if(player was moving right) { 1. }
else { 2 }

if(player was moving up) { 3}
else { 4 }

if(player was moving right) { 1. }
else { 2 }

if(player was moving up) { 3}
else { 4 }

so we have

Vector A; // velocity before

//apply gravity
//apply movement from input
//apply any other physics

Vector B; // velocity after

Vector2 vel = B - A;

//Terrain collision check
if(vel.X > 0) { wall.Left - player.Width }
else { player.X = wall.Right }

if(vel.Y > 0) { player.Y = wall.Bottom }
else { player.Y = wall.Bottom - player.Height }

Vector A; // velocity before

//apply gravity
//apply movement from input
//apply any other physics

Vector B; // velocity after

Vector2 vel = B - A;

//Terrain collision check
if(vel.X > 0) { player.X = wall.Left - player.Width }
if(vel.X < 0) { player.X = wall.Right }
if(vel.Y > 0) { player.Y = wall.Bottom }
if(vel.Y < 0) { player.Y = wall.Bottom - player.Height }

so we have

Vector A; // position of the player at the beggining of the frame

//apply gravity
//apply movement from input
//apply any other physics

Vector B; // position of the player after gravity / input / etc is applied

Vector2 vel = B - A;

//Terrain collision check
if(vel.X > 0) { player.X = wall.Left - player.Width }
else if(vel.X < 0) { player.X = wall.Right }
if(vel.Y > 0) { player.Y = wall.Bottom }
else if(vel.Y < 0) { player.Y = wall.Bottom - player.Height }

awesome