--- Procedural generation methods

function init_mapgen()
   uid_seed = 2229 -- arbitrarily chosen number

   -- Metadata for different biomes
   -- tile_frequencies tuples are {frequency, sprite_index}, see index_map.md
   -- frequencies by convention add up to 1000, but this is arbitrary, the
   -- only concern is the space consumed by the resulting tables.
   biome_data = {
      meadow = {
         biome_frequency = 20,
         tile_frequencies = { {525, 2}, {200, 5}, {200, 6}, {55, 18}, {19, 3}, {1, 4} }
      },
      grassland = {
         biome_frequency = 55,
         tile_frequencies = { {500, 2}, {345, 18}, {4, 3}, {1, 4}, {100, 5}, {50, 6} }
      },
      forest = {
         biome_frequency = 20,
         tile_frequencies = { {600, 2}, {200, 4}, {50, 3}, {50, 5}, {40, 7}, {59, 6}, {1, 8} }
      },
      desert = {
         biome_frequency = 5,
         tile_frequencies = { {800, 9}, {109, 11}, {60, 13}, {30, 12}, {1, 10} },
      }
   }

   -- Why is this hard-coded separately from the biome_data? glad you asked.
   -- Lua's pairs() function appears not to guarantee a consistent return order,
   -- and we want our world to be deterministically generated,
   -- so the biome_metadata array needs to have its entries appear consistently.
   biome_list = {"grassland", "meadow", "forest", "desert"}

   -- this is the frequency list for the biomes themselves
   biome_metadata = {}

   for i=1,#biome_list do
      local biome = biome_list[i]

      -- add the biome's name N times to the biome metadata 'hat'
      for i=1,biome_data[biome].biome_frequency do
         add(biome_metadata, biome)
      end

      build_biome(biome, biome_data[biome])
   end
end

-- build the lookup table for a given biome, based on the biome_meta data for
-- that string.
function build_biome(biome_name, data)
   local meta_frequencies = data.tile_frequencies
   local tile_lookup = {}

   for i=1,#meta_frequencies do
      local tuple = meta_frequencies[i]
      for j=1,tuple[1] do
         add(tile_lookup, tuple[2])
      end
   end

   data.tile_lookup = tile_lookup
end

-- generates a unique identifier for a position
-- uses srand() and rand() to get an unpredictable value (is this too slow?)
-- the two seed values are randomly chosen. Change them and change the world.
function generate_uid(pos_x, pos_y)
   srand((pos_x + 2229) * (pos_y + 12295))
   return flr(rnd(0xffff))
end

-- given an {x,y} position, calculates the aligned starting position for the
-- biome that position is in.
-- biomes are currently defined to be 128x128
function calculate_biome_pos(pos_x, pos_y)
   return flr(pos_x / 128), flr(pos_y / 128)
end

-- determines which biome a given world map position should be,
-- returns the object out of the biome_data table
function get_biome_name(pos_x, pos_y)
   local biome_pos_x, biome_pos_y = calculate_biome_pos(pos_x, pos_y)
   local uid = generate_uid(biome_pos_x, biome_pos_y)
   return biome_metadata[(uid % #biome_metadata) + 1]
end

-- determine what sprite to render for a given position.
-- todo: this needs the ability to have a list of 'changed' tiles to check against.
function get_tile(pos_x, pos_y)
   local biome_name = get_biome_name(pos_x, pos_y)
   local biome = biome_data[biome_name]
   local uid = generate_uid(pos_x, pos_y)

   return biome.tile_lookup[(uid % #biome.tile_lookup) + 1]
end

-- generate the map and writes to the map area from 0 - block_size,
-- assuming 'start' as the top-left corner of the map area to generate.
-- writes block_size x block_size tiles
-- after a call to generate_map you should always center the camera/player over the map, i.e.
-- camera at { (block_size / 2) - 8, (block_size / 2) - 8 }
function generate_map(start_x, start_y)
   for x=0,63 do
      for y=0,63 do
         mset(x, y, get_tile(start_x+x, start_y+y))
      end
   end
end