[PokeCC] Save/Load code commented

--[[
  log2 is used to reverse 2^x this is basic math I wont explain this
--]]
local function log2(colour)
  return math.log(colour) / math.log(2)
end

--[[
  iterates through a 2D table, finding the longest row
  this is so we can calculate the full width and height of a 2D table, incase some rows have more columns than others
--]]
local function findLongest( t )
  local l = #t[1]
  for _,v in pairs( t ) do
    l = math.max( l, #v )
  end
  return l
end

--[[
  Creates a single byte from a colour and a true/false value
  a byte is 8 bits, we can store a colour in 4 bytes (a nibble) and a boolean in 1 bit
  this means that we have 3 spare bits in the nibble we're using to store the state,
  you could use this for other states if you wish
--]]
local function buildByte( color, state )
  --# turn the boolean into a number and then shift it 4 places to the left so its a high nibble
  --# so we have 00000001 to begin, but after shifting we have 00010000
  local high = bit.blshift(state == true and 1 or 0, 4)
  --# turn the colour into a number 0-15
  local low = log2( color )
  --# combine the low and high bits together into a byte
  return bit.bor( high, low )
end

--[[
  unpacks a single byte back into a colour and a true/false value
--]]
local function unpackByte( byte )
  --# shift the boolean value back into a range, so lets assume we have a byte that is colors.red and true
  --# the binary would be 00011110 performing this shift would make it 00000001 which is obviously what we
  --# want 'cause that's how we saved our boolean
  local high = bit.brshift( byte, 4)
  --# read the colour, assuming the same example with 00011110 applying a bitwise AND of 00001111 to it would
  --# result in 00001110 which is colors.red
  local low = bit.band( byte, 0xF )
  --# return the table in the form it was, converting the high bit back to a boolean by checking it its 00000001
  return { c = 2^low, s = high == 1 }
end

--[[
  Save the map to a file
--]]
function save(filename, map)
  --# open the file in binary writing mode
  local file = fs.open(filename, "wb")
  file.write(#map) --# height
  file.write(findLongest(map)) --# width
  --# iterate over the 2D table
  for _,row in pairs(map) do
    for _,col in pairs(row) do
      --# write out the byte
      file.write(buildByte(col.c, col.s))
    end
    --# make note that we dont make a newline here, newlines are useless in binary mode
    --# we will have to deal with this in `load`
  end
  --# done
  file.close()
end

--[[
  Load the map to a file
--]]
function load(filename)
  --# the map
  local map = {}
  --# open the file in binary read mode
  local file = fs.open(filename, "rb")
  --# read the width and height
  local height = file.read()
  local width = file.read()
  --# tracking information, `bc` is the number of bytes we've read
  local row, col, bc = 0,0,0
  --# read the file, 1 byte at a time
  for byte in file.read do
    --# calculate which row and column it belongs in
    row, col = (bc % height) + 1, (bc % width) + 1
    --# if there is no table for this row, make it
    map[row] = map[row] or {}
    --# unpack the byte into the row/column
    map[row][col] = unpackByte(byte)
    --# increment, we've read the byte
    bc = bc + 1
  end
  --# return the map and close the file, file.close returns nil, so you wont get a value from this
  return map, file.close()
end