Rework and expand the Items section (#39)

Author: IchHabeHunger54

docs/datastorage/nbt.md

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+# Named Binary Tag (NBT)
+
+NBT is a format introduced in the earliest days of Minecraft, written by Notch himself. It is widely used throughout the Minecraft codebase for data storage.
+
+## Specification
+
+The NBT spec is similar to the JSON spec, with a few differences:
+
+- Distinct types for bytes, shorts, longs and floats exist, suffixed by `b`, `s`, `l` and `f`, respectively, similar to how they would be represented in Java code.
+  - Doubles may also be suffixed with `d`, but this is not required, similar to Java code. The optional `i` suffix available in Java for integers is not permitted.
+  - The suffixes are not case-sensitive. So for example, `64b` is the same as `64B`, and `0.5F` is the same as `0.5f`.
+- Booleans do not exist, they are instead represented by bytes. `true` becomes `1b`, `false` becomes `0b`.
+  - The current implementation treats all non-zero values as `true`, so `2b` would be treated as `true` as well.
+- There is no `null` equivalent in NBT.
+- Quotes around keys are optional. So a JSON property `"duration": 20` can become both `duration: 20` and `"duration": 20` in NBT.
+- What is known in JSON as a sub-object is known in NBT as a **compound tag** (or just compound).
+- NBT lists cannot mix and match types, unlike in JSON. The list type is determined by the first element, or defined in code.
+  - However, lists of lists can mix and match different list types. So a list of two lists, where the first one is a list of strings and the second one is a list of bytes, is allowed.
+- There are special **array** types that are different from lists, but follow their scheme of containing elements in square brackets. There are three array types:
+  - Byte arrays, denoted by a `B;` at the beginning of the array. Example: `[B;0b,30b]`
+  - Integer arrays, denoted by a `I;` at the beginning of the array. Example: `[I;0,-300]`
+  - Long arrays, denoted by an `L;` at the beginning of the array. Example: `[L;0l,240l]`
+- Trailing commas in lists, arrays and compound tags are allowed.
+
+## NBT Files
+
+Minecraft uses `.nbt` files extensively, for example for structure files in [datapacks][datapack]. Region files (`.mca`) that contain the contents of a region (i.e. a collection of chunks), as well as the various `.dat` files used in different places by the game, are NBT files as well.
+
+NBT files are typically compressed with GZip. As such, they are binary files and cannot be edited directly.
+
+## NBT in Code
+
+Like in JSON, all NBT objects are children of an enclosing object. So let's create one:
+
+```java
+CompoundTag tag = new CompoundTag();
+```
+
+We can now put our data into that tag:
+
+```java
+tag.putInt("Color", 0xffffff);
+tag.putString("Level", "minecraft:overworld");
+tag.putDouble("IAmRunningOutOfIdeasForNamesHere", 1d);
+```
+
+Several helpers exist here, for example, `putIntArray` also has a convenience method that takes a `List<Integer>` in addition to the standard variant that takes an `int[]`.
+
+Of course, we can also get values from that tag:
+
+```java
+int color = tag.getInt("Color");
+String level = tag.getString("Level");
+double d = tag.getDouble("IAmRunningOutOfIdeasForNamesHere");
+```
+
+Number types will return 0 if absent. Strings will return `""` if absent. More complex types (lists, arrays, compounds) will throw an exception if absent.
+
+As such, we want to safeguard by checking if a tag element exists:
+
+```java
+boolean hasColor = tag.contains("Color");
+boolean hasColorMoreExplicitly = tag.contains("Color", Tag.TAG_INT);
+```
+
+The `TAG_INT` constant is defined in `Tag`, which is the super interface for all tag types. Most tag types besides `CompoundTag` are mostly internal, for example `ByteTag` or `StringTag`, though the direct `CompoundTag#get` and `#put` methods can work with them if you ever stumble across some.
+
+There is one obvious exception, though: `ListTag`s. Working with these is special because when getting a list tag through `CompoundTag#getList`, you must also specify the list type. So getting a list of strings, for example, would work like this:
+
+```java
+ListTag list = tag.getList("SomeListHere", Tag.TAG_STRING);
+```
+
+Similarly, when creating a `ListTag`, you must also specify the list type during creation:
+
+```java
+ListTag list = new ListTag(List.of("Value1", "Value2"), Tag.TAG_STRING);
+```
+
+Finally, working with `CompoundTag`s inside other `CompoundTag`s directly utilizes `CompoundTag#get` and `#put`:
+
+```java
+tag.put("Tag", new CompoundTag());
+tag.get("Tag");
+```
+
+## Usages of NBT
+
+NBT is used in a lot of places in Minecraft. Some of the most common examples include [`ItemStack`][itemstack]s, [`BlockEntity`][blockentity]s and `Entity`s.
+
+## See Also
+
+- [NBT Format on the Minecraft Wiki][nbtwiki]
+
+[blockentity]: ../blockentities/index.md
+[datapack]: ../resources/server/index.md
+[itemstack]: ../items/index.md#itemstacks
+[nbtwiki]: https://minecraft.wiki/w/NBT_format