// Copyright 2014, 2015, 2017 by Sascha L. Teichmann
// Use of this source code is governed by the MIT license
// that can be found in the LICENSE file.

package common

import (
	"math"
)

type area struct {
	X1, Z1 int16
	X2, Z2 int16
}

func (a area) contains(x, z int16) bool {
	return x >= a.X1 && x <= a.X2 && z >= a.Z1 && z <= a.Z2
}

func (a area) higher() bool {
	return a.Z2-a.Z1 > a.X2-a.X1
}

func areasContain(areas []area, x, z int16) bool {
	for _, r := range areas {
		if r.contains(x, z) {
			return true
		}
	}
	return false
}

// recalculate implements a greedy algorithm to figure out
// a list of disjunct areas of free regions in the domain
// to the (x, z) block plane.
// oldAreas are searched and found free areas are appended
// to newAreas which ist return.
// This is useful to spatial query only blocks from db
// that are not below already rendered blocks.
func (a area) recalculate(r *Renderer, nareas []area) []area {
	yM := r.yMin

	const ex = 1
	const ez = 2

	nas := len(nareas)

	for z := a.Z1; z <= a.Z2; z++ {
		row := z * int16(r.width)
		for x := a.X1; x <= a.X2; x++ {
			// Uncovered and not in list of new areas?
			if yM[row+x] > math.MinInt32 || areasContain(nareas[nas:], x, z) {
				continue
			}
			ar := area{X1: x, Z1: z, X2: x, Z2: z}
			// Try to extend the area in x and/or z till no further extension is possible.
		ext:
			for extend := ex | ez; extend != 0; {
				// If we extending in both directions a the current area
				// is higher than wide we gain more block if extend
				// in the x direction first.
				if (extend == ex|ez && ar.higher()) || extend&ex == ex { // check x
					nx := ar.X2 + 1
					if nx > a.X2 { // reached border of area
						extend &= ^ex
						continue
					}
					// Check column right of the current area if its fully free.
					for nz := ar.Z1; nz <= ar.Z2; nz++ {
						if yM[nz*int16(r.width)+nx] > math.MinInt32 ||
							areasContain(nareas[nas:], nx, nz) {
							extend &= ^ex
							continue ext
						}
					}
					// free -> extend
					ar.X2 = nx
				} else if extend&ez == ez { // check z
					nz := ar.Z2 + 1
					if nz > a.Z2 {
						extend &= ^ez
						continue
					}
					// Check line right below the current area if its free.
					row2 := nz * int16(r.width)
					for nx := ar.X1; nx <= ar.X2; nx++ {
						if yM[row2+nx] > math.MinInt32 ||
							areasContain(nareas[nas:], nx, nz) {
							extend &= ^ez
							continue ext
						}
					}
					// free -> extend
					ar.Z2 = nz
				}
			}
			// At this point the area is extended to max.
			nareas = append(nareas, ar)
		}
	}
	return nareas
}