add a "prev map" key binding

Fixes #2007

Author: fabiangreffrath

src/g_game.c

@@ -1160,6 +1160,53 @@ int G_GotoNextLevel(int *pEpi, int *pMap)
   return false;
 }
 
+int G_GotoPrevLevel(void)
+{
+    const int cur_epsd = gameepisode;
+    const int cur_map = gamemap;
+    int epsd_count, map_count;
+    int ret = false;
+
+    for (epsd_count = 0; epsd_count < 10; epsd_count++, gameepisode = (gameepisode + 9) % 10)
+    {
+        for (map_count= 0, gamemap--; map_count < 100; map_count++, gamemap = (gamemap + 99) % 100)
+        {
+            int next_epsd, next_map;
+            G_GotoNextLevel(&next_epsd, &next_map);
+
+            if (next_epsd == cur_epsd && next_map == cur_map &&
+                (gameepisode != cur_epsd || gamemap != cur_map))
+            {
+                char *name = MapName(gameepisode, gamemap);
+
+                if (W_CheckNumForName(name) != -1)
+                {
+                    G_DeferedInitNew(gameskill, gameepisode, gamemap);
+                    ret = true;
+                    break;
+                }
+            }
+        }
+
+        // only check one episode in Doom 2
+        if (gamemode == commercial)
+        {
+            break;
+        }
+    }
+
+    gameepisode = cur_epsd;
+    gamemap = cur_map;
+
+    if (ret == false)
+    {
+        char *name = MapName(gameepisode, gamemap);
+        displaymsg("Previous level not found for %s", name);
+    }
+
+    return ret;
+}
+
 static boolean G_StrictModeSkipEvent(event_t *ev)
 {
   static boolean enable_mouse = false;

src/g_game.h

@@ -84,6 +84,7 @@ demo_version_t G_GetNamedComplevel(const char *arg);
 const char *G_GetCurrentComplevelName(void);
 
 int G_GotoNextLevel(int *pEpi, int *pMap);
+int G_GotoPrevLevel(void);
 
 void G_BindGameInputVariables(void);
 void G_BindGameVariables(void);

src/m_input.c

@@ -702,6 +702,7 @@ void M_BindInputVariables(void)
 
     BIND_INPUT(input_menu_reloadlevel, "Restart current level/demo");
     BIND_INPUT(input_menu_nextlevel, "Go to next level");
+    BIND_INPUT(input_menu_prevlevel, "Go to previous level");
 
     BIND_INPUT(input_hud_timestats, "Toggle display of level stats and time");
 

src/m_input.h

@@ -68,6 +68,7 @@ enum
     input_menu_clear,
     input_menu_reloadlevel,
     input_menu_nextlevel,
+    input_menu_prevlevel,
 
     input_hud_timestats,
 

src/mn_menu.c

@@ -2608,6 +2608,18 @@ boolean M_ShortcutResponder(const event_t *ev)
         }
     }
 
+    if (M_InputActivated(input_menu_prevlevel))
+    {
+        if (demoplayback && singledemo && !PLAYBACK_SKIP)
+        {
+            return false;
+        }
+        else if (G_GotoPrevLevel())
+        {
+            return true;
+        }
+    }
+
     if (M_InputActivated(input_demo_fforward))
     {
         if (demoplayback && !PLAYBACK_SKIP && !fastdemo && !D_CheckNetConnect())