Add u_buy_monster talk effect

doc/NPCs.md

@@ -449,6 +449,8 @@ Effect | Description
 `add_debt: mod_list` | Increases the NPC's debt to the player by the values in the `mod_list`.<br/>The following would increase the NPC's debt to the player by 1500x the NPC's altruism and 1000x the NPC's opinion of the player's value: `{ "effect": { "add_debt": [ [ "ALTRUISM", 3 ], [ "VALUE", 2 ], [ "TOTAL", 500 ] ] } }`
 `u_consume_item`, `npc_consume_item: item_string`, (*optional* `count: count_num`) | You or the NPC will delete the item or `count_num` copies of the item from their inventory.<br/>This effect will fail if the you or NPC does not have at least `count_num` copies of the item, so it should be checked with `u_has_items` or `npc_has_items`.
 `u_remove_item_with`, `npc_remove_item_with: item_string` | You or the NPC will delete any instances of item in inventory.<br/>This is an unconditional remove and will not fail if you or the NPC does not have the item.
+`u_buy_monster: monster_type_string`, (*optional* `cost: cost_num`, *optional* `count: count_num`, *optional* `name: name_string`, *optional* `pacified: pacified_bool`) | The NPC will give your character `count_num` (default 1) instances of the monster as pets and will subtract `cost_num` from `op_of_u.owed` if specified.  If the `op_o_u.owed` is less than `cost_num`, the trade window will open and the player will have to trade to make up the difference; the NPC will not give the player the item unless `cost_num` is satisfied.<br/>If cost isn't present, the NPC gives your character the item at no charge.<br/>If `name_string` is specified the monster(s) will have the specified name. If `pacified_bool` is set to true, the monster will have the pacified effect applied.
+
 
 #### Behaviour / AI
 

src/dialogue.h

@@ -121,6 +121,8 @@ struct talk_effect_fun_t {
         void set_bulk_trade_accept( bool is_trade, bool is_npc = false );
         void set_npc_gets_item( bool to_use );
         void set_add_mission( std::string mission_id );
+        void set_u_buy_monster( const std::string &monster_id, int cost, int count, bool pacified,
+                                const std::string &name );
 
         void operator()( const dialogue &d ) const {
             if( !function ) {

src/npctalk.cpp

@@ -32,6 +32,7 @@
 #include "json.h"
 #include "line.h"
 #include "map.h"
+#include "map_iterator.h"
 #include "mapgen_functions.h"
 #include "martialarts.h"
 #include "messages.h"
@@ -2097,6 +2098,55 @@ void talk_effect_fun_t::set_add_mission( const std::string mission_id )
     };
 }
 
+void talk_effect_fun_t::set_u_buy_monster( const std::string &monster_type_id, int cost, int count,
+        bool pacified, const std::string &name )
+{
+    function = [monster_type_id, cost, count, pacified, name]( const dialogue & d ) {
+        npc &p = *d.beta;
+        player &u = *d.alpha;
+        if( !npc_trading::pay_npc( p, cost ) ) {
+            popup( _( "You can't afford it!" ) );
+            return;
+        }
+
+        const mtype_id mtype( monster_type_id );
+        const efftype_id effect_pet( "pet" );
+        const efftype_id effect_pacified( "pacified" );
+        const tripoint_range points = g->m.points_in_radius( u.pos(), 3 );
+
+        for( int i = 0; i < count; i++ ) {
+            monster tmp( mtype );
+
+            // Our monster is always a pet.
+            tmp.friendly = -1;
+            tmp.add_effect( effect_pet, 1_turns, num_bp, true );
+
+            if( pacified ) {
+                tmp.add_effect( effect_pacified, 1_turns, num_bp, true );
+            }
+
+            if( !name.empty() ) {
+                tmp.unique_name = name;
+            }
+
+            if( const cata::optional<tripoint> pos = random_point( points, [&]( const tripoint & p ) {
+            return g->is_empty( p ) && tmp.can_move_to( p );
+            } ) ) {
+                tmp.spawn( *pos );
+                g->add_zombie( tmp );
+            } else {
+                add_msg( m_debug, "Cannot place u_buy_monster, no valid placement locations." );
+            }
+        }
+
+        if( name.empty() ) {
+            popup( _( "%1$s gives you %2$d %3$s." ), p.name, count, mtype.obj().nname( count ) );
+        } else {
+            popup( _( "%1$s gives you %2$s." ), p.name, name );
+        }
+    };
+}
+
 void talk_effect_t::set_effect_consequence( const talk_effect_fun_t &fun, dialogue_consequence con )
 {
     effects.push_back( fun );
@@ -2293,6 +2343,13 @@ void talk_effect_t::parse_sub_effect( JsonObject jo )
         subeffect_fun.set_npc_cbm_recharge_rule( setting );
     } else if( jo.has_member( "mapgen_update" ) ) {
         subeffect_fun.set_mapgen_update( jo, "mapgen_update" );
+    } else if( jo.has_string( "u_buy_monster" ) ) {
+        const std::string &monster_type_id = jo.get_string( "u_buy_monster" );
+        const int cost = jo.get_int( "cost", 0 );
+        const int count = jo.get_int( "count", 1 );
+        const bool pacified = jo.get_bool( "pacified", false );
+        const std::string &name = jo.get_string( "name", "" );
+        subeffect_fun.set_u_buy_monster( monster_type_id, cost, count, pacified, name );
     } else {
         jo.throw_error( "invalid sub effect syntax :" + jo.str() );
     }