day9 part2

day9/part2.jl

@@ -0,0 +1,146 @@
+#!/usr/bin/env julia
+
+#= --- Part Two ---
+
+Next, you need to find the largest basins so you know what areas are most important to avoid.
+
+A basin is all locations that eventually flow downward to a single low point. Therefore, every low point has a basin, although some basins are very small. Locations of height 9 do not count as being in any basin, and all other locations will always be part of exactly one basin.
+
+The size of a basin is the number of locations within the basin, including the low point. The example above has four basins.
+
+The top-left basin, size 3:
+
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+The top-right basin, size 9:
+
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+The middle basin, size 14:
+
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+The bottom-right basin, size 9:
+
+2199943210
+3987894921
+9856789892
+8767896789
+9899965678
+
+Find the three largest basins and multiply their sizes together. In the above example, this is 9 * 14 * 9 = 1134.
+
+What do you get if you multiply together the sizes of the three largest basins?
+=#
+
+infile = length(ARGS) > 0 ? ARGS[1] : "input.txt"
+println("infile = ", infile)
+
+depth_map = reduce(vcat, [parse.(Int, split(line, ""))'
+                          for line in  eachline(infile)])
+
+function neighbor_idx(nrows, ncols, y, x)
+    v = CartesianIndex[]
+    if x > 1
+        push!(v, CartesianIndex(y, x-1))
+    end
+    if x < ncols
+        push!(v, CartesianIndex(y, x+1))
+    end
+    if y > 1
+        push!(v, CartesianIndex(y-1, x))
+    end
+    if y < nrows
+        push!(v, CartesianIndex(y+1, x))
+    end
+    return v
+end
+
+
+function neighbors(m, y, x)
+    nrows, ncols = size(m)
+    v = eltype(m)[]
+    if x > 1
+        push!(v, m[y, x-1])
+    end
+    if x < ncols
+        push!(v, m[y, x+1])
+    end
+    if y > 1
+        push!(v, m[y-1, x])
+    end
+    if y < nrows
+        push!(v, m[y+1, x])
+    end
+    return v
+end
+
+function flow_neighbors(m, y, x, flow_map, flow_count)
+    nrows, ncols = size(m)
+    nidx = neighbor_idx(nrows, ncols, y, x)
+    v = m[y, x]
+    if v == 9
+        flow_count[y, x] = 0
+        return
+    end
+    smallest_v = v
+    flow_idx = flow_map[y, x]
+    for idx in nidx
+        if m[idx] < smallest_v
+            smallest_v = m[idx]
+            flow_idx = flow_map[idx]
+            while flow_map[flow_idx] != flow_idx
+                flow_idx = flow_map[flow_idx]
+            end
+        end
+    end
+    if smallest_v != v
+        flow_count[flow_idx] += flow_count[y, x]
+        flow_count[y, x] = 0
+        flow_map[y, x] = flow_idx
+    end
+    #println(y, "x", x)
+    #display(flow_count)
+    #println()
+end
+
+function flow_all(m)
+    total = 0
+    nrows, ncols = size(m)
+    flow_count = ones(Int, nrows, ncols)
+    flow_map = reshape([CartesianIndex(y, x)
+                        for x in 1:ncols
+                        for y in 1:nrows], nrows, ncols)
+    for x in 1:ncols
+        for y in 1:nrows
+            flow_neighbors(m, y, x, flow_map, flow_count)
+        end
+    end
+    return flow_map, flow_count
+end
+
+map, count = flow_all(depth_map)
+
+if (length(depth_map) < 100)
+    display(count)
+    println()
+end
+
+basins = sort(filter(x -> x > 0, count))
+display(basins)
+println()
+
+println("nbasins = ", length(basins))
+println("largest_prod = ", prod(basins[end-2:end]))