Add nbtOptionalLengthPrefixed datatype (length-prefixed optional NBT)

src/datatypes/compiler-minecraft.js

@@ -71,7 +71,8 @@ if (n !== 0) {
   }
     `.trim())
     }],
-    lpVec3: ['native', minecraft.lpVec3[0]]
+    lpVec3: ['native', minecraft.lpVec3[0]],
+    nbtOptionalLengthPrefixed: ['native', minecraft.nbtOptionalLengthPrefixed[0]]
   },
   Write: {
     varlong: ['native', minecraft.varlong[1]],
@@ -137,7 +138,8 @@ if (${baseName} != null) {
 return offset
     `.trim())
     }],
-    lpVec3: ['native', minecraft.lpVec3[1]]
+    lpVec3: ['native', minecraft.lpVec3[1]],
+    nbtOptionalLengthPrefixed: ['native', minecraft.nbtOptionalLengthPrefixed[1]]
   },
   SizeOf: {
     varlong: ['native', minecraft.varlong[2]],
@@ -197,6 +199,7 @@ if (${baseName} != null) {
 return size
       `.trim())
     }],
-    lpVec3: ['native', minecraft.lpVec3[2]]
+    lpVec3: ['native', minecraft.lpVec3[2]],
+    nbtOptionalLengthPrefixed: ['native', minecraft.nbtOptionalLengthPrefixed[2]]
   }
 }

src/datatypes/minecraft.js

@@ -13,7 +13,8 @@ module.exports = {
   restBuffer: [readRestBuffer, writeRestBuffer, sizeOfRestBuffer],
   entityMetadataLoop: [readEntityMetadata, writeEntityMetadata, sizeOfEntityMetadata],
   topBitSetTerminatedArray: [readTopBitSetTerminatedArray, writeTopBitSetTerminatedArray, sizeOfTopBitSetTerminatedArray],
-  lpVec3: [readLpVec3, writeLpVec3, sizeOfLpVec3]
+  lpVec3: [readLpVec3, writeLpVec3, sizeOfLpVec3],
+  nbtOptionalLengthPrefixed: [readNbtOptionalLengthPrefixed, writeNbtOptionalLengthPrefixed, sizeOfNbtOptionalLengthPrefixed]
 }
 const PartialReadError = require('protodef').utils.PartialReadError
 
@@ -187,3 +188,25 @@ function sizeOfTopBitSetTerminatedArray (value, { type }) {
   }
   return size
 }
+
+// A network (anonymous) optional NBT tag preceded by its byte length as a VarInt.
+// This is Mojang's `ByteBufCodecs.optionalTagCodec(...).apply(ByteBufCodecs.lengthPrefixed(N))`
+// (e.g. the payload of ServerboundCustomClickActionPacket / the dialog system). An empty tag
+// is a single TAG_END (0) byte, so an absent value encodes as `01 00`.
+function readNbtOptionalLengthPrefixed (buffer, offset) {
+  const { value: length, size: lengthSize } = readVarInt(buffer, offset)
+  if (offset + lengthSize + length > buffer.length) { throw new PartialReadError() }
+  const tag = nbt.proto.read(buffer, offset + lengthSize, 'anonOptionalNbt')
+  return { value: tag.value, size: lengthSize + tag.size }
+}
+
+function writeNbtOptionalLengthPrefixed (value, buffer, offset) {
+  const innerSize = nbt.proto.sizeOf(value, 'anonOptionalNbt')
+  offset = writeVarInt(innerSize, buffer, offset)
+  return nbt.proto.write(value, buffer, offset, 'anonOptionalNbt')
+}
+
+function sizeOfNbtOptionalLengthPrefixed (value) {
+  const innerSize = nbt.proto.sizeOf(value, 'anonOptionalNbt')
+  return sizeOfVarInt(innerSize) + innerSize
+}

test/nbtOptionalLengthPrefixedTest.js

@@ -0,0 +1,38 @@
+/* eslint-env mocha */
+const assert = require('assert')
+const nbt = require('prismarine-nbt')
+const { nbtOptionalLengthPrefixed } = require('../src/datatypes/minecraft')
+const [read, write, sizeOf] = nbtOptionalLengthPrefixed
+
+describe('nbtOptionalLengthPrefixed', () => {
+  // The NBT payload of a real ServerboundCustomClickActionPacket (a dialog form submit),
+  // captured from a 1.21.11 client: a single VarInt byte-length (0x1a = 26) followed by the
+  // anonymous compound { password: "hello world" }.
+  const compound = nbt.comp({ password: nbt.string('hello world') })
+  // 1a | 0a(compound) 08(string) 0008 "password" 000b "hello world" 00(end)
+  const expectedHex = '1a' + '0a' + '08' + '0008' + Buffer.from('password').toString('hex') +
+    '000b' + Buffer.from('hello world').toString('hex') + '00'
+
+  it('writes a present tag as VarInt(byteLength) + anonymous NBT', () => {
+    const buf = Buffer.alloc(sizeOf(compound))
+    const end = write(compound, buf, 0)
+    assert.strictEqual(buf.subarray(0, end).toString('hex'), expectedHex)
+    assert.strictEqual(sizeOf(compound), end)
+  })
+
+  it('round-trips a present tag', () => {
+    const buf = Buffer.alloc(sizeOf(compound))
+    write(compound, buf, 0)
+    const { value, size } = read(buf, 0)
+    assert.strictEqual(size, buf.length)
+    assert.strictEqual(value.value.password.value, 'hello world')
+  })
+
+  it('encodes an absent tag as 01 00 (length 1, TAG_END)', () => {
+    const buf = Buffer.alloc(8)
+    const end = write(undefined, buf, 0)
+    assert.strictEqual(buf.subarray(0, end).toString('hex'), '0100')
+    assert.strictEqual(sizeOf(undefined), 2)
+    assert.strictEqual(read(buf, 0).value, undefined)
+  })
+})