Add support for gamepad mapping input modifiers

This adds support for the + and - and ~ input modifiers for joystick
axes.  It also changes how joystick axes are translated to buttons to
more closely match SDL 2.0.7.

Output modifiers are still not supported but have not yet been seen in
the wild.

docs/input.dox

@@ -804,6 +804,12 @@ a hat bitmask or empty.  Joystick buttons are specified as `bN`, for example
 example `h0.8` for left on the first hat.  More than one bit may be set in the
 mask.
 
+Before an axis there may be a `+` or `-` range modifier, for example `+a3` for
+the positive half of the fourth axis.  This restricts input to only the positive
+or negative halves of the joystick axis.  After an axis or half-axis there may
+be the `~` inversion modifier, for example `a2~` or `-a7~`.  This negates the
+values of the gamepad axis.
+
 The hat bit mask match the [hat states](@ref hat_state) in the joystick
 functions.
 
@@ -822,8 +828,9 @@ rightstick:b9,leftx:a0,lefty:a1,rightx:a2,righty:a3,lefttrigger:a4,
 righttrigger:a5,dpup:h0.1,dpright:h0.2,dpdown:h0.4,dpleft:h0.8,
 @endcode
 
-@note GLFW does not yet support the range and inversion modifiers `+`, `-` and
+@note GLFW does not yet support the output range and modifiers `+` and `-` that
-`~` that were recently added to SDL.
+were recently added to SDL.  The input modifiers `+`, `-` and `~` are supported
+and described above.
 
 
 @section time Time input

src/input.c

@@ -167,6 +167,10 @@ static GLFWbool parseMapping(_GLFWmapping* mapping, const char* string)
 
     while (*c)
     {
+        // TODO: Implement output modifiers
+        if (*c == '+' || *c == '-')
+            return GLFW_FALSE;
+
         for (i = 0;  i < sizeof(fields) / sizeof(fields[0]);  i++)
         {
             length = strlen(fields[i].name);
@@ -177,23 +181,50 @@ static GLFWbool parseMapping(_GLFWmapping* mapping, const char* string)
 
             if (fields[i].element)
             {
+                _GLFWmapelement* e = fields[i].element;
+                int8_t minimum = -1;
+                int8_t maximum = 1;
+
+                if (*c == '+')
+                {
+                    minimum = 0;
+                    c += 1;
+                }
+                else if (*c == '-')
+                {
+                    maximum = 0;
+                    c += 1;
+                }
+
                 if (*c == 'a')
-                    fields[i].element->type = _GLFW_JOYSTICK_AXIS;
+                    e->type = _GLFW_JOYSTICK_AXIS;
                 else if (*c == 'b')
-                    fields[i].element->type = _GLFW_JOYSTICK_BUTTON;
+                    e->type = _GLFW_JOYSTICK_BUTTON;
                 else if (*c == 'h')
-                    fields[i].element->type = _GLFW_JOYSTICK_HATBIT;
+                    e->type = _GLFW_JOYSTICK_HATBIT;
                 else
                     break;
 
-                if (fields[i].element->type == _GLFW_JOYSTICK_HATBIT)
+                if (e->type == _GLFW_JOYSTICK_HATBIT)
                 {
                     const unsigned long hat = strtoul(c + 1, (char**) &c, 10);
                     const unsigned long bit = strtoul(c + 1, (char**) &c, 10);
-                    fields[i].element->value = (uint8_t) ((hat << 4) | bit);
+                    e->value = (uint8_t) ((hat << 4) | bit);
                 }
                 else
-                    fields[i].element->value = (uint8_t) strtoul(c + 1, (char**) &c, 10);
+                    e->value = (uint8_t) strtoul(c + 1, (char**) &c, 10);
+
+                if (e->type == _GLFW_JOYSTICK_AXIS)
+                {
+                    e->axisScale = 2 / (maximum - minimum);
+                    e->axisOffset = -(maximum + minimum);
+
+                    if (*c == '~')
+                    {
+                        e->axisScale = -e->axisScale;
+                        e->axisOffset = -e->axisOffset;
+                    }
+                }
             }
             else
             {
@@ -1188,35 +1219,41 @@ GLFWAPI int glfwGetGamepadState(int jid, GLFWgamepadstate* state)
 
     for (i = 0;  i <= GLFW_GAMEPAD_BUTTON_LAST;  i++)
     {
-        if (js->mapping->buttons[i].type == _GLFW_JOYSTICK_AXIS)
+        const _GLFWmapelement* e = js->mapping->buttons + i;
+        if (e->type == _GLFW_JOYSTICK_AXIS)
         {
-            if (fabsf(js->axes[js->mapping->buttons[i].value]) > 0.5f)
+            const float value = js->axes[e->value] * e->axisScale + e->axisOffset;
+            if (value > 0.f)
                 state->buttons[i] = GLFW_PRESS;
         }
-        else if (js->mapping->buttons[i].type == _GLFW_JOYSTICK_HATBIT)
+        else if (e->type == _GLFW_JOYSTICK_HATBIT)
         {
-            const unsigned int hat = js->mapping->buttons[i].value >> 4;
+            const unsigned int hat = e->value >> 4;
-            const unsigned int bit = js->mapping->buttons[i].value & 0xf;
+            const unsigned int bit = e->value & 0xf;
             if (js->hats[hat] & bit)
                 state->buttons[i] = GLFW_PRESS;
         }
-        else if (js->mapping->buttons[i].type == _GLFW_JOYSTICK_BUTTON)
+        else if (e->type == _GLFW_JOYSTICK_BUTTON)
-            state->buttons[i] = js->buttons[js->mapping->buttons[i].value];
+            state->buttons[i] = js->buttons[e->value];
     }
 
     for (i = 0;  i <= GLFW_GAMEPAD_AXIS_LAST;  i++)
     {
-        if (js->mapping->axes[i].type == _GLFW_JOYSTICK_AXIS)
+        const _GLFWmapelement* e = js->mapping->axes + i;
-            state->axes[i] = js->axes[js->mapping->axes[i].value];
+        if (e->type == _GLFW_JOYSTICK_AXIS)
-        else if (js->mapping->buttons[i].type == _GLFW_JOYSTICK_HATBIT)
         {
-            const unsigned int hat = js->mapping->axes[i].value >> 4;
+            const float value = js->axes[e->value] * e->axisScale + e->axisOffset;
-            const unsigned int bit = js->mapping->axes[i].value & 0xf;
+            state->axes[i] = fminf(fmaxf(value, -1.f), 1.f);
+        }
+        else if (e->type == _GLFW_JOYSTICK_HATBIT)
+        {
+            const unsigned int hat = e->value >> 4;
+            const unsigned int bit = e->value & 0xf;
             if (js->hats[hat] & bit)
                 state->axes[i] = 1.f;
         }
-        else if (js->mapping->buttons[i].type == _GLFW_JOYSTICK_BUTTON)
+        else if (e->type == _GLFW_JOYSTICK_BUTTON)
-            state->axes[i] = (float) js->buttons[js->mapping->axes[i].value];
+            state->axes[i] = (float) js->buttons[e->value];
     }
 
     return GLFW_TRUE;

src/internal.h

@@ -457,6 +457,8 @@ struct _GLFWmapelement
 {
     uint8_t         type;
     uint8_t         value;
+    int8_t          axisScale;
+    int8_t          axisOffset;
 };
 
 // Gamepad mapping structure