doodle/pkg/collision/collide_level.go

package collision

import (
	"sync"

	"git.kirsle.net/apps/doodle/lib/render"
	"git.kirsle.net/apps/doodle/pkg/doodads"
	"git.kirsle.net/apps/doodle/pkg/level"
)

// Collide describes how a collision occurred.
type Collide struct {
	Top         bool
	TopPoint    render.Point
	TopPixel    *level.Swatch
	Left        bool
	LeftPoint   render.Point
	LeftPixel   *level.Swatch
	Right       bool
	RightPoint  render.Point
	RightPixel  *level.Swatch
	Bottom      bool
	BottomPoint render.Point
	BottomPixel *level.Swatch
	MoveTo      render.Point
}

// Reset a Collide struct flipping all the bools off, but keeping MoveTo.
func (c *Collide) Reset() {
	c.Top = false
	c.Left = false
	c.Right = false
	c.Bottom = false
}

// Side of the collision box (top, bottom, left, right)
type Side uint8

// Options for the Side type.
const (
	Top Side = iota
	Bottom
	Left
	Right
)

/*
CollidesWithGrid checks if a Doodad collides with level geometry.

The `target` is the point the actor wants to move to on this tick.
*/
func CollidesWithGrid(d doodads.Actor, grid *level.Chunker, target render.Point) (*Collide, bool) {
	var (
		P = d.Position()
		S = d.Size()

		result = &Collide{
			MoveTo: P,
		}
		ceiling   bool  // Has hit a ceiling?
		capHeight int32 // Stop vertical movement thru a ceiling
		capLeft   int32 // Stop movement thru a wall
		capRight  int32
		capFloor  int32 // Stop movement thru the floor
		hitLeft   bool  // Has hit an obstacle on the left
		hitRight  bool  // or right
		hitFloor  bool
	)

	// Test all of the bounding boxes for a collision with level geometry.
	if ok := result.ScanBoundingBox(doodads.GetBoundingRect(d), grid); ok {
		// We've already collided! Try to wiggle free.
		if result.Bottom {
			if !d.Grounded() {
				d.SetGrounded(true)
			} else {
				// result.Bottom = false
			}
		} else {
			d.SetGrounded(false)
		}
		if result.Top {
			// Never seen it touch the top.
		}
		if result.Left {
			P.X++
		}
		if result.Right {
			P.X--
		}
	}

	// If grounded, cap our Y position.
	if d.Grounded() {
		if !result.Bottom {
			// We've fallen off a ledge.
			d.SetGrounded(false)
		} else if target.Y < P.Y {
			// We're moving upward.
			d.SetGrounded(false)
		} else {
			// Cap our downward motion to our current position.
			target.Y = P.Y
		}
	}

	// Cap our horizontal movement if we're touching walls.
	if (result.Left && target.X < P.X) || (result.Right && target.X > P.X) {
		// If the step is short enough, try and jump up.
		height := P.Y + S.H
		if result.Left && target.X < P.X {
			height -= result.LeftPoint.Y
		} else {
			height -= result.RightPoint.Y
		}
		if height <= 8 {
			target.Y -= height
			if target.X < P.X {
				target.X-- // push along to the left
			} else if target.X > P.X {
				target.X++ // push along to the right
			}
		} else {
			target.X = P.X
		}
	}

	// Cap our vertical movement if we're touching ceilings.
	if ceiling {
		// The existing box intersects a ceiling, this will almost never
		// happen because gravity will always pull you away at the last frame.
		// But if we do somehow get here, may as well cap it where it's at.
		capHeight = P.Y
	}

	// Trace a line from where we are to where we wanna go.
	result.Reset()
	result.MoveTo = P
	for point := range render.IterLine2(P, target) {
		if has := result.ScanBoundingBox(render.Rect{
			X: point.X,
			Y: point.Y,
			W: S.W,
			H: S.H,
		}, grid); has {
			if result.Bottom {
				if !hitFloor {
					hitFloor = true
					capFloor = result.BottomPoint.Y - S.H
				}
				d.SetGrounded(true)
			}

			if result.Top && !ceiling {
				// This is a newly discovered ceiling.
				ceiling = true
				capHeight = result.TopPoint.Y
			}

			if result.Left && !hitLeft {
				hitLeft = true
				capLeft = result.LeftPoint.X
			}
			if result.Right && !hitRight {
				hitRight = true
				capRight = result.RightPoint.X - S.W
			}
		}

		// So far so good, keep following the MoveTo to
		// the last good point before a collision.
		result.MoveTo = point

	}

	// If they hit the roof, cap them to the roof.
	if ceiling && result.MoveTo.Y < capHeight {
		result.Top = true
		result.MoveTo.Y = capHeight
	}
	if hitFloor && result.MoveTo.Y > capFloor {
		result.Bottom = true
		result.MoveTo.Y = capFloor
	}
	if hitLeft {
		result.Left = true
		result.MoveTo.X = capLeft
	}
	if hitRight {
		result.Right = true
		result.MoveTo.X = capRight
	}

	return result, result.IsColliding()
}

// IsColliding returns whether any sort of collision has occurred.
func (c *Collide) IsColliding() bool {
	return c.Top || c.Bottom || c.Left || c.Right
}

// ScanBoundingBox scans all of the pixels in a bounding box on the grid and
// returns if any of them intersect with level geometry.
func (c *Collide) ScanBoundingBox(box render.Rect, grid *level.Chunker) bool {
	col := GetCollisionBox(box)

	// Check all four edges of the box in parallel on different CPU cores.
	type jobSide struct {
		p1   render.Point // p2 is perpendicular to p1 along a straight edge
		p2   render.Point // of the collision box.
		side Side
	}
	jobs := []jobSide{ // We'll scan each side of the bounding box in parallel
		jobSide{col.Top[0], col.Top[1], Top},
		jobSide{col.Bottom[0], col.Bottom[1], Bottom},
		jobSide{col.Left[0], col.Left[1], Left},
		jobSide{col.Right[0], col.Right[1], Right},
	}

	var wg sync.WaitGroup
	for _, job := range jobs {
		wg.Add(1)
		go func(job jobSide) {
			defer wg.Done()
			c.ScanGridLine(job.p1, job.p2, grid, job.side)
		}(job)
	}

	wg.Wait()
	return c.IsColliding()
}

// ScanGridLine scans all of the pixels between p1 and p2 on the grid and tests
// for any pixels to be set, implying a collision between level geometry and the
// bounding boxes of the doodad.
func (c *Collide) ScanGridLine(p1, p2 render.Point, grid *level.Chunker, side Side) {
	for point := range render.IterLine2(p1, p2) {
		if _, err := grid.Get(point); err == nil {
			// A hit!
			switch side {
			case Top:
				c.Top = true
				c.TopPoint = point
			case Bottom:
				c.Bottom = true
				c.BottomPoint = point
			case Left:
				c.Left = true
				c.LeftPoint = point
			case Right:
				c.Right = true
				c.RightPoint = point
			}
		}
	}
}