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]: 8d57a70322/script/game/modules/randomizers.js

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