A* (#98)

And here’s my solution - nothing particularl unusua;, beyond the
definition of distance I’d mentioned already. I used a simple
priority queue of my own design, though this could easily be adapted
to use some off-the-shelf version.

require ‘enumerator’

is easier to follow.

class PriorityQueue
def initialize
@list = []
end
# Add @list.length so that sort is always using Fixnum comparisons,
# which should be fast, rather than whatever is comparison on `item’
@list << [priority, @list.length, item]
@list.sort!
self
end
def <<(pritem)
end
def next
@list.shift[2]
end
def empty?
@list.empty?
end
end

class Astar
def do_quiz_solution(puzzle)
@terrain = []
instr = “”
puzzle.each {|rowstr|
next if rowstr =~ /^\s*\$/
rowstr.gsub!(/[^.@~X*^]/,’’)
instr += rowstr
instr += “\n”
row = []
rowstr.scan(/[.@~X*^]/) {|terrain|
case terrain
when /[.@X]/; row << 1
when /[*]/; row << 2
when /^/; row << 3
when /~/; row << nil
end
}
xind = rowstr.index(‘X’)
aind = rowstr.index(’@’)
if (aind)
@start = [@terrain.length, aind]
end
if (xind)
@goal = [@terrain.length, xind]
end
@terrain << row
}
if do_find_path
outarr = instr.split(/\n/)
@path.each{|row,col| outarr[row][col]=’#’}
return outarr.join("\n")
else
return nil
end
end

def do_find_path
been_there = {}
pqueue = PriorityQueue.new
pqueue << [1,[@start,[],1]]
while !pqueue.empty?
spot,path_so_far,cost_so_far = pqueue.next
next if been_there[spot]
newpath = [path_so_far, spot]
if (spot == @goal)
@path = []
newpath.flatten.each_slice(2) {|i,j| @path << [i,j]}
return @path
end
been_there[spot] = 1
spotsfrom(spot).each {|newspot|
next if been_there[newspot]
tcost = @terrain[newspot[0]][newspot[1]]
newcost = cost_so_far + tcost
pqueue << [newcost + estimate(newspot),
[newspot,newpath,newcost]]
}
end
return nil
end

def estimate(spot)
# quiz statement version
# (spot[0] - @goal[0]).abs + (spot[1] - @goal[1]).abs
# my version
[(spot[0] - @goal[0]).abs, (spot[1] - @goal[1]).abs].max
end

def spotsfrom(spot)
retval = []
if (spot[0] > 0) then vertadds << -1; end
if (spot[1] > 0) then horizadds << -1; end
if (v != 0 or h != 0) then
ns = [spot[0]+v,spot[1]+h]
if (@terrain[ns[0]] and @terrain[ns[0]][ns[1]]) then
retval << ns
end
end
}}
retval
end
end

if FILE == \$0
puts Astar.new.do_quiz_solution(ARGF)
end