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

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

local node_list = {}
local graph_map = {}
local function collect (fp)
  local f = assert(io.open(fp), "\n" ..
    "[ERROR] could not open " .. fp .. " for reading.")
  table.insert(node_list, fp)
  graph_map[fp] = { order = #node_list, nodes = {} }
  local data = f:read("a")
  f:close()
  local visited = {}
  read(data):walk({ Link = function (link)
    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(graph_map[fp]["nodes"], link.target)
    if not graph_map[link.target] then collect(link.target) end
  end })
end

collect(arg[1])

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

local layout = {}
local r_wing = {}
local l_wing = {}
for i,node in ipairs(node_list) do
  r_wing[i] = {}
  layout[i] = node
  l_wing[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 = graph_map[node]
  for _,node in ipairs(a_node["nodes"]) do
    local a = a_node["order"]
    local b = graph_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 r_wing or l_wing
    local r_first, l_first = true, true
    local first = a < b and r_first or l_first

    local col = find_free_col(wing, ia, ib)
    if col == 1 then col = 2 end
    local a_row = wing[ia]
    if a_row[1] == nil or a_row[1] == " " then
      a_row[1] = a < b and "-" or "<"
    end
    for i=1,col-1 do
      if a_row[i] == nil or a_row[i] == L["X"] then a_row[i] = "-" end
    end
    a_row[col] = "."
    for i=ia+1,ib-1 do
      for j=1,col-1 do
        if wing[i][j] == nil then wing[i][j] = " " end
      end
      wing[i][col] = "|"
    end
    local b_row = wing[ib]
    if b_row[1] == nil or b_row[1] == " " then
      b_row[1] = a < b and ">" or "-"
    end
    for i=1,col-1 do
      if b_row[i] == nil or b_row[i] == L["X"] then b_row[i] = "-" end
    end
    b_row[col] = "'"
  end
end

-- write
local data = {}
table.insert(data, "<nav id=\"sitemap\">")
table.insert(data, "<header>")
table.insert(data, "<h1>sitemap</h1>")
table.insert(data, "</header>")
table.insert(data, "<pre>")

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

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

  table.insert(data,table.concat(row))
end

table.insert(data, "</pre>")
table.insert(data, "</nav>")

local f = assert(io.open(arg[2], "w"), "\n" ..
  "[ERROR] could not open " .. arg[2] .. " for writing.")
f:write(table.concat(data, "\n"))
f:close()