a*
A算法，A（A-Star)算法是一种静态路网中求解最短路径最有效的直接搜索方法，也是解决许多搜索问题的有效算法。算法中的距离估算值与实际值越接近，最终搜索速度越快。
A* （A-Star)算法是一种静态路网中求解最短路径最有效的直接搜索方法，也是许多其他问题的常用启发式算法。注意——是最有效的直接搜索算法，之后涌现了很多预处理算法（如ALT，CH，HL等等），在线查询效率是A*算法的数千甚至上万倍。
公式表示为： f(n)=g(n)+h(n),
其中， f(n) 是从初始状态经由状态n到目标状态的代价估计，
g(n) 是在状态空间中从初始状态到状态n的实际代价，
h(n) 是从状态n到目标状态的最佳路径的估计代价。
（对于路径搜索问题，状态就是图中的节点，代价就是距离）
h(n)的选取
保证找到最短路径（最优解的）条件，关键在于估价函数f(n)的选取（或者说h(n)的选取）。
我们以d(n)表达状态n到目标状态的距离，那么h(n)的选取大致有如下三种情况：
- 如果h(n)< d(n)到目标状态的实际距离，这种情况下，搜索的点数多，搜索范围大，效率低。但能得到最优解。
- 如果h(n)=d(n)，即距离估计h(n)等于最短距离，那么搜索将严格沿着最短路径进行， 此时的搜索效率是最高的。
- 如果 h(n)>d(n)，搜索的点数少，搜索范围小，效率高，但不能保证得到最优解。
A*同样可以用于其他搜索问题，只需要对应状态和状态的距离即可。
package main
import (
"container/heap"
"fmt"
"math"
"strings"
)
import "strconv"
type OpenList []*_AstarPoint
func (self OpenList) Len() int { return len(self) }
func (self OpenList) Less(i, j int) bool { return self[i].fVal < self[j].fVal }
func (self OpenList) Swap(i, j int) { self[i], self[j] = self[j], self[i] }
func (this *OpenList) Push(x interface{}) {
// Push and Pop use pointer receivers because they modify the slice's length,
// not just its contents.
*this = append(*this, x.(*_AstarPoint))
}
func (this *OpenList) Pop() interface{} {
old := *this
n := len(old)
x := old[n-1]
*this = old[0 : n-1]
return x
}
type _Point struct {
x int
y int
view string
}
//========================================================================================
// 保存地图的基本信息
type Map struct {
points [][]_Point
blocks map[string]*_Point
maxX int
maxY int
}
func NewMap(charMap []string) (m Map) {
m.points = make([][]_Point, len(charMap))
m.blocks = make(map[string]*_Point, len(charMap)*2)
for x, row := range charMap {
cols := strings.Split(row, " ")
m.points[x] = make([]_Point, len(cols))
for y, view := range cols {
m.points[x][y] = _Point{x, y, view}
if view == "X" {
m.blocks[pointAsKey(x, y)] = &m.points[x][y]
}
} // end of cols
} // end of row
m.maxX = len(m.points)
m.maxY = len(m.points[0])
return m
}
func (this *Map) getAdjacentPoint(curPoint *_Point) (adjacents []*_Point) {
if x, y := curPoint.x, curPoint.y-1; x >= 0 && x < this.maxX && y >= 0 && y < this.maxY {
adjacents = append(adjacents, &this.points[x][y])
}
if x, y := curPoint.x+1, curPoint.y-1; x >= 0 && x < this.maxX && y >= 0 && y < this.maxY {
adjacents = append(adjacents, &this.points[x][y])
}
if x, y := curPoint.x+1, curPoint.y; x >= 0 && x < this.maxX && y >= 0 && y < this.maxY {
adjacents = append(adjacents, &this.points[x][y])
}
if x, y := curPoint.x+1, curPoint.y+1; x >= 0 && x < this.maxX && y >= 0 && y < this.maxY {
adjacents = append(adjacents, &this.points[x][y])
}
if x, y := curPoint.x, curPoint.y+1; x >= 0 && x < this.maxX && y >= 0 && y < this.maxY {
adjacents = append(adjacents, &this.points[x][y])
}
if x, y := curPoint.x-1, curPoint.y+1; x >= 0 && x < this.maxX && y >= 0 && y < this.maxY {
adjacents = append(adjacents, &this.points[x][y])
}
if x, y := curPoint.x-1, curPoint.y; x >= 0 && x < this.maxX && y >= 0 && y < this.maxY {
adjacents = append(adjacents, &this.points[x][y])
}
if x, y := curPoint.x-1, curPoint.y-1; x >= 0 && x < this.maxX && y >= 0 && y < this.maxY {
adjacents = append(adjacents, &this.points[x][y])
}
return adjacents
}
func (this *Map) PrintMap(path *SearchRoad) {
fmt.Println("map's border:", this.maxX, this.maxY)
for x := 0; x < this.maxX; x++ {
for y := 0; y < this.maxY; y++ {
if path != nil {
if x == path.start.x && y == path.start.y {
fmt.Print("S")
goto NEXT
}
if x == path.end.x && y == path.end.y {
fmt.Print("E")
goto NEXT
}
for i := 0; i < len(path.TheRoad); i++ {
if path.TheRoad[i].x == x && path.TheRoad[i].y == y {
fmt.Print("*")
goto NEXT
}
}
}
fmt.Print(this.points[x][y].view)
NEXT:
}
fmt.Println()
}
}
func pointAsKey(x, y int) (key string) {
key = strconv.Itoa(x) + "," + strconv.Itoa(y)
return key
}
//========================================================================================
type _AstarPoint struct {
_Point
father *_AstarPoint
gVal int
hVal int
fVal int
}
func NewAstarPoint(p *_Point, father *_AstarPoint, end *_AstarPoint) (ap *_AstarPoint) {
ap = &_AstarPoint{*p, father, 0, 0, 0}
if end != nil {
ap.calcFVal(end)
}
return ap
}
func (this *_AstarPoint) calcGVal() int {
if this.father != nil {
deltaX := math.Abs(float64(this.father.x - this.x))
deltaY := math.Abs(float64(this.father.y - this.y))
if deltaX == 1 && deltaY == 0 {
this.gVal = this.father.gVal + 10
} else if deltaX == 0 && deltaY == 1 {
this.gVal = this.father.gVal + 10
} else if deltaX == 1 && deltaY == 1 {
this.gVal = this.father.gVal + 14
} else {
panic("father point is invalid!")
}
}
return this.gVal
}
func (this *_AstarPoint) calcHVal(end *_AstarPoint) int {
this.hVal = int(math.Abs(float64(end.x-this.x)) + math.Abs(float64(end.y-this.y)))
return this.hVal
}
func (this *_AstarPoint) calcFVal(end *_AstarPoint) int {
this.fVal = this.calcGVal() + this.calcHVal(end)
return this.fVal
}
//========================================================================================
type SearchRoad struct {
theMap *Map
start _AstarPoint
end _AstarPoint
closeLi map[string]*_AstarPoint
openLi OpenList
openSet map[string]*_AstarPoint
TheRoad []*_AstarPoint
}
func NewSearchRoad(startx, starty, endx, endy int, m *Map) *SearchRoad {
sr := &SearchRoad{}
sr.theMap = m
sr.start = *NewAstarPoint(&_Point{startx, starty, "S"}, nil, nil)
sr.end = *NewAstarPoint(&_Point{endx, endy, "E"}, nil, nil)
sr.TheRoad = make([]*_AstarPoint, 0)
sr.openSet = make(map[string]*_AstarPoint, m.maxX+m.maxY)
sr.closeLi = make(map[string]*_AstarPoint, m.maxX+m.maxY)
heap.Init(&sr.openLi)
heap.Push(&sr.openLi, &sr.start) // 首先把起点加入开放列表
sr.openSet[pointAsKey(sr.start.x, sr.start.y)] = &sr.start
// 将障碍点放入关闭列表
for k, v := range m.blocks {
sr.closeLi[k] = NewAstarPoint(v, nil, nil)
}
return sr
}
func (this *SearchRoad) FindoutRoad() bool {
for len(this.openLi) > 0 {
// 将节点从开放列表移到关闭列表当中。
x := heap.Pop(&this.openLi)
curPoint := x.(*_AstarPoint)
delete(this.openSet, pointAsKey(curPoint.x, curPoint.y))
this.closeLi[pointAsKey(curPoint.x, curPoint.y)] = curPoint
//fmt.Println("curPoint :", curPoint.x, curPoint.y)
adjacs := this.theMap.getAdjacentPoint(&curPoint._Point)
for _, p := range adjacs {
//fmt.Println("\t adjact :", p.x, p.y)
theAP := NewAstarPoint(p, curPoint, &this.end)
if pointAsKey(theAP.x, theAP.y) == pointAsKey(this.end.x, this.end.y) {
// 找出路径了, 标记路径
for theAP.father != nil {
this.TheRoad = append(this.TheRoad, theAP)
theAP.view = "*"
theAP = theAP.father
}
return true
}
_, ok := this.closeLi[pointAsKey(p.x, p.y)]
if ok {
continue
}
existAP, ok := this.openSet[pointAsKey(p.x, p.y)]
if !ok {
heap.Push(&this.openLi, theAP)
this.openSet[pointAsKey(theAP.x, theAP.y)] = theAP
} else {
oldGVal, oldFather := existAP.gVal, existAP.father
existAP.father = curPoint
existAP.calcGVal()
// 如果新的节点的G值还不如老的节点就恢复老的节点
if existAP.gVal > oldGVal {
// restore father
existAP.father = oldFather
existAP.gVal = oldGVal
}
}
}
}
return false
}
//========================================================================================
func main() {
presetMap := []string{
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
"X . X X X X X X X X X X X X X X X X X X X X X X X X X",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
"X X X X X X X X X X X X X X X X X X X X X X X X . X X",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
". . . . . . . . . . . . . . . . . . . . . . . . . . .",
}
m := NewMap(presetMap)
m.PrintMap(nil)
searchRoad := NewSearchRoad(0, 0, 18, 10, &m)
if searchRoad.FindoutRoad() {
fmt.Println("找到了， 你看！")
m.PrintMap(searchRoad)
} else {
fmt.Println("找不到路径！")
}
}
原文地址：http://www.byteedu.com/forum.php?mod=viewthread&tid=436&page=1&extra=#pid552
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