--- Procedural generation methods

function init_world()
   -- 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

   -- the indices here are sprite numbers.
   object_interaction_map = {
      -- bush
      [3] = {
         replacement = 17,
         sfx = 13,
         drop = 68
      },

      -- tree
      [4] = {
         replacement = 14,
         sfx = 11,
         drop = 64
      },

      -- big mushroom
      [8] = {
         replacement = 16,
         sfx = 12,
         drop = 65
      },

      -- cactus w/ flower
      [10] = {
         replacement = 15,
         sfx = 12,
         drop = 67
      },

      -- cactus
      [13] = {
         replacement = 15,
         sfx = 12,
         drop = 66
      }
   }

   -- initialize a ring buffer of changed positions. In use, this will be keyed
   -- using strings of the form mod_buffer["x:y"], using absolute world
   -- coordinates. this is to flatten the buffer so that #mod_cache is useful
   -- for checking against the number of allowed entries
   mod_buffer = {}
   mod_queue = {}
end

-- draw the sprites for this part of the world to the screen
-- this calculates everything about the world fresh every frame,
-- but pico-8 handles this just fine!
function draw_world(start_x, start_y)
   for x=0,15 do
      for y=0,15 do
         spr(get_tile(start_x-8+x, start_y-8+y), x*8, y*8)
      end
   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.
-- pos_x and pos_y are global coordinates.
function get_tile(pos_x, pos_y)
   -- lookup changes in the change buffer
   local modded_sprite = mod_buffer[get_mod_key(pos_x, pos_y)]
   if (modded_sprite) return modded_sprite

   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


---
--- mod buffer functions - these handle parts of the world map that have
--- changed from their 'default' position
---
--- todo: the mod buffer eventually needs to be a bit more elaborate.
---   we need to make it possible to cull only old and unimportant changes,
---   so each entry needs a 'critical' flag and something to indicate order
---   added. We could keep 2 buffers for the latter, one that's just keys
---   in an array... expensive though.
---

-- x and y are global coords
function get_mod_key(x, y)
   return tostr(x) .. ":" .. tostr(y)
end

-- x and y are map-local coords
function write_map_change(new_sprite, x, y, perm)
   if #mod_buffer >= 8192 then
      cull_mod_buffer()
   end
   local key = get_mod_key(x, y)
   mod_buffer[key] = new_sprite

   -- the queue gives us a time-ordered list of items to delete.
   -- anything that should persist is simply not added to the queue,
   -- making it un-deletable.
   -- obviously if we end up with a *very large number* of persistent
   -- objects we can run into trouble, but this is functionally a design
   -- limitation.
   if not perm then
      add(mod_queue, key)
   end
end

function cull_mod_buffer()
   -- we cull 512 entries at a time.
   -- stub: implement me!
end