alexanderscott · January 26, 2015 08:51 · Jan 26, 2015
diff --git a/ZSPREAD.lua b/ZSPREAD.lua
@@ -0,0 +1,56 @@
+-- @desc:   Spread members between two ZSETs so they are even size
+-- @usage:  redis-cli eval "$(cat ZSPREAD.lua)" 2 <zset1> <zset2>
+-- @return: list of spread members
+
+local function round(num) 
+  if num >= 0 then return math.floor(num+.5) 
+  else return math.ceil(num-.5) end
+end
+
+local function randomFromTo(from, to)
+  local diff = to - from
+  return round((math.random() * diff) + from)
+end
+
+local function ZSPREAD(zset1, zset2)
+  local zset1size = redis.call("ZCARD", zset1)
+  if zset1size == 0 then return {} end
+
+  local zset2size = redis.call("ZCARD", zset2)
+  if zset2size == 0 then return {} end
+
+  local fromZset, fromZsetSize = zset1, zset1size
+  local toZsetSize, toZsetSize = zset2, zset2size
+
+  if zset1size > zset2size then 
+    fromZset, fromZsetSize = toZset, toZsetSize
+  elseif zset1size == zset2size then
+    return {}
+  end
+
+
+  local spreadCount = round(((fromZsetSize - toZsetSize) / 2))
+  if spreadCount == 0 then return {} end
+
+  local members = redis.call("ZREVRANGEBYSCORE", zset1, "+INF", "-INF", "WITHSCORES")
+
+  local tmpMembers = {}
+
+  for i=1,spreadCount do
+    local randIdx = randomFromTo(0, (fromZsetSize - 1))
+    local tmpMember = redis.call("ZREVRANGE", fromZset, randIdx, randIdx, "WITHSCORES")
+
+    for idx,val in ipairs(tmpMember) do
+      if idx % 2 == 0 then 
+        redis.call("ZADD", toZset, tonumber(val), tmpMembers[#tmpMembers])
+      else 
+        table.insert(tmpMembers, val)
+        redis.call("ZREM", fromZset, val)
+      end
+    end
+  end
+
+  return tmpMembers
+end
+
+return ZSPREAD(KEYS[1], KEYS[2])