quota/tokenCount: Implement the "request tokens as half" case

src/quota/tokenCount.ts

@@ -65,9 +65,34 @@ export default class tokenCount implements IQuota {
     return this.createUserQuotaData(this.defaultQuota, usedUnits);
   }
 
+  /**
+   * Helper function to use proper query based on considerInputTokensAsHalf setting
+   * @param user The user to find nth usage
+   * @param requestTimestamp the timestamp of the request
+   * @param unitCount the unit count to check
+   * @returns promise of giving out the record
+   */
   findNthUsage(user: string, requestTimestamp: number, unitCount: number) {
     if (this.considerInputTokensAsHalf)
-      throw("Not implemented");
+      return database.$queryRaw<Array<Usage & {usage: number}>>`
+        SELECT t1.*, (
+            SELECT
+              SUM(usageResponse + usageRequest/2) AS usage
+            FROM \`usage\`
+            WHERE 
+              user = ${user} AND
+              timestamp >= ${requestTimestamp - this.lookback} AND
+              timestamp <= t1.timestamp
+          ) as usage
+        FROM
+          \`usage\` AS t1
+        WHERE 
+          user = ${user} AND
+          timestamp >= ${requestTimestamp - this.lookback} AND
+          usage >= ${unitCount}
+        ORDER BY timestamp ASC
+        LIMIT 1
+      `;
     return database.$queryRaw<Array<Usage & {usage: bigint}>>`
       SELECT t1.*, (
           SELECT