Lua Indexed-List Demo

index_idea.lua

#!/usr/bin/env lua

-- I'm using luajson here
local json = require("json");
local list_file = "data.lst";
local index_file = "data.idx";

-- returns just the list index
function list_load_index()
  local index;
  local ifile, err = io.open(index_file,"r");
  if not ifile then
      return nil;
  end

  index = json.decode(ifile:read("*a"));
  ifile:close();

  return index;
end

-- store a passed index
function list_store_index(index)
  local ifile = io.open(index_file,"w");
  ifile:write(json.encode(index).."\n");
  ifile:close()
end

-- appends data to a list
-- optional [i] argument replaces item
function list_append(data,i)
  local index = list_load_index();
  if not index then
      -- appending to a brand new list, just call list_store
      list_store(data);
      return 1;
  end

  local i = i or #index+1;
  index[i] = {};

  datafile = io.open(list_file,"a");

  index[i].pos = datafile:seek();
  datafile:write(json.encode(data).."\n")
  index[i].length = datafile:seek() - index[i].pos;
  datafile:close();
  list_store_index(index);
end

-- loads the list in its entirety
function list_load()
  local index = list_load_index();
  local datatable = {};
  if not index then
      return datatable;
  end

  datafile = io.open(list_file,"r");

  if #index < 1 then
      return datatable;
  end

  for i=1,#index do
      datafile:seek("set",index[i].pos);
      datatable[i] = json.decode(datafile:read(index[i].length));
  end
  return datatable;
end

-- overwrites the list entirely
function list_store(data)
  local index = {};
  local file, err = io.open(list_file,"w");
  for i, item in ipairs(data) do
      index[i] = {}
      index[i].pos = file:seek();
      file:write(json.encode(item).."\n");
      index[i].length = file:seek() - index[i].pos;
  end
  file:close()

  list_store_index(index);
end

print("Creating datalist with two messages");
local data = {};
data[#data+1] = { message = "first message", somenumber = 1 };
data[#data+1] = { message = "second message", somenumber = 5 };
list_store(data);

print("Dump of list as json")
print("---\n" .. json.encode(list_load()) .. "\n---");

print("Appending a third message");
list_append( { message = "third message", somenumber = 20 } )

print("Dump of list as json")
print("---\n" .. json.encode(list_load()) .. "\n---");

print("Replacing the second message");
list_append( { message = "new second message", somenumber = 15}, 2);

print("Dump of list as json")
print("---\n" .. json.encode(list_load()) .. "\n---");