-- local l = require("pandoc.logging")
local read = pandoc.read

assert(#arg == 2, "\n" ..
  "[ERROR] usage: pandoc lua sitemap.lua <index file> <output file>")
local index_file, output_file = table.unpack(arg)

local node_len = 1
local node_list, node_map = {}, {}
local function collect (fp)
  local f = assert(io.open(fp), "\n" ..
    "[ERROR] could not open " .. fp .. " for reading.")
  table.insert(node_list, fp)
  node_map[fp] = { order = #node_list, nodes = {} }
  if fp:len() > node_len then node_len = fp:len() end -- track node len
  local data = f:read("a")
  f:close()
  local visited = {}
  read(data):walk({ Link = function (link)
    if not link.target:find(".%.md$") then return end
    local f = io.open(link.target)
    if f == nil then return else f:close() end
    if visited[link.target] then return end
    visited[link.target] = true
    table.insert(node_map[fp]["nodes"], link.target)
    if not node_map[link.target] then collect(link.target) end
  end })
end
collect(index_file)

local L = {
  L = "<", 
  R = ">", 
  NW = ",", N = "-", NE = ".",
  W  = "|", X = " ", E  = "|",
  SW = "'", S = "-", SE = "'",
}

local arm_a, arm_b = {}, {}
for i,_ in ipairs(node_list) do
  arm_a[i], arm_b[i] = {}, {}
end


local arm_row_len = 1
for _,node in ipairs(node_list) do
  local node_a = node_map[node]
  for _,node in ipairs(node_a["nodes"]) do
    local a, b = node_a["order"], node_map[node]["order"]
    local i_a, i_b = math.min(a,b), math.max(a,b)
    local arm = a < b and arm_a or arm_b

    -- find the first free column
    local col = 1
    while true do
      for i=i_a,i_b do
        local cell = arm[i][col]
        if cell ~= nil and cell ~= L.X then
          col = col + 1
          goto continue
        end
      end
      break
      ::continue::
    end
    col = col == 1 and 2 or col
    if col > arm_row_len then arm_row_len = col end -- track row len

    -- layout starting row
    local row_a = arm[i_a]
    if row_a[1] == nil or row_a[1] == L.X then
      row_a[1] = a < b and L.N or L.L
    end
    for i=2,col-1 do
      if row_a[i] == nil or row_a[i] == L.X then row_a[i] = L.N end
    end
    row_a[col] = a < b and L.NW or L.NE

    -- layout rows between start and end
    for i=i_a+1,i_b-1 do
      local row_i = arm[i]
      for j=1,col-1 do
        if row_i[j] == nil then row_i[j] = L.X end
      end
      row_i[col] = L.W
    end

    -- layout ending row
    local row_b = arm[i_b]
    if row_b[1] == nil or row_b[1] == L.X then
      row_b[1] = a < b and L.R or L.S
    end
    for i=2,col-1 do
      if row_b[i] == nil or row_b[i] == L.X then row_b[i] = L.S end
    end
    row_b[col] = a < b and L.SW or L.SE
  end
end

-- construct data lines
local lines = {}
table.insert(lines, "<article id=\"sitemap\" class=\"post tail\">")
table.insert(lines, "<header>")
table.insert(lines, "<h1>sitemap</h1>")
table.insert(lines, "</header>")
table.insert(lines, "<pre>")

for i,node in ipairs(node_list) do
  local line = {}
  local node_padded = ("%-"..node_len.."s"):format(node)
  local node_anchor = node_padded:gsub("(.+)%.md", "<a href=\"%1.html\">%1</a>")
  local row_a, row_b = arm_a[i], arm_b[i]

  -- reverse arm a row
  for j=1,#row_a//2 do
    row_a[j], row_a[#row_a-j+1] = row_a[#row_a-j+1], row_a[j]
  end

  table.insert(line, (("%"..arm_row_len.."s"):format(table.concat(row_a))))
  table.insert(line, ((" %s "):format(node_anchor)))
  table.insert(line, (table.concat(row_b)))
  table.insert(lines, table.concat(line))
end
table.insert(lines, "</pre>")
table.insert(lines, "</article>")
local data = table.concat(lines, "\n")

-- read existing data from output_file
local f = io.open(output_file)
local data_old
if f then data_old = f:read("a"); f:close() end

-- write to output_file if no output_file yet or data changed
if not data_old or data ~= data_old then
  local f = assert(io.open(output_file, "w"), "\n" ..
    "[ERROR] could not open " .. output_file .. " for writing.")
  f:write(data)
  f:close()
end