Provide new function gnc_price_invert.

To invert a price when it's currency is what we want to use as a commodity.
Generalizes the source-type PRICE_SOURCE_INVOICE to PRICE_SOURCE_TEMPORARY
for all cases like this where we don't want to save the price.

src/backend/sql/gnc-price-sql.c

@@ -203,7 +203,7 @@ write_price( GNCPrice* p, gpointer data )
     g_return_val_if_fail( p != NULL, FALSE );
     g_return_val_if_fail( data != NULL, FALSE );
 
-    if ( s->is_ok && gnc_price_get_source(p) != PRICE_SOURCE_INVOICE)
+    if ( s->is_ok && gnc_price_get_source(p) != PRICE_SOURCE_TEMP)
     {
         s->is_ok = save_price( s->be, QOF_INSTANCE(p) );
     }

src/business/business-gnome/dialog-invoice.c

@@ -898,7 +898,7 @@ gnc_invoice_post(InvoiceWindow *iw, struct post_invoice_params *post_params)
             gnc_price_set_commodity (convprice, account_currency);
             gnc_price_set_currency (convprice, gncInvoiceGetCurrency (invoice));
             gnc_price_set_time (convprice, postdate);
-            gnc_price_set_source (convprice, PRICE_SOURCE_INVOICE);
+            gnc_price_set_source (convprice, PRICE_SOURCE_TEMP);
             gnc_price_set_typestr (convprice, PRICE_TYPE_LAST);
             gnc_price_set_value (convprice, exch_rate);
             gncInvoiceAddPrice(invoice, convprice);

src/engine/engine.i

@@ -382,7 +382,7 @@ KvpValue* qof_book_get_option (QofBook *book, GSList *key_path);
     SET_ENUM("PRICE-SOURCE-XFER-DLG-VAL");
     SET_ENUM("PRICE-SOURCE-SPLIT-REG");
     SET_ENUM("PRICE-SOURCE-STOCK-SPLIT");
-    SET_ENUM("PRICE-SOURCE-INVOICE");
+    SET_ENUM("PRICE-SOURCE-TEMP");
     SET_ENUM("PRICE-SOURCE-INVALID");
 
 #undef SET_ENUM

src/engine/gnc-pricedb.c

@@ -95,11 +95,10 @@ gnc_price_init(GNCPrice* price)
 
 /* Array of char constants for converting price-source enums. Be sure to keep in
  * sync with the enum values in gnc-pricedb.h The string user:price-editor is
- * explicitly used by price_to_gui() in dialog-price-editor.c and the string
+ * explicitly used by price_to_gui() in dialog-price-editor.c. Beware
- * Finance::Quote is explicitly used by fq-results->commod-tz-quote-triples in
- * price-quotes.scm. Take care to keep them in sync if you make changes. Beware
  * that the strings are used to store the enum values in the backends so any
  * changes will affect backward data compatibility.
+ * The last two values, temporary and invalid, are *not* used.
  */
 static const char* source_names[] =
 {
@@ -112,7 +111,7 @@ static const char* source_names[] =
     "user:xfer-dialog",
     "user:split-register",
     "user:stock-split",
-    "user:invoice-post",
+    "temporary",
     "invalid"
 };
 
@@ -383,6 +382,23 @@ gnc_price_clone (GNCPrice* p, QofBook *book)
     return(new_p);
 }
 
+GNCPrice *
+gnc_price_invert (GNCPrice *p)
+{
+    QofBook *book = qof_instance_get_book (QOF_INSTANCE(p));
+    GNCPrice *new_p = gnc_price_create (book);
+    qof_instance_copy_version(new_p, p);
+    gnc_price_begin_edit(new_p);
+    gnc_price_set_time(new_p, gnc_price_get_time(p));
+    gnc_price_set_source(new_p, PRICE_SOURCE_TEMP);
+    gnc_price_set_typestr(new_p, gnc_price_get_typestr(p));
+    gnc_price_set_commodity(new_p, gnc_price_get_currency(p));
+    gnc_price_set_currency(new_p, gnc_price_get_commodity(p));
+    gnc_price_set_value(new_p, gnc_numeric_invert(gnc_price_get_value(p)));
+    gnc_price_commit_edit(new_p);
+    return new_p;
+}
+
 /* ==================================================================== */
 
 void

src/engine/gnc-pricedb.h

@@ -169,7 +169,8 @@ typedef enum
     PRICE_SOURCE_SPLIT_REG,        // "user:split-register"
     PRICE_SOURCE_STOCK_SPLIT,      // "user:stock-split"
     PRICE_SOURCE_INVOICE,          // "user:invoice-post"
-    PRICE_SOURCE_INVALID,
+    PRICE_SOURCE_TEMP,             // "temporary"
+    PRICE_SOURCE_INVALID,          // "invalid"
 } PriceSource;
 
 #define PRICE_TYPE_LAST "last"
@@ -188,6 +189,17 @@ GNCPrice *gnc_price_create(QofBook *book);
    content-wise duplicate of the given price, p.  The returned clone
    will have a reference count of 1. */
 GNCPrice *gnc_price_clone(GNCPrice* p, QofBook *book);
+
+/** Return a newly-allocated price that's the inverse of the given price, p.
+ *
+ * Inverse means that the commodity and currency are swapped and the value is
+ * the numeric inverse of the original's. The source is set to PRICE_SOURCE_TEMP
+ * to prevent it being saved in the pricedb.
+ * @param p The price to invert
+ * @return a new price, with a ref-count of 1. Don't forget to unref it!
+ */
+GNCPrice *gnc_price_invert(GNCPrice *p);
+
 /**  @} */
 
 /* ------------------ */