day15 p2: proper Bellman-Ford

day15/part2.jl

@@ -171,10 +171,35 @@ function neighbor_idx(nrows, ncols, y, x)
     return v
 end
 
+function relax(nrows, ncols, R, costs, row, col; first=false)
+    idx = neighbor_idx(nrows, ncols, row, col)
+    c = minimum(costs[i] for i in idx) + R[row, col]
+    if first || c < costs[row, col]
+        costs[row, col] = c
+        return true
+    end
+    return false
+end
+
+function relax_all(nrows, ncols, R, costs)
+    relaxed = false
+    for row in 1:nrows
+        for col in 1:ncols
+            v_relaxed = relax(nrows, ncols, R, costs, row, col)
+            relaxed = relaxed || v_relaxed
+        end
+    end
+    return relaxed
+end
+
 function get_costs(R)
     costs = zeros(Int, size(R))
     dirs = zeros(Int, size(R))
     nrows, ncols = size(R)
+
+    # first pass, unconditional update looking only right/down. Avoids
+    # having to add an infinity value
+    costs[1,1] = 0
     for col in 2:ncols
         costs[1, col] = costs[1, col-1] + R[1, col]
     end
@@ -190,90 +215,24 @@ function get_costs(R)
         end
     end
 
-    if false
-    # more passes
-    for i in 1:max(nrows, ncols)
-        for col in ncols-1:-1:1
-            rcost = costs[end, col+1] + R[end, col]
-            if rcost < costs[end, col]
-                costs[end, col] = rcost
-            end
-        end
-        for row in nrows-1:-1:1
-            dcost = costs[row+1, end] + R[row, end]
-            if dcost < costs[row, end]
-                costs[row, end] = dcost
-            end
-            for col in ncols-1:-1:1
-                rcost = costs[row, col+1] + R[row, col]
-                if rcost < costs[row, col]
-                    costs[row, col] = rcost
-                end
-                dcost = costs[row+1, col] + R[row, col]
-                if dcost < costs[row, col]
-                    costs[row, col] = dcost
-                end
-            end
-        end
-    end
-    end
-
-    for i in 1:max(nrows, ncols)
+    edges = ( (nrows-2)*(ncols-2)*4
+             +(nrows+ncols-4)*3
+             +4*2 )
+    max_passes = (nrows*ncols - 1) * edges
 
-        for row in nrows:-1:1
-            for col in ncols:-1:1
-                idx = neighbor_idx(nrows, ncols, row, col)
-                c = minimum(costs[i] for i in idx) + R[row, col]
-                if c < costs[row, col]
-                    costs[row, col] = c
-                end
-            end
-        end
-
-        for row in nrows:-1:1
-            for col in 1:ncols
-                idx = neighbor_idx(nrows, ncols, row, col)
-                c = minimum(costs[i] for i in idx) + R[row, col]
-                if c < costs[row, col]
-                    costs[row, col] = c
-                end
-            end
-        end
-
-        for row in 1:nrows
-            for col in ncols:-1:1
-                idx = neighbor_idx(nrows, ncols, row, col)
-                c = minimum(costs[i] for i in idx) + R[row, col]
-                if c < costs[row, col]
-                    costs[row, col] = c
-                end
-            end
-        end
-
-        for row in 1:nrows
-            for col in 1:ncols
-                idx = neighbor_idx(nrows, ncols, row, col)
-                c = minimum(costs[i] for i in idx) + R[row, col]
-                if c < costs[row, col]
-                    costs[row, col] = c
-                end
-            end
+    i = 1
+    while i <= max_passes
+        relaxed = relax_all(nrows, ncols, R, costs)
+        if !relaxed
+            break
         end
-
+        i += 1
     end
+    println("passes ", i)
 
     return costs, dirs
 end
 
-#=
-cost, path = least_risk(risk_map)
-
-display(path)
-println()
-
-println("cost = ", cost)
-=#
-
 costs, dirs = get_costs(risk_map)
 
 if length(costs) < 1000