Player physics psuedocode

X player coordinate is the middle of the player horizontally
Y player coordinate is one pixel under the bottom of the player

Constants:
  BlockSize         : Width/Height of a map block

Variables:
  PlayerRunning     : Player is currently running
  PlayerOnGround    : Player is on the ground this frame
  PlayerWasOnGround : Player was on the ground last frame
  MaxSpeed          : Maximum allowed speed for walking/running
  PlayerPX          : Player X position
  PlayerPY          : Player Y position
  PlayerVX          : Player X speed
  PlayerVY          : Player Y speed

GetSlopeHeight(X, Y):
  BlockID = Level[X / BlockSize][Y / BlockSize]
  return the Y position corresponding to the highest point on the block, using X to select a column within the block
  (If the column is the maximum height, check if the block above is a slope, and use that block's highest point if so)

Player update():
  PlayerWasOnGround = PlayerOnGround
  PlayerOnGround = 0

  // Get the maximum horizontal speed
  If PlayerRunning:
    MaxSpeed = Run speed
  Else:
    MaxSpeed = Walk speed

  // Speed up if pressing left/right
  If pressing left:
    If PlayerVX > -MaxSpeed:
      PlayerVX -= Acceleration speed

  If pressing right:
    If PlayerVX < MaxSpeed:
      PlayerVX += Acceleration speed

  // Slow down if not pressing left/right
  Temp = Deceleration speed
  If pressing left:
    If PlayerVX < 0:
      goto SkipDeceleration
    Temp /= 2
  If pressing right:
    If PlayerVX > 0:
      goto SkipDeceleration
    Temp /= 2
  If PlayerVX > 0:
    PlayerVX -= Temp
    If PlayerVX < 0:
      PlayerVX = 0
  Else If PlayerVX < 0:
    PlayerVX += Temp
    If PlayerVX > 0:
      PlayerVX = 0
  SkipDeceleration:

  PlayerPX += PlayerVX

  // Going downhill requires extra help
  If PlayerWasOnGround && !JumpButtonPressed && PlayerVX != 0:
    If Level[PlayerPX / BlockSize][PlayerPY / BlockSize] is slope:
      PlayerPY = GetSlopeHeight(PlayerPX, PlayerPY)
      PlayerOnGround = True
      PlayerVY = 0
    Else If Level[PlayerPX / BlockSize][(PlayerPY+BlockSize) / BlockSize] is slope:
      PlayerPY = GetSlopeHeight(PlayerPX, PlayerPY + BlockSize)
      PlayerOnGround = True
      PlayerVY = 0

  // Interact with walls
  If PlayerVX >= 0:
    SideXPos = PlayerPX + 3 pixels
  If PlayerVX < 0:
    SideXPos = PlayerPX - 3 pixels

  TrySideInteraction(SideXPos, PlayerPY - 16 pixels)
  TrySideInteraction(SideXPos, PlayerPY - 23 pixels)

  // Vertical movement
  If PlayerVY < MaxGravity:
    PlayerVY += Gravity
  PlayerPY += PlayerVY

  // Original code allows canceling a jump here, for variable height jumps

  // Slope interaction
  If Level[PlayerPX / BlockSize][PlayerPY / BlockSize] is slope:
    Temp = GetSlopeHeight(PlayerPX, PlayerPY)
    If PlayerPY >= Temp:
      PlayerPY = Temp
      PlayerVY = 0
      PlayerOnGround = 1
      goto SkipGroundCheck

  // Collide with blocks on the bottom
  If PlayerVY >= 0:
    TryAboveInteraction(PlayerPX - 4 pixels, PlayerPY)
    TryAboveInteraction(PlayerPX + 3 pixels, PlayerPY)
  SkipGroundCheck:

  // Collide with blocks above the player
  TryBelowInteraction(PlayerPX, PlayerPY - 30 pixels)

  If PlayerOnGround:
    If JumpButtonPressed:
      PlayerVY = Jump velocity
    Update PlayerRunning for the current run button status

TryBelowInteraction(X, Y):
  If Level[X / BlockSize][Y / BlockSize] is solid on the bottom:
    PlayerVY = 0
    Snap PlayerPY to the bottom of the block

TrySideInteraction(X, Y):
  If Level[X / BlockSize][Y / BlockSize] is solid on the side:
    PlayerVX = 0
    Snap PlayerPX to the side of the block

TryAboveInteraction(X, Y):
  If Level[X / BlockSize][Y / BlockSize] is solid on the top:
    PlayerVY = 0
    Snap PlayerPY to the top of the block