day 15, part 2 - thanks again alderwick!

15/15_2.awk

@@ -0,0 +1,37 @@
+# algorithm proposed by alderwick :)
+BEGIN{ FS = "" }
+
+function p(x,y){ return x","y }
+
+function risk(x,y){
+	x--; y--; # zero based
+	return (r[p(x%NF+1,y%NF+1)] + int(x/NF) + int(y/NR)-1)%9 + 1
+}
+
+{ for(i=1;i<=NF;i++) r[p(i,NR)] = $i }
+
+END{
+	s[p(1,1)] = 0
+	target = 1
+	while(!(p(NF*5,NR*5) in s)){
+		if(target%100==0) print "target:",target
+		delete ns
+		for(i in s){
+			split(i,arr,",")
+			x = arr[1]; y=arr[2]
+			cs = s[p(x,y)]
+			if(cs<target-9) continue # too far from target score
+			if(x>1 && risk(x-1,y)+cs==target && s[p(x-1,y)]=="" )
+				ns[p(x-1,y)] = target # left
+			if(x<NF*5 && risk(x+1,y)+cs==target && s[p(x+1,y)]=="" )
+				ns[p(x+1,y)] = target  #right
+			if(y>1 && risk(x,y-1)+cs==target && s[p(x,y-1)]=="" )
+				ns[p(x,y-1)] = target # up
+			if(y<NR*5 && risk(x,y+1)+cs==target && s[p(x,y+1)]=="" )
+				ns[p(x,y+1)] = target # down
+		}
+		for(i in ns) s[i] = ns[i]
+		target++
+	}
+	print s[p(NF*5,NR*5)]
+}