[Julia] Day 18 (#67)

inputs/18_example.txt

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+R 6 (#70c710)
+D 5 (#0dc571)
+L 2 (#5713f0)
+D 2 (#d2c081)
+R 2 (#59c680)
+D 2 (#411b91)
+L 5 (#8ceee2)
+U 2 (#caa173)
+L 1 (#1b58a2)
+U 2 (#caa171)
+R 2 (#7807d2)
+U 3 (#a77fa3)
+L 2 (#015232)
+U 2 (#7a21e3)

solutions/18_example.txt

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+62
+952408144115

solutions/18_moelf.txt

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+56678
+79088855654037

src/julia/18_moelf.jl

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+const CI = CartesianIndex{2}
+const L, R, U, D = CI(0, -1), CI(0, 1), CI(-1, 0), CI(1, 0)
+const DIRS = Dict(
+    "R" => R, "D" => D, "L" => L, "U" => U,
+    '0' => R, '1' => D, '2' => L, '3' => U)
+
+function make_poly(dirs, nums)
+    POS = CI(0, 0)
+    PATH = CI[]
+    for (D, N) in zip(dirs, nums)
+        for _ in 1:N
+            POS += D
+            push!(PATH, POS)
+        end
+    end
+    return PATH
+end
+# https://en.wikipedia.org/wiki/Shoelace_formula
+function shoelace(poly)
+    a = 0
+    for i in 1:lastindex(poly)-1
+        p1 = poly[i]
+        p2 = poly[i+1]
+        a += p1[1] * p2[2] - p2[1] * p1[2]
+    end
+    return abs(a ÷ 2)
+end
+# https://en.wikipedia.org/wiki/Pick's_theorem
+function in_area(poly)
+    A = shoelace(poly)
+    b = length(poly)
+    return A + 1 - b ÷ 2
+end
+
+function impl(dirs, nums)
+    poly = make_poly(dirs, nums)
+    return in_area(poly) + length(poly)
+end
+
+function main(path)
+    M_str = stack(split.(readlines(path)); dims=1)
+    dir_str, num_str, color_str = eachcol(M_str)
+
+    dirs = [DIRS[s] for s in dir_str]
+    nums = parse.(Int, num_str)
+    println(impl(dirs, nums))
+
+    empty!(dirs); empty!(nums)
+
+    for s in color_str
+        num_str..., dir_str = replace(s, r"(#|\)|\()" => "")
+        push!(dirs, DIRS[dir_str])
+        push!(nums, parse(Int, num_str; base=16))
+    end
+    println(impl(dirs, nums))
+end
+
+(abspath(PROGRAM_FILE) == @__FILE__) && main(ARGS[1])