Add piece RNG document

Author: lemoncove

Piece_RNG.md

@@ -0,0 +1,93 @@
+# Piece RNG
+
+Though TETR.IO has five different types of [randomizer](Room_Config.md#Bag-type), all of them are based upon the same pseudorandom number generator. This RNG is the [Lehmer random number generator](https://en.wikipedia.org/wiki/Lehmer_random_number_generator), specifically MINSTD (`a = 16807` and `m = 2147483647`). The `shuffleArray` function uses the [Fisher–Yates shuffle](https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle).
+
+A JavaScript reference implementation closely matching that found in [tetrio.js](https://tetr.io/js/tetrio.js) follows:
+
+```js
+function rng(seed) {
+    let t = seed % 2147483647;
+
+    if (t <= 0) {
+        t += 2147483646;
+    }
+
+    return {
+        next: function () {
+            return t = 16807 * t % 2147483647;
+        },
+        nextFloat: function () {
+            return (this.next() - 1) / 2147483646;
+        },
+        shuffleArray: function (array) {
+            if (array.length == 0) {
+                return array;
+            }
+
+            for (let i = array.length - 1; i != 0; i--) {
+                console.log(i)
+                const r = Math.floor(this.nextFloat() * (i + 1));
+                [array[i], array[r]] = [array[r], array[i]];
+            }
+
+            return array;
+        }
+    }
+}
+```
+
+In singleplayer, replays with `no_szo` set to true (currently accessible via stride mode) will skip the first piece if it is any of S, Z, or O.
+
+# `"7-bag"`
+
+The 7-bag randomization system generates a set of the seven tetrominos and shuffles them before putting them into the queue. This guarantees that there will never be a gap of more than 12 tetrominos between any two tetrominos of the same type. As with any shuffling algorithm, the order of the input array matters. TETR.IO uses the ordering `ZLOSIJT` (a rainbow, how cute!) for all purposes.
+
+```js
+queue.push(...rng.shuffleArray(["Z", "L", "O", "S", "I", "J", "T"]));
+```
+
+# `"14-bag"`
+
+The 14-bag randomization system is a variant of the 7-bag system with less of a guarantee against droughts. The input ordering is `ZLOSIJTZLOSIJT`.
+
+```js
+queue.push(...rng.shuffleArray(["Z", "L", "O", "S", "I", "J", "T", "Z", "L", "O", "S", "I", "J", "T"]));
+```
+
+# `"classic"`
+
+This is the randomizer used in *Tetris* for the NES. For each piece, the randomizer rolls an eight-sided die, with seven sides representing seven tetrominos and the eighth side representing a reroll. If a reroll or the same tetromino that was previously generated is rolled, then the randomizer selects randomly from the tetrominos.
+
+```js
+const TETROMINOS = ["Z", "L", "O", "S", "I", "J", "T"];
+
+let index = Math.floor(rng.nextFloat() * (TETROMINOS.length + 1));
+
+if (index === lastGenerated || index >= TETROMINOS.length) {
+    index = Math.floor(rng.nextFloat() * TETROMINOS.length);
+}
+
+lastGenerated = index;
+queue.push(TETROMINOS[index]);
+```
+
+# `"pairs"`
+
+This randomizer picks pairs of tetromino types and gives three of each in a random order.
+
+```js
+const s = rng.shuffleArray(["Z", "L", "O", "S", "I", "J", "T"]),
+const pairs = [s[0], s[0], s[0], s[1], s[1], s[1]];
+rng.shuffleArray(pairs);
+
+queue.push(...pairs)
+```
+
+# `"total mayhem"`
+
+This randomizer is entirely random. <!-- xD -->
+
+```js
+const TETROMINOS = ["Z", "L", "O", "S", "I", "J", "T"];
+queue.push(TETROMINOS[Math.floor(rng.nextFloat() * TETROMINOS.length)]);
+```

README.md

@@ -49,6 +49,8 @@ Once you're done with that, the core of a TETR.IO bot is interfacing with the [R
     * [Resuming a connection](Resuming_a_connection.md)
 * Guides
     * [Chat](Chat.md)
+* Game mechanics
+    * [Piece RNG](Piece_RNG.md)
 * Ribbon messages
     * Client
         * [authorize](Messages/client_authorize.md)