Make the item iterator capable of iterating over a cargo list

New snxtitem_cargo() initializes an iterator for a cargo list, with
new enum ns_seltype member NS_GROUP and new struct nstr_item member
next.  Extend nxtitem() and nxtitemp() to step through the list.

include/nsc.h

@@ -144,7 +144,8 @@ enum ns_seltype {
     NS_AREA,			/* rectangular area */
     NS_ALL,			/* everything */
     NS_XY,			/* one sector area */
-    NS_GROUP			/* group, i.e. fleet, wing, army */
+    NS_GROUP,			/* group, i.e. fleet, wing, army */
+    NS_CARGO			/* loaded on the same carrier */
 };
 
 /* Sector iterator */
@@ -171,6 +172,7 @@ struct nstr_item {
     int dist;			/* NS_DIST: distance selector */
     coord cx, cy;		/* NS_DIST: center x-y, NS_XY: xy */
     char group;			/* NS_GROUP: fleet/wing match */
+    short next;			/* NS_CARGO: next item */
     int size;			/* NS_LIST: size of list */
     int index;			/* NS_LIST: index */
     int list[NS_LSIZE];		/* NS_LIST: item list */

include/prototypes.h

@@ -674,6 +674,7 @@ extern void snxtitem_all(struct nstr_item *, int);
 extern void snxtitem_group(struct nstr_item *, int, char);
 extern void snxtitem_rewind(struct nstr_item *);
 extern int snxtitem_list(struct nstr_item *, int, int *, int);
+extern void snxtitem_cargo(struct nstr_item *, int, int, int);
 /* snxtsct.c */
 extern int snxtsct(struct nstr_sect *, char *);
 extern void snxtsct_area(struct nstr_sect *, struct range *);

src/lib/subs/nxtitem.c

@@ -41,6 +41,7 @@
 #include "player.h"
 #include "prototypes.h"
 #include "ship.h"
+#include "unit.h"
 #include "xy.h"
 
 int
@@ -58,6 +59,11 @@ nxtitem(struct nstr_item *np, void *ptr)
 	    if (np->index >= np->size)
 		return 0;
 	    np->cur = np->list[np->index];
+	} else if (np->sel == NS_CARGO) {
+	    if (np->next < 0)
+		return 0;
+	    np->cur = np->next;
+	    np->next = unit_cargo_next(np->type, np->next);
 	} else {
 	    np->cur++;
 	}
@@ -66,7 +72,7 @@ nxtitem(struct nstr_item *np, void *ptr)
 	selected = 1;
 	switch (np->sel) {
 	case NS_LIST:
-	    break;
+	case NS_CARGO:
 	case NS_ALL:
 	    break;
 	case NS_DIST:

src/lib/subs/snxtitem.c

@@ -41,6 +41,7 @@
 #include "nsc.h"
 #include "file.h"
 #include "prototypes.h"
+#include "unit.h"
 
 /*
  * setup the nstr structure for sector selection.
@@ -222,3 +223,21 @@ snxtitem_list(struct nstr_item *np, int type, int *list, int len)
     np->size = len;
     return 1;
 }
+
+/*
+ * Initialize NP to iterate over the items of type TYPE in a carrier.
+ * The carrier has file type CARRIER_TYPE and uid CARRIER_UID.
+ * Note: you can take an item gotten with nxtitem() off its carrier
+ * without disturbing the iterator.  Whether the iterator will pick up
+ * stuff you load onto the carrier during iteration is unspecified.
+ */
+void
+snxtitem_cargo(struct nstr_item *np, int type,
+		int carrier_type, int carrier_uid)
+{
+    memset(np, 0, sizeof(*np));
+    np->cur = -1;
+    np->type = type;
+    np->sel = NS_CARGO;
+    np->next = unit_cargo_first(carrier_type, carrier_uid, type);
+}

src/lib/update/nxtitemp.c

@@ -37,6 +37,7 @@
 #include "land.h"
 #include "nsc.h"
 #include "ship.h"
+#include "unit.h"
 #include "update.h"
 
 void *
@@ -53,6 +54,11 @@ nxtitemp(struct nstr_item *np)
 	    if (np->index >= np->size)
 		return 0;
 	    np->cur = np->list[np->index];
+	} else if (np->sel == NS_CARGO) {
+	    if (np->next < 0)
+		return 0;
+	    np->cur = np->next;
+	    np->next = unit_cargo_next(np->type, np->next);
 	} else {
 	    np->cur++;
 	}
@@ -63,7 +69,7 @@ nxtitemp(struct nstr_item *np)
 	selected = 1;
 	switch (np->sel) {
 	case NS_LIST:
-	    break;
+	case NS_CARGO:
 	case NS_ALL:
 	    break;
 	case NS_DIST: