Add the hash datatype

doc/datatypes.md

@@ -18,3 +18,4 @@
 * * [bitfield](./datatypes/utils.md)
 * * [mapper](./datatypes/utils.md)
 * * [pstring](./datatypes/utils.md)
+* * [hash](./datatypes/utils.md)

doc/datatypes/utils.md

@@ -124,3 +124,35 @@ Example: A string length prefixed by a varint.
 ]
 ```
 Example of value: `"my string"`
+
+### **hash** ({ alg: String, type: Type, body: Type })
+Arguments:
+* alg : the hash algorithm, currently only `crc32c`
+* type : the type the hash is written and read as
+* body : the type the value is serialized as before being hashed
+
+Represents a hash of a value instead of the value itself: writing serializes the value as `body`, hashes the bytes and writes the hash as `type`; reading reads `type` and yields the hash. The value is not recoverable from the buffer, so the read side sees only the hash.
+
+`alg` is an explicit list rather than whatever hashes the implementation's platform happens to offer, so that a protocol using `hash` means the same thing in every implementation:
+
+| alg | digest |
+| --- | --- |
+| `crc32c` | a 4 byte unsigned integer (CRC-32C, the Castagnoli polynomial, reflected, all-ones init and final xor) |
+
+Because the digest's width is fixed by `alg`, the size of a `hash` field is known without hashing anything. `type` must therefore be a type of constant size, at least as wide as the digest; a variable-length `type` such as `varint` is an error.
+
+A CRC is unsigned. When `type` is signed (for example `i32`) the hash is written in two's complement, so it reads back the way a language with only signed integers would produce it.
+
+Example: a CRC32C of an item component, written as a signed int like Minecraft's `HashedSlot` carries it.
+```json
+[
+  "hash",
+  {
+    "alg": "crc32c",
+    "type": "i32",
+    "body": "ItemComponent"
+  }
+]
+```
+
+Example of value: `{ "id": 5, "level": 3 }` (whatever `body` accepts) / reads as `-486237565`

schemas/datatype.json

@@ -45,6 +45,7 @@
     { "$ref": "buffer" },
     { "$ref": "bitfield" },
     { "$ref": "bitflags" },
-    { "$ref": "mapper" }
+    { "$ref": "mapper" },
+    { "$ref": "hash" }
   ]
 }

schemas/utils.json

@@ -144,5 +144,31 @@
       }
     ],
     "additionalItems": false
+  },
+  "hash": {
+    "title": "hash",
+    "type": "array",
+    "items": [
+      {
+        "enum": ["hash"]
+      },
+      {
+        "type": "object",
+        "properties": {
+          "alg": {
+            "enum": ["crc32c"]
+          },
+          "type": {
+            "$ref": "dataType"
+          },
+          "body": {
+            "$ref": "dataType"
+          }
+        },
+        "required": ["alg", "type", "body"],
+        "additionalProperties": false
+      }
+    ],
+    "additionalItems": false
   }
 }