.get("targets").is_some() {
@@ -3702,6 +3707,30 @@ pub(super) fn block_input_matches(
             .all(|(k, v)| s.properties.get(k) == Some(v))
 }
 
+/// The names `native_difficulty_changed` hears for a change from `from` to
+/// `to`: the four difficulties of `options.difficulty.*`, the index kept in
+/// range as the command and the options keep it.
+fn difficulty_names(from: u8, to: u8) -> (&'static str, &'static str) {
+    (
+        DIFFICULTIES[usize::from(from) & 3],
+        DIFFICULTIES[usize::from(to) & 3],
+    )
+}
+
+#[cfg(test)]
+mod difficulty_tests {
+    use super::difficulty_names;
+
+    #[test]
+    fn a_difficulty_change_names_both_ends() {
+        assert_eq!(difficulty_names(0, 3), ("peaceful", "hard"));
+        assert_eq!(difficulty_names(2, 1), ("normal", "easy"));
+        // The four difficulties are the only indexes the command and the
+        // options keep; an index outside them still names one.
+        assert_eq!(difficulty_names(7, 4), ("hard", "peaceful"));
+    }
+}
+
 #[cfg(test)]
 mod summon_tests {
     #[test]

crates/horizon-client/src/game/native_hooks.rs

@@ -3115,6 +3115,16 @@ impl Game<'_> {
     pub(super) fn native_level_unload(&mut self, dimension: &str) {
         let _ = dimension;
     }
+    /// The server's difficulty changed, from `from` to `to` (`peaceful`,
+    /// `easy`, `normal`, `hard`; NeoForge's `DifficultyChangeEvent`). It is
+    /// vanilla's `MinecraftServer.setDifficulty`, which `/difficulty`, a
+    /// dedicated server's management and a test environment's difficulty all
+    /// go through, and it is told only when the difficulty is another one. It
+    /// only hears: the change has already happened. Server-side. API
+    /// @API:difficulty_changed@.
+    pub(super) fn native_difficulty_changed(&mut self, from: &str, to: &str) {
+        let _ = (from, to);
+    }
     /// Chunk `chunk` (`[x, z]`, in chunks) of `dimension` came into the world
     /// (NeoForge's `ChunkEvent.Load`): `new` when it was just generated, false
     /// when read from the save. A flat world's chunk that leaves before it's

tools/native-hook-fragments/difficulty_changed.json

@@ -0,0 +1,13 @@
+{
+  "hooks": [
+    {
+      "name": "difficulty_changed",
+      "target": "horizon_client::game::native_hooks::Game<'_>::native_difficulty_changed",
+      "neoforge": "DifficultyChangeEvent",
+      "side": "server"
+    }
+  ],
+  "rust_summary": "the server's difficulty changing (`native_difficulty_changed`).",
+  "doc_summary": "the server's difficulty changed, from one of `peaceful`, `easy`, `normal`, `hard` to another, through `native_difficulty_changed` (NeoForge's `DifficultyChangeEvent`). It is vanilla's `MinecraftServer.setDifficulty`, which `/difficulty`, a dedicated server's management and a test environment's difficulty all go through, and it is told only when the difficulty is another one. It only hears: the change has already happened. Bundles that use it need \"hooks\": @API:difficulty_changed@.",
+  "comments": []
+}