local fmt = require "fmt"

local digitL <const> = {
    ["   ## #"] = 0,
    ["  ##  #"] = 1,
    ["  #  ##"] = 2,
    [" #### #"] = 3,
    [" #   ##"] = 4,
    [" ##   #"] = 5,
    [" # ####"] = 6,
    [" ### ##"] = 7,
    [" ## ###"] = 8,
    ["   # ##"] = 9
}

local digitR <const> = {
    ["###  # "] = 0,
    ["##  ## "] = 1,
    ["## ##  "] = 2,
    ["#    # "] = 3,
    ["# ###  "] = 4,
    ["#  ### "] = 5,
    ["# #    "] = 6,
    ["#   #  "] = 7,
    ["#  #   "] = 8,
    ["### #  "] = 9
}

local end_sentinel <const> = "# #"  --  also at start
local mid_sentinel <const> = " # # "

local function decode_upc(s)
    local function decode_upc_impl(input)
        local upc = input:strip()
        if #upc != 95 then return false end
        local pos = 1
        local digits = {}
        local sum = 0
        local one_three = {1, 3}

        -- end sentinel
        if upc:sub(pos, pos + 2) == end_sentinel then
            pos += 3
        else
            return false
        end

        -- 6 left hand digits
        for _ = 1, 6 do
            local digit = digitL[upc:sub(pos, pos + 6)]
            if !digit then return false end
            digits:insert(digit)
            sum += digit * one_three[#digits % 2 + 1]
            pos += 7
        end

        -- mid sentinel
        if upc:sub(pos, pos + 4) == mid_sentinel then
            pos += 5
        else
            return false
        end

        -- 6 right hand digits
        for _ = 1, 6 do
            local digit = digitR[upc:sub(pos, pos + 6)]
            if !digit then return false end
            digits:insert(digit)
            sum += digit * one_three[#digits % 2 + 1]
            pos += 7
        end

        -- end sentinel
        if upc:sub(pos, pos + 2) != end_sentinel then return false end

        if sum % 10 == 0 then
            fmt.write("%s ", fmt.swrite(digits))
            return true
        end
        io.write("Failed Checksum ")
        return false
    end

    if decode_upc_impl(s) then
        print("Rightside Up")
    elseif decode_upc_impl(s:reverse()) then
        print("Upside Down")
    else
        print("Invalid digit(s)")
    end
end

local barcodes <const> = {
    "         # #   # ##  #  ## #   ## ### ## ### ## #### # # # ## ##  #   #  ##  ## ###  # ##  ## ### #  # #       ",
    "        # # #   ##   ## # #### #   # ## #   ## #   ## # # # ###  # ###  ##  ## ###  # #  ### ###  # # #         ",
    "         # #    # # #  ###  #   #    # #  #   #    # # # # ## #   ## #   ## #   ##   # # #### ### ## # #         ",
    "       # # ##  ## ##  ##   #  #   #  # ###  # ##  ## # # #   ## ##  #  ### ## ## #   # #### ## #   # #        ",
    "         # # ### ## #   ## ## ###  ##  # ##   #   # ## # # ### #  ## ##  #    # ### #  ## ##  #      # #          ",
    "          # #  #   # ##  ##  #   #   #  # ##  ##  #   # # # # #### #  ##  # #### #### # #  ##  # #### # #         ",
    "         # #  #  ##  ##  # #   ## ##   # ### ## ##   # # # #  #   #   #  #  ### # #    ###  # #  #   # #        ",
    "        # # #    # ##  ##   #  # ##  ##  ### #   #  # # # ### ## ## ### ## ### ### ## #  ##  ### ## # #         ",
    "         # # ### ##   ## # # #### #   ## # #### # #### # # #   #  # ###  #    # ###  # #    # ###  # # #       ",
    "        # # # #### ##   # #### # #   ## ## ### #### # # # #  ### # ###  ###  # # ###  #    # #  ### # #         "
}
local n = 0
for barcodes as barcode do
    ++n
    fmt.write("%2d: ", n)
    decode_upc(barcode)
end