Implement fast falling (#44)

Reviewed-on: #44
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/state/air.rs

@@ -2,10 +2,44 @@ use crate::fighter::state::ground::GroundedStateCommon;
 use crate::fighter::state::land::{LandingState, LandingType};
 use crate::fighter::state::{CollisionContext, FighterState};
 use crate::fighter::*;
+use crate::input::FighterCommands;
 
-pub fn integrate_gravity(velocity: &mut FighterVelocity, attributes: &FighterAttributes) {
+#[derive(Default, Debug, Clone, Copy, Hash)]
+pub struct AirborneStateCommon {
+    pub is_fast_falling: bool,
+}
+
+pub fn process_fast_fall(
+    airborne_state_common: &mut AirborneStateCommon,
+    input: &mut FighterInput,
+    velocity: &mut FighterVelocity,
+    attributes: &FighterAttributes,
+) {
+    if velocity.y.is_negative() {
+        if input.has_command(FighterCommands::SmashMoveDown) {
+            airborne_state_common.is_fast_falling = true;
+        }
+        input.clear_command(FighterCommands::SmashMoveDown);
+    }
+
+    if airborne_state_common.is_fast_falling {
+        velocity.y = -attributes.fast_fall_speed;
+    }
+}
+
+pub fn integrate_gravity(
+    velocity: &mut FighterVelocity,
+    attributes: &FighterAttributes,
+    fast_fall: bool
+) {
     velocity.y -= attributes.gravity;
-    velocity.y = velocity.y.max(-attributes.terminal_velocity);
+
+    let terminal_vel = if fast_fall {
+        -attributes.fast_fall_speed
+    } else {
+        -attributes.terminal_velocity
+    };
+    velocity.y = velocity.y.max(terminal_vel);
 }
 
 pub fn apply_air_drift(
@@ -56,8 +90,12 @@ pub fn apply_air_motion(
     translation.0 += velocity.0;
 }
 
-pub fn update_standard_motion(ctx: &mut crate::fighter::state::UpdateContext) {
-    integrate_gravity(ctx.velocity, ctx.fighter_attributes);
+pub fn update_standard_motion(
+    ctx: &mut crate::fighter::state::UpdateContext,
+    airborne_common: &mut AirborneStateCommon,
+) {
+    process_fast_fall(airborne_common, ctx.input, &mut ctx.velocity, ctx.fighter_attributes);
+    integrate_gravity(ctx.velocity, ctx.fighter_attributes, airborne_common.is_fast_falling);
     apply_air_drift(ctx.velocity, ctx.fighter_attributes, ctx.input);
     apply_air_motion(ctx.velocity, ctx.translation, ctx.prev_translation);
 }
@@ -110,6 +148,7 @@ mod tests {
             air_drift_max: FGi32::lit("1.2"),
             ground_friction: FGi32::ZERO,
             terminal_velocity: FGi32::ZERO,
+            fast_fall_speed: FGi32::ZERO,
             gravity: FGi32::ZERO,
             weight: FGi32::lit("1"),
             air_friction: FGi32::lit("0.001"),