Add menu_anim_t and easing helper functions

src/k_menu.h

@@ -178,6 +178,12 @@ struct menu_t
 	boolean		 (*inputroutine)(INT32); // if set, called every frame in the input handler. Returning true overwrites normal input handling.
 };
 
+struct menu_anim_t
+{
+	tic_t start;
+	INT16 dist;
+};
+
 typedef enum
 {
 	MM_NOTHING = 0, // is just displayed until the user do someting

src/k_menudraw.c

@@ -67,6 +67,9 @@
 // And just some randomness for the exits.
 #include "m_random.h"
 
+#include "i_time.h"
+#include "m_easing.h"
+
 #ifdef PC_DOS
 #include <stdio.h> // for snprintf
 int	snprintf(char *str, size_t n, const char *fmt, ...);
@@ -77,6 +80,29 @@ int	snprintf(char *str, size_t n, const char *fmt, ...);
 #include "discord.h"
 #endif
 
+static fixed_t M_TimeFrac(tic_t tics, tic_t duration)
+{
+	return tics < duration ? (tics * FRACUNIT + rendertimefrac) / duration : FRACUNIT;
+}
+
+static fixed_t M_ReverseTimeFrac(tic_t tics, tic_t duration)
+{
+	return FRACUNIT - M_TimeFrac(duration - tics, duration);
+}
+
+static fixed_t M_DueFrac(tic_t start, tic_t duration)
+{
+	tic_t t = I_GetTime();
+	tic_t n = t - start;
+	return M_TimeFrac(min(n, duration), duration);
+}
+
+// FIXME: C++ template
+#define M_EaseWithTransition(EasingFunc, N) \
+	(menutransition.tics != menutransition.dest ? EasingFunc(menutransition.in ?\
+		M_ReverseTimeFrac(menutransition.tics, menutransition.endmenu->transitionTics) :\
+		M_TimeFrac(menutransition.tics, menutransition.startmenu->transitionTics), 0, N) : 0)
+
 #define SKULLXOFF -32
 #define LINEHEIGHT 16
 #define STRINGHEIGHT 8

src/typedef.h

@@ -200,6 +200,7 @@ TYPEDEF (trackingResult_t);
 TYPEDEF (menucolor_t);
 TYPEDEF (menuitem_t);
 TYPEDEF (menu_t);
+TYPEDEF (menu_anim_t);
 TYPEDEF (menucmd_t);
 TYPEDEF (setup_player_colors_t);
 TYPEDEF (setup_player_t);