readers.clj

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(ns net.bowmap.server.protocol.readers
  "Functions for reading values from io.netty.ByteBufs.
   Buffers are assumed to be properly framed and contain exactly 1 packet.
   Functions look like ->type, where 'type' is a Minecraft protocol type.
   See https://minecraft.wiki/w/Java_Edition_protocol/Packets#Data_types"
  (:require
   [medley.core :refer [assoc-some]]
   [net.bowmap.server.protocol.types :refer [max-varint-size]])
  (:import
   [io.netty.buffer ByteBuf]
   [java.nio.charset Charset]))

(def ->byte ByteBuf/.readByte)
(def ->ubyte (comp unchecked-byte ByteBuf/.readByte))
(def ->short ByteBuf/.readShort)
(def ->ushort (comp unchecked-short ByteBuf/.readShort))
(def ->int ByteBuf/.readInt)
(def ->long ByteBuf/.readLong)
(def ->float ByteBuf/.readFloat)
(def ->double ByteBuf/.readDouble)

(defn ->varint [buf]
  (loop [num-read 0
         result 0]
    (if (>= num-read max-varint-size)
      (throw (Exception. "VarInt too long"))
      (let [b (ByteBuf/.readByte buf)
            result' (bit-or result (bit-shift-left (bit-and b 0x7F) (* 7 num-read)))]
        (if (zero? (bit-and b 0x80))
          result'
          (recur (inc num-read) result'))))))

(defn ->string [buf]
  (let [len (->varint buf)]
    (if (neg? len)
      (throw (Exception. "String length cannot be negative"))
      (ByteBuf/.readCharSequence buf len (Charset/forName "UTF-8")))))

(defn ->enum [reader m]
  (fn [buf]
    (m (reader buf))))

(defn ->table [schema]
  (fn [buf]
    (reduce (fn [acc [field reader]]
              (assoc-some acc field (reader buf)))
            {} (partition 2 schema))))

(defn to-reader
  "Attempts to coerces 'reader' into a reader. Throws if it cannot.
   Vectors with pairs of [:key reader] are turned into tables readers.
   Nil is coerced to the empty table reader."
  [reader]
  (cond
    (or (vector? reader) (nil? reader)) (->table reader)
    (fn? reader) reader
    :else (throw (IllegalArgumentException. (str "Cannot coerce into reader: " (pr-str reader))))))