diff --git a/CRM/Tbusainvoicegen/Generator.php b/CRM/Tbusainvoicegen/Timebank.php
similarity index 55%
rename from CRM/Tbusainvoicegen/Generator.php
rename to CRM/Tbusainvoicegen/Timebank.php
index 0968811..08cd2e7 100644
--- a/CRM/Tbusainvoicegen/Generator.php
+++ b/CRM/Tbusainvoicegen/Timebank.php
@@ -47,6 +47,19 @@ class CRM_Tbusainvoicegen_Timebank {
    */
   private $price;
 
+  /**
+   * An array of all the contact IDs for whom a contribution already exists in this billing period.
+   * @var array
+   */
+  public static $contributionExists = [];
+
+  /**
+   * A string that represents the current billing period - e.g. "2019-2".
+   * First number is the year; second is 1 for Jan-Jun, 2 for Jul-Dec.
+   * @var string
+   */
+  public static $billingPeriod = NULL;
+
   /**
    * Class constructor.
    */
@@ -60,32 +73,52 @@ class CRM_Tbusainvoicegen_Timebank {
    * @param int $cid
    * @return array APIv3 standard response.
    */
-  public static function generate($cid = NULL) {
+  public static function generate($cid = NULL, $billingPeriod = NULL) {
+    self::setBillingPeriod($billingPeriod);
     // Get a list of contact IDs for everyone to generate an invoice for.
-    $result = civicrm_api3('Contact', 'get', [
+    $contacts = civicrm_api3('Contact', 'get', [
       'return' => ["id", self::BILLABLEMEMBERFIELD],
       'contact_id' => $cid,
       'contact_type' => 'Organization',
       self::TBBILLABLEFIELD => 1,
       'options' => ['limit' => 0],
-    ]);
+    ])['values'];
+    self::setContributionExists();
     foreach ($result as $k => $contact) {
       $tb = new CRM_Tbusainvoicegen_Timebank($k, $contact[self::BILLABLEMEMBERFIELD]);
+      $price = $tb->setPrice();
     }
   }
 
+  public static function setBillingPeriod($billingPeriod) {
+    if ($billingPeriod) {
+      self::$billingPeriod = $billingPeriod;
+    }
+    else {
+      // Should we try to auto-calculate it here?  Or nah?
+    }
+  }
+
+  /**
+   * Generate an array of contacts who already have a contribution created in this billing period.
+   */
+  public static function setContributionExists() {
+    // API call to contribution.get to find all records with this billing period
+    // Return an array of contact IDs.
+  }
+
   private function setPrice() {
-    if (self::$price) {
-      return self::$price;
+    if ($this->price) {
+      return $this->price;
     }
-    $adjustedMemberCount = $memberCount - 2;
-    foreach ($priceArray as $members => $cost) {
+    $adjustedMemberCount = $this->memberCount - 2;
+    foreach ($this->priceArray as $members => $cost) {
       if ($adjustedMemberCount <= $members) {
-        self::$price = $cost;
+        $this->price = $cost;
         break;
       }
     }
-    return self::$price;
+    return $this->price;
   }
 
 }
diff --git a/api/v3/Invoicegen/Generate.php b/api/v3/Invoicegen/Generate.php
index 121cecf..b6763ca 100644
--- a/api/v3/Invoicegen/Generate.php
+++ b/api/v3/Invoicegen/Generate.php
@@ -10,7 +10,8 @@ use CRM_Tbusainvoicegen_ExtensionUtil as E;
  * @see http://wiki.civicrm.org/confluence/display/CRMDOC/API+Architecture+Standards
  */
 function _civicrm_api3_invoicegen_Generate_spec(&$spec) {
-  $spec['magicword']['api.required'] = 1;
+  $spec['contact_id']['api.required'] = 0;
+  $spec['billing_period']['api.required'] = 0;
 }
 
 /**
@@ -23,20 +24,6 @@ function _civicrm_api3_invoicegen_Generate_spec(&$spec) {
  * @throws API_Exception
  */
 function civicrm_api3_invoicegen_Generate($params) {
-  if (array_key_exists('magicword', $params) && $params['magicword'] == 'sesame') {
-    $returnValues = array(
-      // OK, return several data rows
-      12 => array('id' => 12, 'name' => 'Twelve'),
-      34 => array('id' => 34, 'name' => 'Thirty four'),
-      56 => array('id' => 56, 'name' => 'Fifty six'),
-    );
-    // ALTERNATIVE: $returnValues = array(); // OK, success
-    // ALTERNATIVE: $returnValues = array("Some value"); // OK, return a single value
-
-    // Spec: civicrm_api3_create_success($values = 1, $params = array(), $entity = NULL, $action = NULL)
-    return civicrm_api3_create_success($returnValues, $params, 'NewEntity', 'NewAction');
-  }
-  else {
-    throw new API_Exception(/*errorMessage*/ 'Everyone knows that the magicword is "sesame"', /*errorCode*/ 1234);
-  }
+  $returnValues = CRM_Tbusainvoicegen_Timebank::generate($params['contact_id'], $params['billing_period']);
+  return civicrm_api3_create_success($returnValues, $params, 'Invoicegen', 'generate');
 }