Implement player inventory storage and shared equipment selection

benchmarks/rust-player-inventory-2026-10-07/scoped-parity.json

@@ -0,0 +1,273 @@
+{
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+  "minecraft": "26.3",
+  "manifest_sha1": "4fe1aa1ef8da1cb95c5bad1fb98890ca56dd8ca3",
+  "platform": "Linux-7.0.0-31-generic-x86_64-with-glibc2.43",
+  "java": "openjdk 25.0.4.1 2026-08-18",
+  "rustc": "rustc 1.99.0 (b940084d7 2026-09-28)",
+  "fixtures": 3960,
+  "sources_sha256": {
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+    "crates/horizon-world/examples/player_inventory_fixture.rs": "5e8f93f623b225ad65721058b61269763b8d49179166fe8dcb45b83e5de794fd"
+  },
+  "fixtures_sha256": {
+    ".local/rust-player-inventory-parity/inputs.txt": "01e7711fd5f12f3b3c4e47b263f5a98751a146aec217255ef0182b5e76ba1cc3"
+  },
+  "rust_output_sha256": "003e761e441fad8b82b169f8ac04d228237236fb492e74690d85ded629278297",
+  "references": [
+    {
+      "side": "client",
+      "jar_sha256": "4508d006323f24fa02876310c192d739af56516eb259000ac50f0909a68c9a2d",
+      "output_sha256": "003e761e441fad8b82b169f8ac04d228237236fb492e74690d85ded629278297",
+      "matched_output_lines": 790440
+    },
+    {
+      "side": "server",
+      "jar_sha256": "a362163eec5d1612d520772bc16e5b39c09e3b234fdc045f56bf544284ee8ae6",
+      "output_sha256": "003e761e441fad8b82b169f8ac04d228237236fb492e74690d85ded629278297",
+      "matched_output_lines": 790440
+    }
+  ],
+  "scope": "Actual player Inventory storage, equipment, selected slots, hotbar picking, matching/crafting eligibility, reference removals, clear/replacement, counts and change counter. Actual ItemStack components cover damage, unbreakable, one enchantment and custom names; fixture-controlled equipment-map iteration order isolates native JVM Map.of random ordering. Shared stack and equipment references are traced across two inventories, including self-replacement and failures after selection changes. Pickup, item/equipment ticks, menus, recipes, persistence, arbitrary virtual overrides, full components and complete gameplay integration remain pending."
+}

crates/horizon-world/examples/player_inventory_fixture.rs

@@ -0,0 +1,303 @@
+use horizon_world::{
+    component_patch::{Patch, Prototype},
+    item_stack_refs::{ItemStacks, StackRef},
+    player_inventory::{EntityEquipment, EquipmentSlot, InventoryRules, PlayerInventory},
+};
+use serde_json::Value;
+use std::{
+    fs,
+    io::{self, Write},
+    path::Path,
+};
+type Stack = StackRef<u8, usize, i32>;
+type Inventory = PlayerInventory<u8, usize, i32>;
+struct Rules;
+impl InventoryRules<u8, usize, i32> for Rules {
+    fn is_enchanted(&self, s: &Stack) -> bool {
+        s.read().component(&4).is_some_and(|n| *n != 0)
+    }
+    fn is_damaged(&self, s: &Stack) -> bool {
+        let s = s.read();
+        s.component(&2).is_some()
+            && s.component(&3).is_none()
+            && s.component(&1).is_some()
+            && s.component(&1)
+                .copied()
+                .unwrap_or(0)
+                .max(0)
+                .min(s.component(&2).copied().unwrap_or(0))
+                > 0
+    }
+    fn has_custom_name(&self, s: &Stack) -> bool {
+        s.read().component(&5).is_some()
+    }
+}
+fn n(v: &Value, k: &str) -> i32 {
+    v[k].as_i64().unwrap() as i32
+}
+fn opt(v: Option<i32>) -> String {
+    v.map_or("none".into(), |n| n.to_string())
+}
+struct Output {
+    out: io::BufWriter<fs::File>,
+    ids: Vec<Stack>,
+}
+impl Output {
+    fn snapshot(&mut self, s: &Stack) -> String {
+        let id = self
+            .ids
+            .iter()
+            .position(|v| v.same_reference(s))
+            .unwrap_or_else(|| {
+                self.ids.push(s.clone());
+                self.ids.len() - 1
+            });
+        let damaged = Rules.is_damaged(s);
+        let enchanted = Rules.is_enchanted(s);
+        let s = s.read();
+        format!(
+            "{}:{}:{}:{}:{}:{}:{}:{}:{}:{}",
+            id,
+            s.item(),
+            s.count,
+            s.count(),
+            s.pop_time,
+            s.is_empty(),
+            damaged,
+            enchanted,
+            (0..6)
+                .map(|k| opt(s.component(&k).copied()))
+                .collect::<Vec<_>>()
+                .join(","),
+            s.components
+                .as_patch()
+                .entries()
+                .iter()
+                .map(|(k, v)| format!("{k}={}", opt(*v)))
+                .collect::<Vec<_>>()
+                .join(",")
+        )
+    }
+    fn all(&mut self, refs: &[Stack], inv: &[Inventory; 2], empty: &Stack) {
+        for (i, s) in refs.iter().enumerate() {
+            let s = self.snapshot(s);
+            writeln!(self.out, "ref {i} {s}").unwrap();
+        }
+        for (i, c) in inv.iter().enumerate() {
+            writeln!(
+                self.out,
+                "inventory {} {} {} {} {} {} {} {} {}",
+                i,
+                c.selected(),
+                c.times_changed,
+                c.free_slot(),
+                c.suitable_hotbar(&Rules),
+                c.is_empty(),
+                c.count_item(0),
+                c.count_item(1),
+                c.count_item(2)
+            )
+            .unwrap();
+            let slots = (0..43)
+                .map(|j| self.snapshot(&c.get(j).unwrap()))
+                .collect::<Vec<_>>()
+                .join(" | ");
+            writeln!(self.out, "slots {slots}").unwrap();
+            let eq = c
+                .equipment()
+                .entries()
+                .into_iter()
+                .map(|(s, v)| {
+                    format!(
+                        "{}={}",
+                        match s {
+                            EquipmentSlot::MainHand => "MAINHAND",
+                            EquipmentSlot::OffHand => "OFFHAND",
+                            EquipmentSlot::Feet => "FEET",
+                            EquipmentSlot::Legs => "LEGS",
+                            EquipmentSlot::Chest => "CHEST",
+                            EquipmentSlot::Head => "HEAD",
+                            EquipmentSlot::Body => "BODY",
+                            EquipmentSlot::Saddle => "SADDLE",
+                        },
+                        self.snapshot(&v)
+                    )
+                })
+                .collect::<Vec<_>>()
+                .join(" | ");
+            writeln!(self.out, "equipment {eq}").unwrap();
+            for (j, s) in refs.iter().enumerate() {
+                writeln!(
+                    self.out,
+                    "query {} {} {} {}",
+                    j,
+                    c.contains(s),
+                    c.find_matching(s),
+                    c.find_crafting(1, s, &Rules)
+                )
+                .unwrap();
+            }
+        }
+        let e = self.snapshot(empty);
+        writeln!(self.out, "empty {e}").unwrap();
+    }
+}
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let arg = std::env::args().nth(1).unwrap();
+    let dir = Path::new(&arg);
+    let mut out = io::BufWriter::new(fs::File::create(dir.join("rust.txt"))?);
+    for line in fs::read_to_string(dir.join("inputs.txt"))?.lines() {
+        let f: Value = serde_json::from_str(line)?;
+        let stacks = ItemStacks::new(0);
+        let empty = stacks.empty();
+        let prototypes: Vec<_> = f["prototypes"]
+            .as_array()
+            .unwrap()
+            .iter()
+            .map(|p| {
+                Prototype::new(p.as_array().unwrap().iter().map(|v| {
+                    (
+                        v[0].as_u64().unwrap() as usize,
+                        v[1].as_i64().unwrap() as i32,
+                    )
+                }))
+            })
+            .collect();
+        let mut refs: Vec<_> = f["stacks"]
+            .as_array()
+            .unwrap()
+            .iter()
+            .map(|v| {
+                let item = n(v, "item");
+                if item < 0 {
+                    empty.clone()
+                } else {
+                    let s =
+                        stacks.create(item as u8, n(v, "count"), prototypes[item as usize].clone());
+                    s.write().pop_time = n(v, "pop");
+                    s.write().components.restore_patch(&Patch::new(
+                        v["patch"].as_array().unwrap().iter().map(|v| {
+                            (
+                                v[0].as_u64().unwrap() as usize,
+                                v[1].as_i64().map(|n| n as i32),
+                            )
+                        }),
+                    ));
+                    s
+                }
+            })
+            .collect();
+        let equipment = EntityEquipment::new(empty.clone());
+        let other = if f["shared_equipment"].as_bool().unwrap() {
+            equipment.clone()
+        } else {
+            EntityEquipment::new(empty.clone())
+        };
+        let mut inv = [
+            Inventory::new(stacks.clone(), equipment),
+            Inventory::new(stacks.clone(), other),
+        ];
+        let order: [EquipmentSlot; 7] = std::array::from_fn(|i| {
+            EquipmentSlot::from_inventory_slot(
+                36 + f["equipment_order"][i].as_i64().unwrap() as i32,
+            )
+            .unwrap()
+        });
+        for (i, c) in inv.iter_mut().enumerate() {
+            let initial = &f["inventories"][i];
+            c.set_selected(n(initial, "selected")).unwrap();
+            c.times_changed = n(initial, "changed");
+            for p in initial["slots"].as_array().unwrap() {
+                c.set(
+                    p[0].as_i64().unwrap() as i32,
+                    refs[p[1].as_u64().unwrap() as usize].clone(),
+                )
+                .unwrap();
+            }
+            if n(initial, "mainhand") >= 0 {
+                c.equipment().set(
+                    EquipmentSlot::MainHand,
+                    refs[n(initial, "mainhand") as usize].clone(),
+                );
+            }
+        }
+        let mut w = Output {
+            out,
+            ids: vec![empty.clone()],
+        };
+        w.all(&refs, &inv, &empty);
+        for op in f["ops"].as_array().unwrap() {
+            let target = n(op, "inventory") as usize;
+            let a = n(op, "a") as usize;
+            let b = n(op, "b") as usize;
+            let slot = n(op, "slot");
+            let amount = n(op, "amount");
+            let result = (|| -> Result<(), ()> {
+                match op["kind"].as_str().unwrap() {
+                    "get" => refs[b] = inv[target].get(slot).map_err(|_| ())?,
+                    "set" => inv[target].set(slot, refs[a].clone()).map_err(|_| ())?,
+                    "remove" => refs[b] = inv[target].remove(slot, amount),
+                    "no-update" => refs[b] = inv[target].remove_no_update(slot).map_err(|_| ())?,
+                    "remove-ref" => inv[target].remove_reference(&refs[a], &order),
+                    "select" => inv[target].set_selected(slot).map_err(|_| ())?,
+                    "selected" => refs[b] = inv[target].selected_item(),
+                    "set-selected" => refs[b] = inv[target].set_selected_item(refs[a].clone()),
+                    "remove-selected" => {
+                        refs[b] = inv[target].remove_selected(op["all"].as_bool().unwrap())
+                    }
+                    "pick" => inv[target].pick_slot(slot, &Rules).map_err(|_| ())?,
+                    "add-pick" => inv[target].add_and_pick(refs[a].clone(), &Rules),
+                    "clear" => inv[target].clear(),
+                    "changed" => inv[target].changed(),
+                    "replace" => {
+                        let other = n(op, "other") as usize;
+                        if other == target {
+                            inv[target].replace_self();
+                        } else {
+                            let (first, second) = inv.split_at_mut(1);
+                            if target == 0 {
+                                first[0].replace_with(&second[0]);
+                            } else {
+                                second[0].replace_with(&first[0]);
+                            }
+                        }
+                    }
+                    "equip" => {
+                        refs[b] = inv[target].equipment().set(
+                            EquipmentSlot::ALL[n(op, "equipment") as usize],
+                            refs[a].clone(),
+                        )
+                    }
+                    "get-equip" => {
+                        refs[b] = inv[target]
+                            .equipment()
+                            .get(EquipmentSlot::ALL[n(op, "equipment") as usize])
+                    }
+                    "alias" => refs[b] = refs[a].clone(),
+                    "copy" => refs[b] = stacks.copy(&refs[a]),
+                    "count" => refs[a].write().count = amount,
+                    "component" => {
+                        refs[a]
+                            .write()
+                            .components
+                            .set(
+                                n(op, "key") as usize,
+                                op["value"].as_i64().map(|n| n as i32),
+                            )
+                            .map_err(|_| ())?;
+                    }
+                    _ => panic!("operation"),
+                }
+                Ok(())
+            })();
+            writeln!(
+                w.out,
+                "result {}",
+                if result.is_ok() { "ok" } else { "error" }
+            )?;
+            w.all(&refs, &inv, &empty);
+        }
+        out = w.out;
+    }
+    Ok(())
+}

crates/horizon-world/src/lib.rs

@@ -96,3 +96,5 @@ pub const DATA_VERSION: i32 = 5023;
 
 pub mod item_stack_refs;
 pub mod simple_container;
+
+pub mod player_inventory;

crates/horizon-world/src/player_inventory.rs

@@ -0,0 +1,336 @@
+//! Player inventory storage and selection. Equipment is shared with its entity;
+//! stack references preserve aliases across inventories and external callers.
+//! Pickup, ticking, menus and persistence are implemented separately.
+use crate::item_stack_refs::{ItemStacks, StackRef};
+use std::{cell::RefCell, rc::Rc};
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[repr(usize)]
+pub enum EquipmentSlot {
+    MainHand,
+    OffHand,
+    Feet,
+    Legs,
+    Chest,
+    Head,
+    Body,
+    Saddle,
+}
+impl EquipmentSlot {
+    pub const ALL: [Self; 8] = [
+        Self::MainHand,
+        Self::OffHand,
+        Self::Feet,
+        Self::Legs,
+        Self::Chest,
+        Self::Head,
+        Self::Body,
+        Self::Saddle,
+    ];
+    pub fn inventory_slot(self) -> Option<usize> {
+        match self {
+            Self::MainHand => None,
+            Self::OffHand => Some(40),
+            Self::Feet => Some(36),
+            Self::Legs => Some(37),
+            Self::Chest => Some(38),
+            Self::Head => Some(39),
+            Self::Body => Some(41),
+            Self::Saddle => Some(42),
+        }
+    }
+    pub fn from_inventory_slot(index: i32) -> Option<Self> {
+        match index {
+            36 => Some(Self::Feet),
+            37 => Some(Self::Legs),
+            38 => Some(Self::Chest),
+            39 => Some(Self::Head),
+            40 => Some(Self::OffHand),
+            41 => Some(Self::Body),
+            42 => Some(Self::Saddle),
+            _ => None,
+        }
+    }
+}
+type EquipmentItems<I, K, V> = [Option<StackRef<I, K, V>>; 8];
+#[derive(Debug)]
+pub struct EntityEquipment<I, K, V> {
+    items: Rc<RefCell<EquipmentItems<I, K, V>>>,
+    empty: StackRef<I, K, V>,
+}
+impl<I, K, V> Clone for EntityEquipment<I, K, V> {
+    fn clone(&self) -> Self {
+        Self {
+            items: self.items.clone(),
+            empty: self.empty.clone(),
+        }
+    }
+}
+impl<I, K, V> EntityEquipment<I, K, V> {
+    pub fn new(empty: StackRef<I, K, V>) -> Self {
+        Self {
+            items: Rc::new(RefCell::new(std::array::from_fn(|_| None))),
+            empty,
+        }
+    }
+    pub fn get(&self, slot: EquipmentSlot) -> StackRef<I, K, V> {
+        self.items.borrow()[slot as usize]
+            .clone()
+            .unwrap_or_else(|| self.empty.clone())
+    }
+    pub fn set(&self, slot: EquipmentSlot, stack: StackRef<I, K, V>) -> StackRef<I, K, V> {
+        self.items.borrow_mut()[slot as usize]
+            .replace(stack)
+            .unwrap_or_else(|| self.empty.clone())
+    }
+    /// Native clear preserves which enum-map entries have been populated.
+    pub fn clear(&self) {
+        for s in self.items.borrow_mut().iter_mut().flatten() {
+            *s = self.empty.clone();
+        }
+    }
+    pub fn entries(&self) -> Vec<(EquipmentSlot, StackRef<I, K, V>)> {
+        let items = self.items.borrow();
+        EquipmentSlot::ALL
+            .into_iter()
+            .filter_map(|s| items[s as usize].clone().map(|v| (s, v)))
+            .collect()
+    }
+}
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum InventoryError {
+    Index,
+    Selection,
+}
+pub trait InventoryRules<I, K, V> {
+    fn is_enchanted(&self, stack: &StackRef<I, K, V>) -> bool;
+    fn is_damaged(&self, stack: &StackRef<I, K, V>) -> bool;
+    fn has_custom_name(&self, stack: &StackRef<I, K, V>) -> bool;
+}
+#[derive(Debug)]
+pub struct PlayerInventory<I, K, V> {
+    items: [StackRef<I, K, V>; 36],
+    equipment: EntityEquipment<I, K, V>,
+    stacks: ItemStacks<I, K, V>,
+    selected: usize,
+    pub times_changed: i32,
+}
+impl<I: Copy + Eq, K: Clone + Eq, V: Clone + PartialEq> PlayerInventory<I, K, V> {
+    pub fn new(stacks: ItemStacks<I, K, V>, equipment: EntityEquipment<I, K, V>) -> Self {
+        Self {
+            items: std::array::from_fn(|_| stacks.empty()),
+            equipment,
+            stacks,
+            selected: 0,
+            times_changed: 0,
+        }
+    }
+    pub fn items(&self) -> &[StackRef<I, K, V>; 36] {
+        &self.items
+    }
+    pub fn equipment(&self) -> &EntityEquipment<I, K, V> {
+        &self.equipment
+    }
+    pub fn selected(&self) -> usize {
+        self.selected
+    }
+    pub fn set_selected(&mut self, selected: i32) -> Result<(), InventoryError> {
+        if !(0..9).contains(&selected) {
+            return Err(InventoryError::Selection);
+        }
+        self.selected = selected as usize;
+        Ok(())
+    }
+    pub fn selected_item(&self) -> StackRef<I, K, V> {
+        self.items[self.selected].clone()
+    }
+    pub fn set_selected_item(&mut self, stack: StackRef<I, K, V>) -> StackRef<I, K, V> {
+        std::mem::replace(&mut self.items[self.selected], stack)
+    }
+    pub fn get(&self, index: i32) -> Result<StackRef<I, K, V>, InventoryError> {
+        if index < 0 {
+            return Err(InventoryError::Index);
+        }
+        Ok(if index < 36 {
+            self.items[index as usize].clone()
+        } else {
+            EquipmentSlot::from_inventory_slot(index)
+                .map_or_else(|| self.stacks.empty(), |s| self.equipment.get(s))
+        })
+    }
+    pub fn set(&mut self, index: i32, stack: StackRef<I, K, V>) -> Result<(), InventoryError> {
+        if index < 0 {
+            return Err(InventoryError::Index);
+        }
+        if index < 36 {
+            self.items[index as usize] = stack;
+        } else if let Some(s) = EquipmentSlot::from_inventory_slot(index) {
+            self.equipment.set(s, stack);
+        }
+        Ok(())
+    }
+    pub fn remove(&mut self, index: i32, amount: i32) -> StackRef<I, K, V> {
+        if index < 0 || (index < 36 && amount <= 0) {
+            return self.stacks.empty();
+        }
+        let s = self.get(index).expect("nonnegative index");
+        if s.read().is_empty() {
+            self.stacks.empty()
+        } else {
+            self.stacks.split(&s, amount)
+        }
+    }
+    pub fn remove_no_update(&mut self, index: i32) -> Result<StackRef<I, K, V>, InventoryError> {
+        if index < 0 {
+            return Err(InventoryError::Index);
+        }
+        Ok(if index < 36 {
+            std::mem::replace(&mut self.items[index as usize], self.stacks.empty())
+        } else {
+            EquipmentSlot::from_inventory_slot(index).map_or_else(
+                || self.stacks.empty(),
+                |s| self.equipment.set(s, self.stacks.empty()),
+            )
+        })
+    }
+    /// The native Map.of equipment iteration order varies by JVM. The caller
+    /// supplies its pinned runtime order when reference aliases span equipment.
+    pub fn remove_reference(
+        &mut self,
+        stack: &StackRef<I, K, V>,
+        equipment_order: &[EquipmentSlot; 7],
+    ) {
+        for s in &mut self.items {
+            if s.same_reference(stack) {
+                *s = self.stacks.empty();
+                return;
+            }
+        }
+        for slot in equipment_order {
+            if self.equipment.get(*slot).same_reference(stack) {
+                self.equipment.set(*slot, self.stacks.empty());
+                return;
+            }
+        }
+    }
+    pub fn free_slot(&self) -> i32 {
+        self.items
+            .iter()
+            .position(|s| s.read().is_empty())
+            .map_or(-1, |i| i as i32)
+    }
+    pub fn find_matching(&self, stack: &StackRef<I, K, V>) -> i32 {
+        self.items
+            .iter()
+            .position(|s| !s.read().is_empty() && stack.read().same_item_components(&s.read()))
+            .map_or(-1, |i| i as i32)
+    }
+    pub fn contains(&self, stack: &StackRef<I, K, V>) -> bool {
+        (0..43).any(|i| {
+            let s = self.get(i).expect("valid slot");
+            !s.read().is_empty() && s.read().same_item_components(&stack.read())
+        })
+    }
+    pub fn count_item(&self, item: I) -> i32 {
+        (0..43).fold(0_i32, |n, i| {
+            let s = self.get(i).expect("valid slot");
+            let s = s.read();
+            if s.item() == item {
+                n.wrapping_add(s.count())
+            } else {
+                n
+            }
+        })
+    }
+    pub fn suitable_hotbar<R: InventoryRules<I, K, V>>(&self, rules: &R) -> usize {
+        for delta in 0..9 {
+            let i = (self.selected + delta) % 9;
+            if self.items[i].read().is_empty() {
+                return i;
+            }
+        }
+        for delta in 0..9 {
+            let i = (self.selected + delta) % 9;
+            if !rules.is_enchanted(&self.items[i]) {
+                return i;
+            }
+        }
+        self.selected
+    }
+    pub fn find_crafting<R: InventoryRules<I, K, V>>(
+        &self,
+        item: I,
+        stack: &StackRef<I, K, V>,
+        rules: &R,
+    ) -> i32 {
+        self.items
+            .iter()
+            .position(|s| {
+                !s.read().is_empty()
+                    && s.read().item() == item
+                    && !rules.is_damaged(s)
+                    && !rules.is_enchanted(s)
+                    && !rules.has_custom_name(s)
+                    && (stack.read().is_empty() || stack.read().same_item_components(&s.read()))
+            })
+            .map_or(-1, |i| i as i32)
+    }
+    pub fn add_and_pick<R: InventoryRules<I, K, V>>(
+        &mut self,
+        stack: StackRef<I, K, V>,
+        rules: &R,
+    ) {
+        self.selected = self.suitable_hotbar(rules);
+        if !self.items[self.selected].read().is_empty() {
+            let free = self.free_slot();
+            if free >= 0 {
+                self.items[free as usize] = self.items[self.selected].clone();
+            }
+        }
+        self.items[self.selected] = stack;
+    }
+    pub fn pick_slot<R: InventoryRules<I, K, V>>(
+        &mut self,
+        index: i32,
+        rules: &R,
+    ) -> Result<(), InventoryError> {
+        self.selected = self.suitable_hotbar(rules);
+        // Selection changes even when the following list access fails.
+        let index = usize::try_from(index)
+            .ok()
+            .filter(|i| *i < 36)
+            .ok_or(InventoryError::Index)?;
+        self.items.swap(self.selected, index);
+        Ok(())
+    }
+    pub fn remove_selected(&mut self, all: bool) -> StackRef<I, K, V> {
+        let s = self.selected_item();
+        if s.read().is_empty() {
+            return self.stacks.empty();
+        }
+        let amount = if all { s.read().count() } else { 1 };
+        self.remove(self.selected as i32, amount)
+    }
+    pub fn changed(&mut self) {
+        self.times_changed = self.times_changed.wrapping_add(1);
+    }
+    pub fn clear(&mut self) {
+        self.items.fill(self.stacks.empty());
+        self.equipment.clear();
+    }
+    pub fn is_empty(&self) -> bool {
+        (0..43).all(|i| self.get(i).expect("valid slot").read().is_empty())
+    }
+    pub fn replace_self(&mut self) {
+        for i in 0..43 {
+            self.set(i, self.get(i).expect("valid slot"))
+                .expect("valid slot");
+        }
+    }
+    pub fn replace_with(&mut self, other: &Self) {
+        for i in 0..43 {
+            self.set(i, other.get(i).expect("valid slot"))
+                .expect("valid slot");
+        }
+        self.selected = other.selected;
+    }
+}

docs/collision-geometry.md

@@ -1664,3 +1664,23 @@ the first empty slot clears the remainder before limiting the inserted copy.
 Targeted Clippy passes. Player inventory, menus, crafting accounting, persistence,
 arbitrary getItem/setItem overrides and complete gameplay integration remain
 pending. Reference counting here has not been benchmarked as a performance gain.
+
+
+`player_inventory` implements the 36 ordinary slots, seven mapped equipment
+slots, shared entity equipment, selected hotbar slot, picking, matching/crafting
+eligibility, removals, replacement, clearing and the change counter. Equipment
+retains populated entries when cleared. Stack and equipment references can be
+shared across inventories; replacing an inventory retains stack aliases.
+
+`python3 tools/test_player_inventory_parity.py` compares actual native Inventory,
+EntityEquipment and ItemStack methods. The
+[inventory report](../benchmarks/rust-player-inventory-2026-10-07/scoped-parity.json)
+records **790,440 matching lines per JAR**, including invalid-index failure
+prefixes, selected-slot changes, negative equipment removal amounts, ordinary
+versus equipment identity, self-replacement and shared equipment clearing.
+Components cover damage, unbreakable presence, a fixed enchantment and literal
+custom names. Native Map.of iteration order varies by JVM; fixtures explicitly
+supply equipment-map order to both sides when removing an aliased reference.
+Targeted Clippy passes. Pickup, inventory/equipment ticks, menus, recipe
+accounting, persistence, full component values and complete gameplay integration
+remain pending.

mods/horizon/src/test/java/dev/horizonlab/parity/VanillaPlayerInventoryOracle.java

@@ -0,0 +1,37 @@
+package dev.horizonlab.parity;
+import com.google.gson.*;import java.io.*;import java.lang.reflect.*;import java.nio.file.*;import java.util.*;
+import net.minecraft.core.component.*;import net.minecraft.world.item.*;import net.minecraft.world.entity.*;import net.minecraft.world.entity.player.Inventory;import net.minecraft.world.item.enchantment.*;
+/** Native Inventory storage/selection with real stack/component and equipment aliases. */
+@SuppressWarnings({"unchecked","rawtypes"})
+public final class VanillaPlayerInventoryOracle {
+ static Item[] items;static DataComponentType[] keys;static Field components,count,patchMap,equipmentItems,changed;static Object removed;static ItemEnchantments enchanted;
+ static EquipmentSlot[] mapped={EquipmentSlot.FEET,EquipmentSlot.LEGS,EquipmentSlot.CHEST,EquipmentSlot.HEAD,EquipmentSlot.OFFHAND,EquipmentSlot.BODY,EquipmentSlot.SADDLE};
+ static Field field(Class<?> c,String n)throws Exception{var f=c.getDeclaredField(n);f.setAccessible(true);return f;}
+ static int n(JsonObject o,String k){return o.get(k).getAsInt();}
+ static int key(Object k){for(int i=0;i<keys.length;i++)if(keys[i]==k)return i;throw new AssertionError();}
+ static int item(Item i){for(int k=0;k<items.length;k++)if(items[k]==i)return k;throw new AssertionError();}
+ static Object value(int k,JsonElement v){if(v.isJsonNull())return null;int n=v.getAsInt();return switch(k){case 3->net.minecraft.util.Unit.INSTANCE;case 4->n==0?ItemEnchantments.EMPTY:enchanted;case 5->net.minecraft.network.chat.Component.literal(Integer.toString(n));default->n;};}
+ static String text(int k,Object v){if(v==null||v==removed)return "none";return switch(k){case 3->"0";case 4->((ItemEnchantments)v).isEmpty()?"0":"1";case 5->((net.minecraft.network.chat.Component)v).getString();default->v.toString();};}
+ static DataComponentMap prototype(JsonArray a){var b=DataComponentMap.builder();for(var e:a){var p=e.getAsJsonArray();int k=p.get(0).getAsInt();b.set(keys[k],value(k,p.get(1)));}return b.build();}
+ static DataComponentPatch patch(JsonArray a){var b=DataComponentPatch.builder();for(var e:a){var p=e.getAsJsonArray();int k=p.get(0).getAsInt();if(p.get(1).isJsonNull())b.remove(keys[k]);else b.set(keys[k],value(k,p.get(1)));}return b.build();}
+ static final class Data {
+  PrintWriter out;ItemStack[] refs;Inventory[] inv;EntityEquipment[] equipment;IdentityHashMap<ItemStack,Integer> ids=new IdentityHashMap<>();
+  int id(ItemStack s){return ids.computeIfAbsent(s,k->ids.size());}
+  String snapshot(ItemStack s){try{var raw=(PatchedDataComponentMap)components.get(s);var p=new StringJoiner(",");for(var e:((Map<?,?>)patchMap.get(raw.asPatch())).entrySet())p.add(key(e.getKey())+"="+text(key(e.getKey()),e.getValue()));var v=new StringJoiner(",");for(int k=0;k<keys.length;k++)v.add(text(k,s.get(keys[k])));return id(s)+":"+item(s.getItem())+":"+count.getInt(s)+":"+s.getCount()+":"+s.getPopTime()+":"+s.isEmpty()+":"+s.isDamaged()+":"+s.isEnchanted()+":"+v+":"+p;}catch(Exception e){throw new AssertionError(e);}}
+  void all()throws Exception {
+   for(int i=0;i<refs.length;i++)out.println("ref "+i+" "+snapshot(refs[i]));
+   for(int i=0;i<2;i++){var c=inv[i];out.println("inventory "+i+" "+c.getSelectedSlot()+" "+c.getTimesChanged()+" "+c.getFreeSlot()+" "+c.getSuitableHotbarSlot()+" "+c.isEmpty()+" "+c.countItem(items[0])+" "+c.countItem(items[1])+" "+c.countItem(items[2]));var slots=new StringJoiner(" | ");for(int j=0;j<43;j++)slots.add(snapshot(c.getItem(j)));out.println("slots "+slots);var eq=new StringJoiner(" | ");for(var e:((EnumMap<EquipmentSlot,ItemStack>)equipmentItems.get(equipment[i])).entrySet())eq.add(e.getKey().name()+"="+snapshot(e.getValue()));out.println("equipment "+eq);for(int j=0;j<refs.length;j++)out.println("query "+j+" "+c.contains(refs[j])+" "+c.findSlotMatchingItem(refs[j])+" "+c.findSlotMatchingCraftingIngredient(items[1].builtInRegistryHolder(),refs[j]));}
+   out.println("empty "+snapshot(ItemStack.EMPTY));
+  }
+ }
+ public static void main(String[] args)throws Exception {
+  net.minecraft.SharedConstants.tryDetectVersion();net.minecraft.server.Bootstrap.bootStrap();var lookup=net.minecraft.data.registries.VanillaRegistries.createWorldLookup();var m=new ItemEnchantments.Mutable(ItemEnchantments.EMPTY);m.set(lookup.lookupOrThrow(net.minecraft.core.registries.Registries.ENCHANTMENT).getOrThrow(Enchantments.UNBREAKING),1);enchanted=m.toImmutable();
+  items=new Item[]{Items.AIR,Items.STICK,Items.SHIELD};keys=new DataComponentType[]{DataComponents.MAX_STACK_SIZE,DataComponents.DAMAGE,DataComponents.MAX_DAMAGE,DataComponents.UNBREAKABLE,DataComponents.ENCHANTMENTS,DataComponents.CUSTOM_NAME};components=field(ItemStack.class,"components");count=field(ItemStack.class,"count");patchMap=field(DataComponentPatch.class,"map");removed=field(Class.forName("net.minecraft.core.component.Removed"),"INSTANCE").get(null);equipmentItems=field(EntityEquipment.class,"items");changed=field(Inventory.class,"timesChanged");Path dir=Path.of(args[1]);
+  try(var out=new PrintWriter(Files.newBufferedWriter(dir.resolve(args[0]+".txt")));var lines=Files.lines(dir.resolve("inputs.txt"))){for(var line:(Iterable<String>)lines::iterator){var f=JsonParser.parseString(line).getAsJsonObject();for(int i=0;i<3;i++)items[i].builtInRegistryHolder().bindComponents(prototype(f.getAsJsonArray("prototypes").get(i).getAsJsonArray()));Inventory.EQUIPMENT_SLOT_MAPPING.clear();for(var e:f.getAsJsonArray("equipment_order")){int i=e.getAsInt();Inventory.EQUIPMENT_SLOT_MAPPING.put(36+i,mapped[i]);}ItemStack.EMPTY.setCount(0);ItemStack.EMPTY.setPopTime(0);((PatchedDataComponentMap)components.get(ItemStack.EMPTY)).clearPatch();var d=new Data();d.out=out;d.ids.put(ItemStack.EMPTY,0);d.refs=new ItemStack[4];for(int i=0;i<4;i++){var v=f.getAsJsonArray("stacks").get(i).getAsJsonObject();int type=n(v,"item");d.refs[i]=type<0?ItemStack.EMPTY:new ItemStack(items[type],n(v,"count"));if(type>=0){d.refs[i].setPopTime(n(v,"pop"));((PatchedDataComponentMap)components.get(d.refs[i])).restorePatch(patch(v.getAsJsonArray("patch")));}}
+   d.equipment=new EntityEquipment[]{new EntityEquipment(),new EntityEquipment()};if(f.get("shared_equipment").getAsBoolean())d.equipment[1]=d.equipment[0];d.inv=new Inventory[]{new Inventory(null,d.equipment[0]),new Inventory(null,d.equipment[1])};
+   for(int i=0;i<2;i++){var initial=f.getAsJsonArray("inventories").get(i).getAsJsonObject();d.inv[i].setSelectedSlot(n(initial,"selected"));changed.setInt(d.inv[i],n(initial,"changed"));for(var entry:initial.getAsJsonArray("slots")){var pair=entry.getAsJsonArray();d.inv[i].setItem(pair.get(0).getAsInt(),d.refs[pair.get(1).getAsInt()]);}if(n(initial,"mainhand")>=0)d.equipment[i].set(EquipmentSlot.MAINHAND,d.refs[n(initial,"mainhand")]);}
+   d.all();for(var ov:f.getAsJsonArray("ops")){var op=ov.getAsJsonObject();var c=d.inv[n(op,"inventory")];int a=n(op,"a"),b=n(op,"b"),slot=n(op,"slot"),amount=n(op,"amount");try{switch(op.get("kind").getAsString()){
+    case "get":d.refs[b]=c.getItem(slot);break;case "set":c.setItem(slot,d.refs[a]);break;case "remove":d.refs[b]=c.removeItem(slot,amount);break;case "no-update":d.refs[b]=c.removeItemNoUpdate(slot);break;case "remove-ref":c.removeItem(d.refs[a]);break;case "select":c.setSelectedSlot(slot);break;case "selected":d.refs[b]=c.getSelectedItem();break;case "set-selected":d.refs[b]=c.setSelectedItem(d.refs[a]);break;case "remove-selected":d.refs[b]=c.removeFromSelected(op.get("all").getAsBoolean());break;case "pick":c.pickSlot(slot);break;case "add-pick":c.addAndPickItem(d.refs[a]);break;case "clear":c.clearContent();break;case "changed":c.setChanged();break;case "replace":c.replaceWith(d.inv[n(op,"other")]);break;case "equip":d.refs[b]=d.equipment[n(op,"inventory")].set(EquipmentSlot.values()[n(op,"equipment")],d.refs[a]);break;case "get-equip":d.refs[b]=d.equipment[n(op,"inventory")].get(EquipmentSlot.values()[n(op,"equipment")]);break;case "alias":d.refs[b]=d.refs[a];break;case "copy":d.refs[b]=d.refs[a].copy();break;case "count":d.refs[a].setCount(amount);break;case "component":int k=n(op,"key");d.refs[a].set(keys[k],value(k,op.get("value")));break;default:throw new AssertionError();}out.println("result ok");}catch(IndexOutOfBoundsException|IllegalArgumentException|NullPointerException e){out.println("result error");}d.all();}
+  }}
+ }
+}

tools/test_player_inventory_parity.py

@@ -0,0 +1,84 @@
+#!/usr/bin/env python3
+"""Compare native PlayerInventory transfers, shared stack identity and change callbacks."""
+import argparse
+import json
+import os
+from pathlib import Path
+import platform
+import subprocess
+import random
+import struct
+import math
+try:
+    from . import lab, test_rust_parity as base
+except ImportError:
+    import lab
+    import test_rust_parity as base
+
+
+def corpus():
+    kinds=['get','set','remove','no-update','remove-ref','select','selected','set-selected','remove-selected','pick','add-pick','clear','changed','replace','equip','get-equip','alias','copy','count','component']
+    edges=[-2147483648,-1,0,1,2,8,9,35,36,39,40,41,42,43,64,2147483647]
+    def op(kind,**kw):
+        v=dict(kind=kind,inventory=0,other=1,a=0,b=3,slot=0,amount=1,all=False,equipment=0,key=0,value=64);v.update(kw);return v
+    def stack(item,count,patch=None):return dict(item=item,count=count,pop=7,patch=patch or [])
+    def row(ops,count=5,slots=None,selected=0):return dict(prototypes=[[],[[0,64]],[[0,1],[1,0],[2,100]]],stacks=[stack(1,count),stack(1,60),stack(2,1),stack(-1,0)],equipment_order=list(range(7)),shared_equipment=False,inventories=[dict(selected=selected,changed=2147483647,mainhand=0,slots=slots if slots is not None else [[0,0],[8,1],[35,2],[36,2],[40,0],[42,1]]),dict(selected=8,changed=0,mainhand=-1,slots=[[2,1],[39,0]])],ops=ops)
+    for kind in kinds:
+        for slot in edges:
+            for amount in [-2147483648,-1,0,1,64,2147483647]:
+                yield row([op(kind,slot=slot,amount=amount),op('changed'),op('copy'),op('count',a=3,amount=3)])
+    for enchanted in [False,True]:
+        for selected in range(9):
+            for free in [-1,0,1,8,9,35]:
+                for kind in ['pick','add-pick','replace','remove-ref','remove-selected']:
+                    slots=[[i,1] for i in range(43) if i!=free]
+                    v=row([op(kind,slot=35),op('replace',other=0)],slots=slots,selected=selected)
+                    if enchanted:v['stacks'][1]['patch']=[[4,1]]
+                    yield v
+    rng=random.Random(0x504c41594552494e)
+    def integer():return rng.choice(edges) if rng.randrange(2) else rng.randrange(-2147483648,2147483648)
+    def value(k):return 0 if k==3 else rng.randrange(2) if k==4 else integer()
+    def patch():return [[k,rng.choice([None,value(k)])] for k in rng.sample(range(6),rng.randrange(7))]
+    for _ in range(1500):
+        prototypes=[[[k,value(k)] for k in rng.sample(range(6),rng.randrange(7))] for __ in range(3)]
+        stacks=[dict(item=rng.randrange(-1,3),count=integer(),pop=integer(),patch=patch()) for __ in range(4)]
+        inventories=[dict(selected=rng.randrange(9),changed=integer(),mainhand=rng.randrange(-1,4),slots=[[i,rng.randrange(4)] for i in rng.sample(range(43),rng.randrange(44))]) for __ in range(2)]
+        ops=[]
+        for __ in range(18):
+            k=rng.randrange(6)
+            ops.append(op(rng.choice(kinds),inventory=rng.randrange(2),other=rng.randrange(2),a=rng.randrange(4),b=rng.randrange(4),slot=rng.choice(edges) if rng.randrange(2) else rng.randrange(43),amount=integer(),all=bool(rng.randrange(2)),equipment=rng.randrange(8),key=k,value=rng.choice([None,value(k)])))
+        yield dict(prototypes=prototypes,stacks=stacks,inventories=inventories,equipment_order=rng.sample(range(7),7),shared_equipment=bool(rng.randrange(2)),ops=ops)
+
+
+def main():
+    parser=argparse.ArgumentParser(description=__doc__);parser.add_argument('--prepare',action='store_true');args=parser.parse_args()
+    build=lab.LOCAL/'rust-player-inventory-parity';build.mkdir(parents=True,exist_ok=True)
+    report_path=build/'report.json';report_path.unlink(missing_ok=True)
+    if args.prepare:lab.prepare()
+    lock,jars,libraries=base.verify_artifacts()
+    oracle=lab.ROOT/'mods/horizon/src/test/java/dev/horizonlab/parity/VanillaPlayerInventoryOracle.java'
+    sources=[oracle,Path(__file__),lab.ROOT/'tools/test_rust_parity.py',lab.ROOT/'tools/lab.py',lab.ROOT/'Cargo.toml',lab.ROOT/'Cargo.lock',lab.ROOT/'crates/horizon-core/Cargo.toml',lab.ROOT/'crates/horizon-world/Cargo.toml',*sorted((lab.ROOT/'crates/horizon-core/src').rglob('*.rs')),*sorted((lab.ROOT/'crates/horizon-world/src').rglob('*.rs')),lab.ROOT/'crates/horizon-world/examples/player_inventory_fixture.rs']
+    tested_sources=base.source_hashes(sources)
+    classes=build/'classes';classes.mkdir(exist_ok=True)
+    def run(command,name):
+        with (build/(name+'.log')).open('w',encoding='utf-8') as log:base.run(list(map(str,command)),stdout=log,stderr=subprocess.STDOUT)
+    dependency_path=os.pathsep.join(map(str,libraries))
+    run(['javac','--release',lock['minecraft']['java_major'],'-proc:none','-cp',str(jars['server'])+os.pathsep+dependency_path,'-d',classes,oracle],'compile-java')
+    def java(side,mode,path):
+        run(['java','-Xmx512M','-cp',os.pathsep.join([str(classes),str(jars[side]),dependency_path]),'dev.horizonlab.parity.VanillaPlayerInventoryOracle',mode,path],side+'-'+mode)
+    fixtures=list(corpus());inputs=build/'inputs.txt';inputs.write_text(''.join(json.dumps(value,separators=(',',':'))+'\n' for value in fixtures),encoding='utf-8')
+    fixture_paths=[inputs];tested_fixtures=base.source_hashes(fixture_paths)
+    run(['cargo','build','--release','-p','horizon-world','--example','player_inventory_fixture'],'compile-rust')
+    metadata=json.loads(subprocess.check_output(['cargo','metadata','--no-deps','--format-version','1'],cwd=lab.ROOT,text=True))
+    binary=Path(metadata['target_directory'])/'release/examples'/('player_inventory_fixture.exe' if os.name=='nt' else 'player_inventory_fixture')
+    run([binary,build],'rust');references=[]
+    for side,jar in jars.items():
+        java(side,side,build);reference=build/f'{side}.txt';matched=base.compare(reference,build/'rust.txt')
+        references.append({'side':side,'jar_sha256':lab.digest(jar,'sha256'),'output_sha256':lab.digest(reference,'sha256'),'matched_output_lines':matched})
+        print(f'{side}: {matched:,} player inventory output lines match Rust',flush=True)
+    base.ensure_sources_unchanged(sources,tested_sources);base.ensure_sources_unchanged(fixture_paths,tested_fixtures)
+    report={'schema':1,'minecraft':lock['minecraft']['version'],'manifest_sha1':lock['minecraft']['manifest_sha1'],'platform':platform.platform(),'java':subprocess.check_output(['java','--version'],text=True).splitlines()[0],'rustc':subprocess.check_output(['rustc','--version'],text=True).strip(),'fixtures':len(fixtures),'sources_sha256':tested_sources,'fixtures_sha256':tested_fixtures,'rust_output_sha256':lab.digest(build/'rust.txt','sha256'),'references':references,'scope':'Actual player Inventory storage, equipment, selected slots, hotbar picking, matching/crafting eligibility, reference removals, clear/replacement, counts and change counter. Actual ItemStack components cover damage, unbreakable, one enchantment and custom names; fixture-controlled equipment-map iteration order isolates native JVM Map.of random ordering. Shared stack and equipment references are traced across two inventories, including self-replacement and failures after selection changes. Pickup, item/equipment ticks, menus, recipes, persistence, arbitrary virtual overrides, full components and complete gameplay integration remain pending.'}
+    lab.write_json(report_path,report);print(f'Scoped player inventory parity passed: {report_path}')
+
+
+if __name__=='__main__':main()