-- 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_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 = {} }
  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 arma = {}
local list = {}
local armb = {}
for i,node in ipairs(node_list) do
  arma[i] = {}
  list[i] = node
  armb[i] = {}
end

local function reverse (t)
  for i=1,#t//2 do
    t[i],t[#t-i+1] = t[#t-i+1],t[i]
  end
  return t
end

local function find_free_col(wing, a, b)
  local col = 1
  while true do
    for i=a,b do
      local cell = wing[i][col]
      if cell ~= nil and cell ~= L.X then
        goto continue
      end
    end
    do return col end
    ::continue::
    col = col + 1
  end
  return col
end

for _,node in ipairs(node_list) do
  local a_node = node_map[node]
  for _,node in ipairs(a_node["nodes"]) do
    local a = a_node["order"]
    local b = node_map[node]["order"]

    local i1 = 1
    local ia = math.min(a,b)
    local ib = math.max(a,b)
    local iz = #node_list
    local wing = a < b and arma or armb

    local col = 1
    while true do
      for i=ia,ib do
        local cell = wing[i][col]
        if cell ~= nil and cell ~= L.X then
          goto continue
        end
      end
      break
      ::continue::
      col = col + 1
    end
    if col == 1 then col = 2 end
    local a_row = wing[ia]
    if a_row[1] == nil or a_row[1] == L.X then
      a_row[1] = a < b and L.N or L.L
    end
    for i=1,col-1 do
      if a_row[i] == nil or a_row[i] == L.X then a_row[i] = L.N end
    end
    a_row[col] = a < b and L.NW or L.NE
    for i=ia+1,ib-1 do
      for j=1,col-1 do
        if wing[i][j] == nil then wing[i][j] = L.X end
      end
      wing[i][col] = L.W
    end
    local b_row = wing[ib]
    if b_row[1] == nil or b_row[1] == L.X then
      b_row[1] = a < b and L.R or L.S
    end
    for i=1,col-1 do
      if b_row[i] == nil or b_row[i] == L.X then b_row[i] = L.S end
    end
    b_row[col] = L.SW
  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>")

-- pad right wing
local arma_pad = 1
for _,len in pairs(arma) do
  if #len > arma_pad then arma_pad = #len end
end
-- pad node names
local node_pad = 1
for _,s in pairs(list) do
  if s:len() > node_pad then node_pad = s:len() end
end

for i,v in ipairs(list) do
  local row = {}
  local v = ("%-"..node_pad.."s"):format(v)
  local a = v:gsub("(.+)%.md", "<a href=\"%1.html\">%0</a>")
  table.insert(row, (("%"..arma_pad.."s"):format(table.concat(reverse(arma[i])))))
  table.insert(row, ((" %s "):format(a)))
  table.insert(row, (table.concat(armb[i])))

  table.insert(lines, table.concat(row))
end
table.insert(lines, "</pre>")
table.insert(lines, "</article>")
local data = table.concat(lines, "\n")

-- read existing data from output_file
local f = assert(io.open(output_file), "\n" ..
  "[ERROR] could not open " .. output_file .. " for reading.")
local data_old = f:read("a")
f:close()

-- write to output_file if data changed
if 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