--- Procedural generation methods

function init_mapgen()
   uid_seed = 2229 -- arbitrarily chosen number
   block_size = 64

   -- Sprite ratios for different biomes, of the form { "biome_name": {frequency, sprite index} ... } }
   -- frequencies don't have to add up to 100, but they should by convention
   --
   -- Sprites are:
   --  * 2 - grass
   --  * 3 - bush
   --  * 4 - tree
   --  * 5 - red flowers
   --  * 6 - pink flowers
   --  * 7 - mushrooms
   biomes = { grassland = { {40, 2}, {28, 5}, {28, 6}, {3, 3}, {1, 4} },
              forest = { {55, 2}, {25, 4}, {5, 3}, {5, 5}, {5, 7}, {5, 6} },
            }
   
   build_biome("forest")
end

-- use biome1 if mix == false
function build_biome(biome1, biome2, mix)
   if mix == true then
   else
      frequencies = biomes[biome1]
   end

   -- global variable used for lookup during mapgen
   -- build_biome() writes the biome_lookup table such
   -- that biome_lookup[uid] returns a sprite index,
   -- weighted by the frequency information for current biome.
   biome_lookup = {}
   for i=1,#frequencies do
      local tuple=frequencies[i]
      for j=1,tuple[1] do
         add(biome_lookup, tuple[2])
      end
   end
end

-- generates a unique identifier for a position
-- uses srand() and rand() to get an unpredictable value (is this too slow?)
function generate_uid(pos)
   srand((pos[1] + uid_seed) * (pos[2] + (uid_seed^2)))
   return flr(rnd(-1))
end

-- determine what sprite to render for a given position
function get_tile(pos)
   return biome_lookup[(generate_uid(pos) % #biome_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)
   for x=0,block_size-1 do
      for y=0,block_size-1 do
         mset(x, y, get_tile({start[1]+x, start[2]+y}))
      end
   end
end