Add interactability with some map objects, and store changed map tiles in a buffer for persistence.

data.lua

@@ -0,0 +1,74 @@
+-- data shared between modules goes here, both constants and mutable data.
+-- also contains some functions that act solely on the data
+--
+-- also has commented "fake constants" that are replaced by actual values, for
+-- reference.
+--
+-- also see index_map.md for more "constant" values
+
+function init_data()
+   -- block_size = 64
+   -- biome_size = 128
+
+   -- the indices here are sprite numbers.
+   object_interaction_map = {
+      -- bush
+      [3] = {
+         replacement = 17,
+         sfx = 13,
+         drop = 69
+      },
+
+      -- tree
+      [4] = {
+         replacement = 14,
+         sfx = 11,
+         drop = 65
+      },
+
+      -- big mushroom
+      [8] = {
+         replacement = 16,
+         sfx = 12,
+         drop = 66
+      },
+
+      -- cactus w/ flower
+      [10] = {
+         replacement = 15,
+         sfx = 12,
+         drop = 68
+      },
+
+      -- cactus
+      [13] = {
+         replacement = 15,
+         sfx = 12,
+         drop = 67
+      }
+   }
+
+   -- 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 max_mod_entries.
+   max_mod_entries = 4096
+   mod_buffer = {}
+end
+
+-- x and y are global coords
+function write_map_change(new_sprite, x, y)
+   local global_x, global_y = calculate_world_pos(x, y)
+   if #mod_buffer >= max_mod_entries then
+      -- todo: make the buffer ring
+   end
+
+   mod_buffer[tostr(global_x) .. "+" .. tostr(global_y)] = new_sprite
+end
+
+function calculate_world_pos(x, y)
+   -- player_pos_[xy] is world absolute, so we can get the world pos of map 0,0,
+   -- then add x and y back in.
+   return player_pos_x - camera_pos_x - 8 + x,
+          player_pos_y - camera_pos_y - 8 + y
+end