Merge T Edmunds's 'amortization-rounding' into maint.

CMakeLists.txt

@@ -12,13 +12,13 @@ enable_testing()
 
 # Version number of gnucash
 set (GNUCASH_MAJOR_VERSION 3)
-set (GNUCASH_MINOR_VERSION 5)
+set (GNUCASH_MINOR_VERSION 900)
 set (VERSION "${GNUCASH_MAJOR_VERSION}.${GNUCASH_MINOR_VERSION}")
 set (GNUCASH_LATEST_STABLE_SERIES 3.x)
 
 set (PACKAGE gnucash)
 set (PACKAGE_NAME GnuCash)
-set (PACKAGE_VERSION 3.5)
+set (PACKAGE_VERSION 3.900)
 set (PACKAGE_BUGREPORT "https://bugs.gnucash.org")
 set (PACKAGE_TARNAME ${PACKAGE})
 set (PACKAGE_STRING "${PACKAGE_NAME} ${PACKAGE_VERSION}")

gnucash/gtkbuilder/dialog-import.glade

@@ -14,6 +14,9 @@
     <property name="default_height">600</property>
     <property name="type_hint">dialog</property>
     <signal name="map" handler="gnc_ui_generic_account_picker_map_cb" swapped="no"/>
+    <child>
+      <placeholder/>
+    </child>
     <child internal-child="vbox">
       <object class="GtkBox" id="account_picker_vbox">
         <property name="visible">True</property>
@@ -165,7 +168,6 @@
     <property name="page_increment">10</property>
   </object>
   <object class="GtkDialog" id="format_picker_dialog">
-    <property name="visible">False</property>
     <property name="can_focus">False</property>
     <property name="title" translatable="yes">Choose a format</property>
     <property name="resizable">False</property>
@@ -173,6 +175,9 @@
     <property name="default_width">600</property>
     <property name="default_height">400</property>
     <property name="type_hint">dialog</property>
+    <child>
+      <placeholder/>
+    </child>
     <child internal-child="vbox">
       <object class="GtkBox" id="dialog-vbox16">
         <property name="visible">True</property>
@@ -301,6 +306,9 @@
     <property name="visible">True</property>
     <property name="can_focus">False</property>
     <property name="title" translatable="yes">Preferences</property>
+    <child>
+      <placeholder/>
+    </child>
     <child>
       <object class="GtkGrid" id="matcher_prefs">
         <property name="visible">True</property>
@@ -554,6 +562,9 @@
     <property name="default_width">600</property>
     <property name="default_height">400</property>
     <property name="type_hint">dialog</property>
+    <child>
+      <placeholder/>
+    </child>
     <child internal-child="vbox">
       <object class="GtkBox" id="dialog-vbox14">
         <property name="visible">True</property>
@@ -717,7 +728,8 @@
     <property name="text" translatable="yes">This transaction requires your intervention or it will NOT be imported.</property>
   </object>
   <object class="GtkTextBuffer" id="textbuffer5">
-    <property name="text" translatable="yes">Double click on the transaction to change the matching transaction to reconcile, or the destination account of the auto-balance split (if required).</property>
+    <property name="text" translatable="yes">Double click on the transaction to change the matching transaction to reconcile, or the destination account of the auto-balance split (if required).
+Multiply select rows using Ctrl-Click, Shift-Click and Rubber banding . Right-Click or Shift-F10 to bring up popup menu and click or select to assign a destination account to the selected rows.</property>
   </object>
   <object class="GtkDialog" id="matcher_help_dialog">
     <property name="can_focus">False</property>
@@ -726,6 +738,9 @@
     <property name="resizable">False</property>
     <property name="type_hint">dialog</property>
     <signal name="close" handler="on_matcher_help_close_clicked" swapped="no"/>
+    <child>
+      <placeholder/>
+    </child>
     <child internal-child="vbox">
       <object class="GtkBox" id="dialog-vbox18">
         <property name="visible">True</property>
@@ -764,6 +779,7 @@
           <object class="GtkGrid" id="table1">
             <property name="visible">True</property>
             <property name="can_focus">False</property>
+            <property name="margin_bottom">42</property>
             <property name="row_spacing">6</property>
             <property name="column_spacing">12</property>
             <child>
@@ -990,6 +1006,7 @@
                   <object class="GtkTextView" id="textview5">
                     <property name="visible">True</property>
                     <property name="can_focus">True</property>
+                    <property name="margin_bottom">2</property>
                     <property name="pixels_above_lines">5</property>
                     <property name="pixels_below_lines">5</property>
                     <property name="editable">False</property>
@@ -1107,8 +1124,11 @@
             <property name="reorderable">True</property>
             <property name="rules_hint">True</property>
             <property name="enable_search">False</property>
+            <property name="rubber_banding">True</property>
             <child internal-child="selection">
-              <object class="GtkTreeSelection" id="treeview-selection3"/>
+              <object class="GtkTreeSelection" id="treeview-selection3">
+                <property name="mode">multiple</property>
+              </object>
             </child>
           </object>
         </child>
@@ -1145,6 +1165,9 @@
     <property name="default_height">400</property>
     <property name="type_hint">dialog</property>
     <signal name="close" handler="on_matcher_cancel_clicked" swapped="no"/>
+    <child>
+      <placeholder/>
+    </child>
     <child internal-child="vbox">
       <object class="GtkBox" id="transaction_matcher_vbox">
         <property name="visible">True</property>

gnucash/import-export/import-main-matcher.c

@@ -98,9 +98,28 @@ void on_matcher_help_clicked (GtkButton *button, gpointer user_data);
 void on_matcher_help_close_clicked (GtkButton *button, gpointer user_data);
 
 /* Local prototypes */
-static void
+static void gnc_gen_trans_assign_transfer_account(
-refresh_model_row(GNCImportMainMatcher *gui, GtkTreeModel *model,
+                                                GtkTreeView *treeview, 
+                                                gboolean *first,
+                                                gboolean *is_selection,
+                                                GtkTreePath *path,
+                                                Account **new_acc,
+                                                GNCImportMainMatcher *info);
+static void gnc_gen_trans_assign_transfer_account_to_selection_cb (
+                                          GtkMenuItem *menuitem,
+                                          GNCImportMainMatcher *info);
+static void gnc_gen_trans_view_popup_menu (GtkTreeView *treeview, 
+                                          GdkEvent *event,
+                                          GNCImportMainMatcher *info);
+static gboolean gnc_gen_trans_onButtonPressed_cb (GtkTreeView *treeview,
+                                                  GdkEvent *event,
+                                                  GNCImportMainMatcher *info);
+static gboolean gnc_gen_trans_onPopupMenu_cb (GtkTreeView *treeview,
+                                              GdkEvent *event, 
+                                              GNCImportMainMatcher *info);
+static void refresh_model_row(GNCImportMainMatcher *gui, GtkTreeModel *model,
                   GtkTreeIter *iter, GNCImportTransInfo *info);
+/* end local prototypes */
 
 void gnc_gen_trans_list_delete (GNCImportMainMatcher *info)
 {
@@ -361,54 +380,238 @@ gnc_gen_trans_update_toggled_cb (GtkCellRendererToggle *cell_renderer,
 }
 
 static void
-gnc_gen_trans_row_activated_cb (GtkTreeView           *view,
+gnc_gen_trans_assign_transfer_account(GtkTreeView *treeview, 
-                                GtkTreePath           *path,
+                                      gboolean *first,
-                                GtkTreeViewColumn     *column,
+                                      gboolean *is_selection,
-                                GNCImportMainMatcher  *gui)
+                                      GtkTreePath *path,
+                                      Account **new_acc,
+                                      GNCImportMainMatcher *info)
 {
     GtkTreeModel *model;
     GtkTreeIter iter;
     GNCImportTransInfo *trans_info;
+    Account *old_acc;
+    gboolean ok_pressed;
 
-    model = gtk_tree_view_get_model(gui->view);
+    ENTER("");
-    if (!gtk_tree_model_get_iter(model, &iter, path))
+    DEBUG("first = %s",*first?"true":"false");
-        return;
+    DEBUG("is_selection = %s",*is_selection?"true":"false");
-    gtk_tree_model_get(model, &iter, DOWNLOADED_COL_DATA, &trans_info, -1);
+    DEBUG("path  = %s", gtk_tree_path_to_string(path));
-
+    DEBUG("account passed in = %s", gnc_get_account_name_for_register(*new_acc));
-    switch (gnc_import_TransInfo_get_action (trans_info))
+    model = gtk_tree_view_get_model(treeview);
+    if (gtk_tree_model_get_iter(model, &iter, path))
     {
-    case GNCImport_ADD:
+        gtk_tree_model_get(model, &iter, DOWNLOADED_COL_DATA, &trans_info, -1);
-        if (gnc_import_TransInfo_is_balanced(trans_info) == FALSE)
+
+        switch (gnc_import_TransInfo_get_action (trans_info))
         {
-            run_account_picker_dialog (gui, model, &iter, trans_info);
+        case GNCImport_ADD:
+            if (gnc_import_TransInfo_is_balanced(trans_info) == FALSE)
+            {
+                ok_pressed = TRUE;
+                old_acc  = gnc_import_TransInfo_get_destacc (trans_info);
+                if (*first)
+                {
+                    ok_pressed = FALSE;
+                    *new_acc = gnc_import_select_account(info->main_widget,
+                        NULL,
+                        TRUE,
+                        _("Destination account for the auto-balance split."),
+                        xaccTransGetCurrency(
+                              gnc_import_TransInfo_get_trans(trans_info)),
+                        ACCT_TYPE_NONE,
+                        old_acc,
+                        &ok_pressed);
+                    *first = FALSE;
+                    DEBUG("account selected = %s", 
+                            gnc_account_get_full_name(*new_acc));                    
+                }
+                if (ok_pressed)
+                        gnc_import_TransInfo_set_destacc (trans_info, 
+                                                    *new_acc, TRUE);
+            }
+            break;
+        case GNCImport_CLEAR:
+        case GNCImport_UPDATE:
+            if (first && !is_selection) run_match_dialog (info, trans_info);
+            break;
+        case GNCImport_SKIP:        
+            break;
+        default:
+            PERR("InvalidGNCImportValue");
+            break;
         }
-        break;
+        refresh_model_row(info, model, &iter, trans_info);
-    case GNCImport_CLEAR:
-    case GNCImport_UPDATE:
-        run_match_dialog (gui, trans_info);
-        break;
-    case GNCImport_SKIP:
-        /*The information displayed is only informative, until you select an action*/
-        break;
-    default:
-        PERR("I don't know what to do! (Yet...)");
-        break;
     }
-    refresh_model_row(gui, model, &iter, trans_info);
+    LEAVE("");
+}
+
+static void
+gnc_gen_trans_assign_transfer_account_to_selection_cb (GtkMenuItem *menuitem,
+                                             GNCImportMainMatcher *info)
+{
+    GtkTreeView *treeview;
+    GtkTreeSelection *selection;
+    GtkTreeModel *model;
+    GtkTreeIter iter;
+    GNCImportTransInfo *trans_info;
+    Account *assigned_account;
+    GList *selected_rows, *l;
+    gboolean first, is_selection;
+
+    ENTER("");
+    treeview = GTK_TREE_VIEW(info->view);
+    model = gtk_tree_view_get_model(treeview);
+    selection = gtk_tree_view_get_selection(treeview);
+    selected_rows = gtk_tree_selection_get_selected_rows (selection, &model);
+    assigned_account = NULL;
+    first = TRUE;
+    is_selection = TRUE;    
+    DEBUG("Rows in selection = %i",
+          gtk_tree_selection_count_selected_rows(selection));
+    DEBUG("Entering loop over selection");
+
+    for (l = selected_rows; l != NULL; l = l->next)
+    {
+        DEBUG("passing first = %s",
+                                            first?"true":"false");
+        DEBUG("passing is_selection = %s",
+                                            is_selection?"true":"false");
+        DEBUG("passing path = %s",
+                       gtk_tree_path_to_string(l->data));
+        DEBUG("passing account value = %s",
+                    gnc_account_get_full_name(assigned_account));
+        gnc_gen_trans_assign_transfer_account(treeview,
+                       &first, &is_selection, l->data, &assigned_account, info);
+        DEBUG("returned value of account = %s", 
+                    gnc_account_get_full_name(assigned_account));
+        DEBUG("returned value of first = %s",first?"true":"false");
+        if (assigned_account == NULL) break;
+        gtk_tree_selection_unselect_path(selection, l->data);
+    }
+
+    g_list_free_full (selected_rows, (GDestroyNotify) gtk_tree_path_free);
+    g_list_free_full(l, (GDestroyNotify) gtk_tree_path_free);
+    LEAVE("");
+}
+
+static void
+gnc_gen_trans_row_activated_cb (GtkTreeView *treeview,
+                                GtkTreePath *path,
+                                GtkTreeViewColumn *column,
+                                GNCImportMainMatcher *info)
+{
+    Account *assigned_account;
+    gboolean first, is_selection;
+
+    ENTER("");
+    assigned_account = NULL;
+    first = TRUE;
+    is_selection = FALSE;
+    gnc_gen_trans_assign_transfer_account(treeview, 
+                            &first,  &is_selection, path,
+                            &assigned_account, info);
+    DEBUG("account returned = %s", gnc_account_get_full_name(assigned_account));
+    LEAVE("");
 }
 
 static void
 gnc_gen_trans_row_changed_cb (GtkTreeSelection *selection,
-                              GNCImportMainMatcher *gui)
+                              GNCImportMainMatcher *info)
 {
     GtkTreeModel *model;
     GtkTreeIter iter;
+    GtkSelectionMode mode;
 
-    if (!gtk_tree_selection_get_selected(selection, &model, &iter))
+    ENTER("");
-        return;
+    mode = gtk_tree_selection_get_mode(selection);
-    gtk_tree_selection_unselect_iter(selection, &iter);
+    switch (mode)
+    {
+        case GTK_SELECTION_MULTIPLE:
+            DEBUG("mode = GTK_SELECTION_MULTIPLE, no action");
+            break;
+        case GTK_SELECTION_NONE:
+            DEBUG("mode = GTK_SELECTION_NONE, no action");
+            break;
+        case GTK_SELECTION_BROWSE:
+            DEBUG("mode = GTK_SELECTION_BROWSE->default");
+        case GTK_SELECTION_SINGLE:
+            DEBUG("mode = GTK_SELECTION_SINGLE->default");
+        default:
+            DEBUG("mode = default unselect selected row");
+            if (gtk_tree_selection_get_selected(selection, &model, &iter))
+            {
+                gtk_tree_selection_unselect_iter(selection, &iter);
+            }
+    }
+    LEAVE("");
 }
 
+static void
+gnc_gen_trans_view_popup_menu (GtkTreeView *treeview, 
+                               GdkEvent *event,
+                               GNCImportMainMatcher *info)
+{    
+    GtkWidget *menu, *menuitem;
+    GdkEventButton *event_button;
+
+    ENTER ("");
+    menu = gtk_menu_new();
+    menuitem = gtk_menu_item_new_with_label(_("Assign a transfer account."));
+    g_signal_connect(menuitem, "activate",
+                     G_CALLBACK(
+                     gnc_gen_trans_assign_transfer_account_to_selection_cb),
+                     info);
+    DEBUG("Callback to assign destination account to selection connected");
+    gtk_menu_shell_append(GTK_MENU_SHELL(menu), menuitem);
+    gtk_widget_show_all(menu);
+    event_button = (GdkEventButton *) event;
+    /* Note: event can be NULL here when called from view_onPopupMenu;
+     *  gdk_event_get_time() accepts a NULL argument */
+    gtk_menu_popup(GTK_MENU(menu), NULL, NULL, NULL, NULL, 
+                    (event_button != NULL) ? event_button->button : 0, 
+                    gdk_event_get_time((GdkEvent*)event));
+    LEAVE ("");
+}
+
+static gboolean
+gnc_gen_trans_onButtonPressed_cb (GtkTreeView *treeview,
+                                  GdkEvent *event,
+                                  GNCImportMainMatcher *info)
+{
+    GdkEventButton *event_button;
+    ENTER("");
+    g_return_val_if_fail (treeview != NULL, FALSE);
+    g_return_val_if_fail (event != NULL, FALSE);
+    /* handle single click with the right mouse button? */
+    if (event->type == GDK_BUTTON_PRESS)
+    {
+        event_button = (GdkEventButton *) event;
+        if (event_button->button == GDK_BUTTON_SECONDARY)
+        {
+            DEBUG("Right mouseClick detected- popup the menu.");
+            gnc_gen_trans_view_popup_menu(treeview, event, info);
+            LEAVE("return TRUE");
+            return TRUE;
+        }
+    }
+    LEAVE("return FALSE");
+    return FALSE;
+}
+
+static gboolean
+gnc_gen_trans_onPopupMenu_cb (GtkTreeView *treeview,
+                              GdkEvent *event, 
+                              GNCImportMainMatcher *info)
+{
+    ENTER("");
+    /* respond to Shift-F10 popup menu hotkey */
+    gnc_gen_trans_view_popup_menu(treeview, NULL, info);
+    LEAVE ("");
+    return TRUE;
+}
+
+
 static GtkTreeViewColumn *
 add_text_column(GtkTreeView *view, const gchar *title, int col_num)
 {
@@ -536,6 +739,11 @@ gnc_gen_trans_init_view (GNCImportMainMatcher *info,
                      G_CALLBACK(gnc_gen_trans_row_activated_cb), info);
     g_signal_connect(selection, "changed",
                      G_CALLBACK(gnc_gen_trans_row_changed_cb), info);
+
+    g_signal_connect(view, "button-press-event", 
+                     G_CALLBACK(gnc_gen_trans_onButtonPressed_cb), info);
+    g_signal_connect(view, "popup-menu", 
+                     G_CALLBACK(gnc_gen_trans_onPopupMenu_cb), info);
 }
 
 static void

libgnucash/app-utils/fin.scm

@@ -182,3 +182,171 @@
     )
   )
 )
+
+;; Further options to match what some (several? many?) lenders do (at
+;;  least in Canada):
+;; The posted interest rate is an annual rate that has a specified
+;;  compounding frequency per year (2 for mortgages in Canada).
+;; A payment frequency and amortization length are selected (e.g.
+;;  monthly payments for 25 years).
+;; The posted nominal rate is converted from the specified compounding
+;;  frequency to the equivalent rate at the payment frequency.
+;; The required payment is calculated.
+;; The payment is rounded up to the next dollar (or $10 dollars,
+;;  or whatever...)
+;; Each payment period, interest is calculated on the outstanding
+;;  balance.
+;; The interest is rounded to the nearest cent and added to the
+;;  balance.
+;; The payment is subtracted from the balance.
+;; The final payment will be smaller because all the other payments
+;;  were rounded up.
+;;
+;; For the purpose of creating scheduled transactions that properly
+;;  debit a source account while crediting the loan account and the
+;;  interest expense account, the first part (the calculation of the
+;;  required payment) doesn't really matter.  You have agreed
+;;  with the lender what the payment terms (interest rate, payment
+;;  frequency, payment amount) will be; you keep paying until the
+;;  balance is zero.
+;;
+;; To create the scheduled transactions, we need to build an
+;;  amortization table.
+;; If it weren't for the rounding of the interest to the nearest cent
+;;  each period, we could calculate the ith row of the amortization
+;;  table directly from the general annuity equation (as is done by
+;;  gnc:ipmt and gnc:ppmt).  But to deal with the intermediate
+;;  rounding, the amortization table has to be constructed iteratively
+;;  (as is done by the AMORT worksheet on the TI BA II Plus
+;;  financial calculator).
+;;
+;; =================================
+;; EXAMPLE:
+;; Say you borrow $100,000 at 5%/yr, compounded semi-annually.
+;; You amortize the loan over 2 years with 24 monthly payments.
+;; This calls for payments of $4,384.8418 at the end of each month.
+;; The lender rounds this up to $4,385.
+;;
+;; If you calculate the balance at each period directly using the annuity
+;; formula (like calc-principal does), and then use the those values to calculate
+;; the principal and interest paid, the first 10 rows of the amortization table
+;; look like this (the values are rounded to the nearest cent for _display_, but
+;; not for calculating the next period):
+;;
+;; PERIOD | Open       | Interest | Principal | End
+;;    1   |$100,000.00 |  $412.39 | $3,972.61 | $96,027.39
+;;    2   | $96,027.39 |  $396.01 | $3,988.99 | $92,038.40 
+;;    3   | $92,038.40 |  $379.56 | $4,005.44 | $88,032.96 
+;;    4   | $88,032.96 |  $363.04 | $4,021.96 | $84,011.00 
+;;    5   | $84,011.00 |  $346.45 | $4,038.55 | $79,972.45 
+;;    6   | $79,972.45 |  $329.80 | $4,055.20 | $75,917.25 
+;;    7   | $75,917.25 |  $313.08 | $4,071.92 | $71,845.33 
+;;    8   | $71,845.33 |  $296.28 | $4,088.72 | $67,756.61 
+;;    9   | $67,756.61 |  $279.43 | $4,105.57 | $63,651.04 
+;;   10   | $63,651.04 |  $262.49 | $4,122.51 | $59,528.53
+;;
+;; If you calculate each period sequentially (rounding the interest and balance
+;; at each step), you get:
+;;
+;; PERIOD | Open       | Interest | Principal | End
+;;    1   |$100,000.00 |  $412.39 | $3,972.61 | $96,027.39
+;;    2   | $96,027.39 |  $396.01 | $3,988.99 | $92,038.40 
+;;    3   | $92,038.40 |  $379.56 | $4,005.44 | $88,032.96 
+;;    4   | $88,032.96 |  $363.04 | $4,021.96 | $84,011.00 
+;;    5   | $84,011.00 |  $346.45 | $4,038.55 | $79,972.45 
+;;    6   | $79,972.45 |  $329.80 | $4,055.20 | $75,917.25 
+;;    7   | $75,917.25 |  $313.08 | $4,071.92 | $71,845.33 
+;;    8   | $71,845.33 |  $296.28 | $4,088.72 | $67,756.61 
+;;    9   | $67,756.61 |  $279.42 | $4,105.58 | $63,651.03 <- Different 
+;;   10   | $63,651.03 |  $262.49 | $4,122.51 | $59,528.52 <- still $0.01 off
+;;
+;; =================================
+;;
+;; For the following functions the argument names are:
+;;  py: payment frequency (number of payments per year)
+;;  cy: compounding frequency of the nominal rate (per year)
+;;  iy: nominal annual interest rate
+;;  pv: the present value (opening balance)
+;;  pmt: the size of the periodic payment
+;;  n: the payment period we are asking about (the first payment is n=1)
+;;  places: number of decimal places to round the interest amount to 
+;;          at each payment (999 does no rounding)
+;;
+;; Note: only ordinary annuities are supported (payments at the end of
+;;  each period, not at the beginning of each period)
+;;
+;; Unlike the AMORT worksheet on the BA II Plus, these methods will
+;;  handle the smaller payment (bringing the balance to zero, then
+;;  zeroing future payments)
+;;
+;; The present value (pv) must be non-negative.  If not, the balance will be
+;;  treated as 0.
+;; The payment (pmt) can be positive (paying interest, and hopefully
+;;  reducing the balance each payment), or negative (increasing the balance
+;   each payment).
+;; The payment number (n) must be positive for amort_pmt, amort_ppmt, and
+;;  amort_ipmt.  I.e., the first payment is payment 1.
+;; The payment number (n) must be non-negative for amort_balance.  (In this
+;;  case, payment zero is at the _beginning_ of the first period, so
+;;  amort_balance will just be the initial balance.)
+;; If the above conditions on n are violated, the functions return -1 (#f is
+;;  not used, because it causes gnucash to crash).
+;;
+;; A negative interest rate works (if you can find a lender who charges
+;;  negative rates), but negative compounding frequency, or negative payment
+;;  frequency is a bad idea.
+
+;; Calculate the balance remaining after the nth payment
+;; (n must be greater than or equal to zero)
+(define (gnc:amort_pmt py cy iy pv pmt n places)
+  (if (< n 1) -1 ;; Returning #f here causes gnucash to crash on startup
+    (let* ((prevBal (gnc:amort_balance py cy iy pv pmt (- n 1) places))
+           (balBeforePayment
+		     (amort_balanceAfterInterest prevBal py cy iy places))
+           (balAfterPayment (amort_balanceAfterPayment balBeforePayment pmt)))
+      (- balBeforePayment balAfterPayment))))
+
+;; Calculate the amount of the nth payment that is principal
+;; (n must be greater than zero)
+(define (gnc:amort_ppmt py cy iy pv pmt n places)
+  (if (< n 1) -1
+    (let* ((prevBal (gnc:amort_balance py cy iy pv pmt (- n 1) places))
+	       (bal-after-int (amort_balanceAfterInterest prevBal py cy iy places))
+           (newBal (amort_balanceAfterPayment bal-after-int pmt)))
+      (- prevBal newBal))))
+
+;; Calculate the amount of the nth payment that is interest
+;; (n must be greater than zero)
+(define (gnc:amort_ipmt py cy iy pv pmt n places)
+  (if (< n 1) -1
+    (let* ((prevBal(gnc:amort_balance py cy iy pv pmt (- n 1) places)))
+      (amort_interest prevBal py cy iy places))))
+
+;; "Private" helper functions:
+
+;; Calculate the amount of interest on the current balance,
+;; rounded to the specified number of decimal places
+(define (amort_interest balance py cy iy places)
+  (roundToPlaces (* balance (gnc:periodic_rate iy py cy)) places)
+)
+
+;; Calculate the new balance after applying the interest, but before
+;; applying the payment
+(define (amort_balanceAfterInterest prevBalance py cy iy places)
+  (+ prevBalance (amort_interest prevBalance py cy iy places))
+)
+
+;; Apply the payment to the balance (after the interest has been
+;; added), without letting the balance go below zero.
+(define (amort_balanceAfterPayment balanceBeforePmt pmt)
+  (max 0 (- balanceBeforePmt pmt))
+)
+
+;; Round the value to the specified number of decimal places.
+;; 999 places means no rounding (#f is not used, because only numbers can be
+;; entered in the scheduled transaction editor)
+(define (roundToPlaces value places)
+  (if (= places 999) value
+    (/ (round (* value (expt 10 places))) (expt 10 places))
+  )
+)