package soko

import (
	"sort"

	"opslag.de/schobers/geom"
)

type PathFinder struct {
	state State
}

func NewPathFinder(s State) PathFinder {
	return PathFinder{s}
}

type betterNeighbourFn func(geom.Point, int)

func (p PathFinder) findBetterNeighbours(distances map[geom.Point]int, curr geom.Point, better betterNeighbourFn) {
	currDistance := distances[curr]
	newDistance := currDistance + 1

	neighbours := Neighbours(curr)
	for _, next := range neighbours {
		if !p.state.IsWalkable(next) || p.state.Bricks.Has(next) { // filter neighbours
			continue
		}
		if distance, ok := distances[next]; ok && distance <= newDistance { // skip when shorter path exists
			continue
		}
		distances[next] = newDistance
		better(next, newDistance)
	}
}

func (p PathFinder) Find(target geom.Point) []geom.Point {
	source := p.state.Player.Pos
	frontier := []geom.Point{source}
	distances := map[geom.Point]int{source: 0}
	moves := map[geom.Point]geom.Point{source: source}
	heuristic := func(i int) int {
		pos := frontier[i]
		return distances[pos] + pos.DistInt(target)
	}
	for {
		curr := frontier[0]
		if curr == target {
			break
		}
		frontier = frontier[1:]

		p.findBetterNeighbours(distances, curr, func(next geom.Point, newDistance int) {
			moves[next] = curr
			frontier = append(frontier, next)
		})

		if len(frontier) == 0 {
			return nil // no path
		}

		// apply heuristic to frontier (favor points closer to target)
		sort.Slice(frontier, func(i, j int) bool { return heuristic(i) < heuristic(j) })
	}

	// build reverse path
	curr := target
	path := []geom.Point{curr}
	for {
		curr = moves[curr]
		if curr == source {
			break
		}
		path = append(path, curr)
	}
	// reverse path
	n := len(path)
	for i := 0; i < n/2; i++ {
		path[i], path[n-i-1] = path[n-i-1], path[i]
	}
	return path
}

func (p PathFinder) FindDistances() map[geom.Point]int {
	source := p.state.Player.Pos
	frontier := []geom.Point{source}
	distances := map[geom.Point]int{source: 0}
	for {
		curr := frontier[0]
		frontier = frontier[1:]

		p.findBetterNeighbours(distances, curr, func(next geom.Point, newDistance int) {
			frontier = append(frontier, next)
		})

		if len(frontier) == 0 {
			return distances
		}
	}
}