DX-1261: allow negative rate values (#143)

* feat: allow negative rate values

* chore: format multi.ts

* fix: allow for refunding more than the original limit and fix cache

* fix: remove duplicate tonumber

---------

Co-authored-by: ytkimirti <yusuftaha9@gmail.com>

Author: CahidArda

src/lua-scripts/hash.ts

@@ -12,7 +12,7 @@ type Algorithm = {
   getRemaining: ScriptInfo,
 }
 
-type AlgorithmKind = 
+type AlgorithmKind =
   | "fixedWindow"
   | "slidingWindow"
   | "tokenBucket"
@@ -36,7 +36,7 @@ export const SCRIPTS: {
     slidingWindow: {
       limit: {
         script: Single.slidingWindowLimitScript,
-        hash: "e1391e429b699c780eb0480350cd5b7280fd9213"
+        hash: "9b7842963bd73721f1a3011650c23c0010848ee3"
       },
       getRemaining: {
         script: Single.slidingWindowRemainingTokensScript,
@@ -46,7 +46,7 @@ export const SCRIPTS: {
     tokenBucket: {
       limit: {
         script: Single.tokenBucketLimitScript,
-        hash: "5bece90aeef8189a8cfd28995b479529e270b3c6"
+        hash: "d1f857ebbdaeca90ccd2cd4eada61d7c8e5db1ca"
       },
       getRemaining: {
         script: Single.tokenBucketRemainingTokensScript,
@@ -78,7 +78,7 @@ export const SCRIPTS: {
     slidingWindow: {
       limit: {
         script: Multi.slidingWindowLimitScript,
-        hash: "cb4fdc2575056df7c6d422764df0de3a08d6753b"
+        hash: "1e7ca8dcd2d600a6d0124a67a57ea225ed62921b"
       },
       getRemaining: {
         script: Multi.slidingWindowRemainingTokensScript,
@@ -92,4 +92,4 @@ export const SCRIPTS: {
 export const RESET_SCRIPT: ScriptInfo = {
   script: resetScript,
   hash: "54bd274ddc59fb3be0f42deee2f64322a10e2b50"
-}
+}

src/lua-scripts/multi.ts

@@ -45,7 +45,9 @@ export const slidingWindowLimitScript = `
 	end
 
 	local percentageInCurrent = ( now % window) / window
-	if requestsInPreviousWindow * (1 - percentageInCurrent ) + requestsInCurrentWindow >= tokens then
+
+	-- Only check limit if not refunding (negative rate)
+	if incrementBy > 0 and requestsInPreviousWindow * (1 - percentageInCurrent ) + requestsInCurrentWindow + incrementBy > tokens then
 	  return {currentFields, previousFields, false}
 	end
 

src/lua-scripts/single.ts

@@ -30,7 +30,7 @@ export const slidingWindowLimitScript = `
   local tokens      = tonumber(ARGV[1]) -- tokens per window
   local now         = ARGV[2]           -- current timestamp in milliseconds
   local window      = ARGV[3]           -- interval in milliseconds
-  local incrementBy = ARGV[4]           -- increment rate per request at a given value, default is 1
+  local incrementBy = tonumber(ARGV[4]) -- increment rate per request at a given value, default is 1
 
   local requestsInCurrentWindow = redis.call("GET", currentKey)
   if requestsInCurrentWindow == false then
@@ -44,12 +44,14 @@ export const slidingWindowLimitScript = `
   local percentageInCurrent = ( now % window ) / window
   -- weighted requests to consider from the previous window
   requestsInPreviousWindow = math.floor(( 1 - percentageInCurrent ) * requestsInPreviousWindow)
-  if requestsInPreviousWindow + requestsInCurrentWindow >= tokens then
+
+  -- Only check limit if not refunding (negative rate)
+  if incrementBy > 0 and requestsInPreviousWindow + requestsInCurrentWindow >= tokens then
     return -1
   end
 
   local newValue = redis.call("INCRBY", currentKey, incrementBy)
-  if newValue == tonumber(incrementBy) then
+  if newValue == incrementBy then
     -- The first time this key is set, the value will be equal to incrementBy.
     -- So we only need the expire command once
     redis.call("PEXPIRE", currentKey, window * 2 + 1000) -- Enough time to overlap with a new window + 1 second
@@ -108,15 +110,19 @@ export const tokenBucketLimitScript = `
     refilledAt = refilledAt + numRefills * interval
   end
 
-  if tokens == 0 then
+  -- Only reject if tokens are 0 and we're consuming (not refunding)
+  if tokens == 0 and incrementBy > 0 then
     return {-1, refilledAt + interval}
   end
 
   local remaining = tokens - incrementBy
   local expireAt = math.ceil(((maxTokens - remaining) / refillRate)) * interval
         
   redis.call("HSET", key, "refilledAt", refilledAt, "tokens", remaining)
-  redis.call("PEXPIRE", key, expireAt)
+
+  if (expireAt > 0) then
+    redis.call("PEXPIRE", key, expireAt)
+  end
   return {remaining, refilledAt + interval}
 `;