From 52ee433e7f9ce63b392bc51d90629f4374c33c72 Mon Sep 17 00:00:00 2001
From: nathyong <nathyong@users.noreply.github.com>
Date: Thu, 3 Dec 2020 11:51:20 +1100
Subject: [PATCH 1/2] Add the Tetra Legends Tetra-X randomizer

This randomizer is a slightly cleaned up version of Dr Ocelot's
JavaScript variant[1], including the 1000-piece selection mechanism.

[1]: https://github.com/Dr-Ocelot/tetralegends/blob/8d57a703224cc5aa5260e13160e973755c7551d9/script/game/modules/randomizers.js
---
 tetris/randomizers/tetra.lua | 71 ++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)
 create mode 100644 tetris/randomizers/tetra.lua

diff --git a/tetris/randomizers/tetra.lua b/tetris/randomizers/tetra.lua
new file mode 100644
index 0000000..4b3f682
--- /dev/null
+++ b/tetris/randomizers/tetra.lua
@@ -0,0 +1,71 @@
+-- A randomizer based on Tetra Legends' "Tetra-X" game mode
+-- There are two main rules:
+-- * if piece hasn't been generated for 13+ pieces, force it
+-- * if piece has been generated in the last two pieces, don't give it out
+--
+-- The official implementation also has self-balancing functionality (bias
+-- towards pieces that have appeared less often).
+
+local Randomizer = require 'tetris.randomizers.randomizer'
+
+local TetraXRandomizer = Randomizer:extend()
+
+function TetraXRandomizer:initialize()
+    local pieces = {"I", "J", "L", "O", "S", "T", "Z"}
+
+    self.count = {}
+    self.lastseen = {}
+    self.totalcount = 0
+    self.pieceselection = pieces
+
+    for _, piece in ipairs(pieces) do
+        self.count[piece] = 0
+        self.lastseen[piece] = -1  -- use -1 as magic value for "not seen"
+    end
+
+end
+
+function TetraXRandomizer:generatePiece()
+    local generated = nil
+    while true do
+        generated = self.pieceselection[math.random(#self.pieceselection)]
+        if not (self.lastseen[generated] == 0 or self.lastseen[generated] == 1) then
+            break
+        end
+    end
+
+    -- piece has not been generated in the last 13+
+    for piece, val in pairs(self.lastseen) do
+        if val >= 13 then
+            generated = piece
+            break
+        end
+    end
+
+    for piece, val in pairs(self.lastseen) do
+        if piece == generated then
+            self.lastseen[piece] = 0
+        else
+            if val ~= -1 then
+                self.lastseen[piece] = val + 1
+            end
+        end
+    end
+
+    self.count[generated] = self.count[generated] + 1
+    self.totalcount = self.totalcount + 1
+
+    -- shuffle the piece selection for the next time
+    self.pieceselection = {}
+    for piece, count in pairs(self.count) do
+        local probability = (self.totalcount - count) / (self.totalcount * 6)
+        local chances = math.floor(probability * 1000 + 0.5) -- simulated "round"
+        for _ = 1, chances do
+            table.insert(self.pieceselection, piece)
+        end
+    end
+
+    return generated
+end
+
+return TetraXRandomizer

From a4c650922272b38e9f5504e85119f1a2a577698c Mon Sep 17 00:00:00 2001
From: nathyong <nathyong@users.noreply.github.com>
Date: Thu, 3 Dec 2020 12:56:02 +1100
Subject: [PATCH 2/2] Document magic value in Tetra-X randomizer

---
 tetris/randomizers/tetra.lua | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tetris/randomizers/tetra.lua b/tetris/randomizers/tetra.lua
index 4b3f682..7dcea7e 100644
--- a/tetris/randomizers/tetra.lua
+++ b/tetris/randomizers/tetra.lua
@@ -58,6 +58,7 @@ function TetraXRandomizer:generatePiece()
     -- shuffle the piece selection for the next time
     self.pieceselection = {}
     for piece, count in pairs(self.count) do
+        -- in this case 6 = #piece types - 1, so all probabilities sum to 1
         local probability = (self.totalcount - count) / (self.totalcount * 6)
         local chances = math.floor(probability * 1000 + 0.5) -- simulated "round"
         for _ = 1, chances do