bugfix_line_spell: prevent narrow line spells from being stopped early (

#40242)

* bugfix_line_spell: prevent narrow line spells from being stopped early

Line attack spells with narrow AOE (0 or 1) were being prevented from
actually working as intended. The exception added to the condition
prevents exiting early if the spell is a line attack.

* bugfix_line_spell: Split `spell_effect_area` for testability

Splits spell effect area calculation out of `spell_effect_area` to
allow independent testing without calling the drawing function
`explosion_handler::draw_custom_explosion`

* bugfix_line_spell: Generate test to ensure narrow beam spell consistency

src/magic_spell_effect.cpp

@@ -29,6 +29,7 @@
 #include "item.h"
 #include "line.h"
 #include "magic.h"
+#include "magic_spell_effect_helpers.h"
 #include "magic_teleporter_list.h"
 #include "magic_ter_furn_transform.h"
 #include "map.h"
@@ -351,12 +352,12 @@ std::set<tripoint> spell_effect::spell_effect_line( const spell &, const tripoin
 
 // spells do not reduce in damage the further away from the epicenter the targets are
 // rather they do their full damage in the entire area of effect
-static std::set<tripoint> spell_effect_area( const spell &sp, const tripoint &target,
+std::set<tripoint> calculate_spell_effect_area( const spell &sp, const tripoint &target,
         std::function<std::set<tripoint>( const spell &, const tripoint &, const tripoint &, int, bool )>
-        aoe_func, const Creature &caster, bool ignore_walls = false )
+        aoe_func, const Creature &caster, bool ignore_walls )
 {
     std::set<tripoint> targets = { target }; // initialize with epicenter
-    if( sp.aoe() <= 1 ) {
+    if( sp.aoe() <= 1 && sp.effect() != "line_attack" ) {
         return targets;
     }
 
@@ -371,6 +372,17 @@ static std::set<tripoint> spell_effect_area( const spell &sp, const tripoint &ta
         }
     }
 
+    return targets;
+}
+
+static std::set<tripoint> spell_effect_area( const spell &sp, const tripoint &target,
+        std::function<std::set<tripoint>( const spell &, const tripoint &, const tripoint &, int, bool )>
+        aoe_func, const Creature &caster, bool ignore_walls = false )
+{
+    // calculate spell's effect area
+    std::set<tripoint> targets = calculate_spell_effect_area( sp, target, aoe_func, caster,
+                                 ignore_walls );
+
     // Draw the explosion
     std::map<tripoint, nc_color> explosion_colors;
     for( auto &pt : targets ) {

src/magic_spell_effect_helpers.h

@@ -0,0 +1,18 @@
+#pragma once
+#ifndef CATA_SRC_MAGIC_SPELL_EFFECT_HELPERS_H
+#define CATA_SRC_MAGIC_SPELL_EFFECT_HELPERS_H
+
+#include <functional>
+#include <set>
+
+class Creature;
+class spell;
+struct tripoint;
+
+// spells do not reduce in damage the further away from the epicenter the targets are
+// rather they do their full damage in the entire area of effect
+std::set<tripoint> calculate_spell_effect_area( const spell &sp, const tripoint &target,
+        std::function<std::set<tripoint>( const spell &, const tripoint &, const tripoint &, int, bool )>
+        aoe_func, const Creature &caster, bool ignore_walls = false );
+
+#endif // CATA_SRC_MAGIC_SPELL_EFFECT_HELPERS_H

tests/magic_spell_effect_test.cpp

@@ -0,0 +1,49 @@
+#include <sstream>
+
+#include "catch/catch.hpp"
+#include "json.h"
+#include "magic.h"
+#include "magic_spell_effect_helpers.h"
+#include "npc.h"
+
+TEST_CASE( "line_attack", "[magic]" )
+{
+    // manually construct a testable spell
+    std::istringstream str(
+        "  {\n"
+        "    \"id\": \"test_line_spell\",\n"
+        "    \"name\": { \"str\": \"Test Line Spell\" },\n"
+        "    \"description\": \"Spews a line of magic\",\n"
+        "    \"valid_targets\": [ \"ground\" ],\n"
+        "    \"damage_type\": \"none\",\n"
+        "    \"min_range\": 5,\n"
+        "    \"max_range\": 5,\n"
+        "    \"effect\": \"line_attack\",\n"
+        "    \"min_aoe\": 0,\n"
+        "    \"max_aoe\": 0,\n"
+        "    \"flags\": [ \"VERBAL\", \"NO_HANDS\", \"NO_LEGS\" ]\n"
+        "  }\n" );
+
+    JsonIn in( str );
+    JsonObject obj( in );
+    spell_type::load_spell( obj, "" );
+
+    spell sp( spell_id( "test_line_spell" ) );
+
+    // set up Character to test with, only need position
+    npc c;
+    c.setpos( tripoint_zero );
+
+    // target point 5 tiles east of zero
+    tripoint target = tripoint_east * 5;
+
+    // Ensure that AOE=0 spell covers the 5 tiles along vector towards target
+    SECTION( "aoe=0" ) {
+        const std::set<tripoint> reference( { tripoint_east * 1, tripoint_east * 2, tripoint_east * 3, tripoint_east * 4, tripoint_east * 5 } );
+
+        std::set<tripoint> targets = calculate_spell_effect_area( sp, target,
+                                     spell_effect::spell_effect_line, c, true );
+
+        CHECK( reference == targets );
+    }
+}