day9: use point array instead of knot pairs

more efficient way to model > 2 knots

day9/day9.jl

@@ -7,11 +7,9 @@ struct Move
   count :: Int
 end
 
-mutable struct KnotPair
-  head_x :: Int
-  head_y :: Int
-  tail_x :: Int
-  tail_y :: Int
+mutable struct Point{T}
+  x::T
+  y::T
 end
 
 function parse_move(line)
@@ -19,71 +17,54 @@ function parse_move(line)
   return Move(parts[1][1], parse(Int, parts[2]))
 end
 
-
-function state_changed(old_state, new_state)
-  return (new_state.head_x != old_state.head_x
-          || new_state.head_y != old_state.head_y
-          || new_state.tail_x != old_state.tail_x
-          || new_state.tail_y != old_state.tail_y)
-end
-
 function read_moves(io)
   return [parse_move(line) for line in readlines(io)]
 end
 
-function apply_move(state, direction)
+function apply_move(head, tail, direction)
   if direction == 'U'
-    state.head_y += 1
+    head.y += 1
   elseif direction == 'D'
-    state.head_y -= 1
+    head.y -= 1
   elseif direction == 'L'
-    state.head_x -= 1
+    head.x -= 1
   elseif direction == 'R'
-    state.head_x += 1
+    head.x += 1
   elseif direction != 'O'
     throw(DomainError("Unknown direction"))
   end
-  x_diff = state.head_x - state.tail_x
-  y_diff = state.head_y - state.tail_y
+  x_diff = head.x - tail.x
+  y_diff = head.y - tail.y
   if abs(x_diff) <= 1 && abs(y_diff) <= 1
-    return state
+    return head, tail
   end
   if abs(x_diff) <= 1
-    state.tail_x += x_diff
-    state.tail_y += sign(y_diff) * (abs(y_diff) - 1)
+    tail.x += x_diff
+    tail.y += sign(y_diff) * (abs(y_diff) - 1)
   elseif abs(y_diff) <= 1
-    state.tail_y += y_diff
-    state.tail_x += sign(x_diff) * (abs(x_diff) - 1)
+    tail.y += y_diff
+    tail.x += sign(x_diff) * (abs(x_diff) - 1)
   elseif abs(x_diff) == 2 && abs(y_diff) == 2
     # possible with more than 2 knots
-    state.tail_x += sign(x_diff) * (abs(x_diff) - 1)
-    state.tail_y += sign(y_diff) * (abs(y_diff) - 1)
+    tail.x += sign(x_diff) * (abs(x_diff) - 1)
+    tail.y += sign(y_diff) * (abs(y_diff) - 1)
   else
-    throw(DomainError("invalid state " * string(state)))
+    throw(DomainError("invalid knots " * string(head) * " " * string(tail)))
   end 
-  return state
+  return head, tail
 end
 
-function get_visited(moves; npairs::Int=1)
+function get_visited(moves; nknots::Int=2)
   visited = Set{NTuple{2, Int}}()
-  states = [KnotPair(0, 0, 0, 0) for _ in 1:npairs]
-  push!(visited, (states[npairs].tail_x, states[npairs].tail_y))
+  knots = [Point(0, 0) for _ in 1:nknots]
+  push!(visited, (knots[nknots].x, knots[nknots].y))
   for move in moves
-    for m in 1:move.count
-      # println(move, " ", m)
-      # print(" 1 ", states[1], " -> ")
-      states[1] = apply_move(states[1], move.direction)
-      # println(states[1])
-      for i in 2:npairs
-        states[i].head_x = states[i-1].tail_x
-        states[i].head_y = states[i-1].tail_y
-        # old_state = deepcopy(states[i])
-        states[i] = apply_move(states[i], 'O')
-        # if state_changed(states[i], old_state)
-        #   println(" ", i, " ", old_state, " -> ", states[i])
-        # end
-      end
-      push!(visited, (states[npairs].tail_x, states[npairs].tail_y))
+    for _ in 1:move.count
+      knots[1], knots[2] = apply_move(knots[1], knots[2], move.direction)
+      for i in 2:nknots
+        _, knots[i] = apply_move(knots[i-1], knots[i], 'O')
+      end
+      push!(visited, (knots[nknots].x, knots[nknots].y))
     end
   end
   return visited
@@ -107,8 +88,8 @@ function test()
     @test length(get_visited(read_moves("input.txt"))) == 6037
   end
   @testset "visited count 10 knot" verbose=true begin
-    @test length(get_visited(read_moves("example.txt"), npairs=9)) == 1
-    @test length(get_visited(read_moves("input.txt"), npairs=9)) == 2485
+    @test length(get_visited(read_moves("example.txt"), nknots=10)) == 1
+    @test length(get_visited(read_moves("input.txt"), nknots=10)) == 2485
   end
 end
 
@@ -126,7 +107,7 @@ function main()
     display(visited_map(visited))
     println()
 
-    visited10 = get_visited(moves, npairs=9)
+    visited10 = get_visited(moves, nknots=10)
     println("visited10: ", visited10)
     println("count10  : ", length(visited10))
     display(visited_map(visited10))