diff --git a/constants.lua b/constants.lua
deleted file mode 100644
index 3a3335d..0000000
--- a/constants.lua
+++ /dev/null
@@ -1,52 +0,0 @@
--- unavoidable table lookups used by multiple files are stored here.
--- nothing in here should
--- be changed at runtime.
---
--- also has commented "fake constants" that are replaced by actual values, for
--- reference.
---
--- see index_map.md for more "constant" values
-
-function init_constants()
-   -- 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
-      }
-   }
-
-end
diff --git a/data.lua b/data.lua
new file mode 100644
index 0000000..3496d10
--- /dev/null
+++ b/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
diff --git a/infimap.p8 b/infimap.p8
index d583fe5..55aaa40 100644
--- a/infimap.p8
+++ b/infimap.p8
@@ -1,7 +1,7 @@
 pico-8 cartridge // http://www.pico-8.com
 version 18
 __lua__
-#include constants.lua
+#include data.lua
 #include sound.lua
 #include mapgen.lua
 #include player.lua
@@ -72,7 +72,7 @@ __gfx__
 00000000000000000000000000000000000000000000000000000000000880000008800000088000000880000008800000088000000880000008800000088000
 00000000000000000000000000000000000000000000000000000000000880000001100000011000001100000000110000011000000110000001100000011000
 __gff__
-0000000303000000020003000103000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+0000000303000000020003000103000000010000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 __sfx__
 012000000961009610096100961007600096000a6000b6000c6000e6000f60010600116001260013600146001560017600196001b6001d6001f6002160024600276002a6002d6003060034600396003c6003e600
diff --git a/main.lua b/main.lua
index 40f2f63..ae45c13 100644
--- a/main.lua
+++ b/main.lua
@@ -1,5 +1,5 @@
 function _init()
-   init_constants()
+   init_data()
    init_sound()
    init_mapgen()
    init_player(32, 32)
@@ -11,7 +11,6 @@ end
 function _update()
    handle_input()
    handle_map_update()
-   debug_input()
 end
 
 function _draw()
@@ -41,41 +40,24 @@ function out_of_bounds(pos_x, pos_y)
       pos_y < 0 or pos_y > 48
 end
 
-function debug_input()
-   if btn"5" and btn"4" then
-      if (btnp"0") debug = "res"
-      if (btnp"1") debug = "sfx"
-      if (btnp"2") debug = "map"
-      if (btnp"3") debug = nil
-   end
-
+function debug_res()
+   print("mem: " + stat(0))
+   print("cpu: " + stat(1))
+   print("fps: " .. stat(9) .. " / " .. stat(8))
 end
 
-function render_debug_info()
-   if debug == "res" then
-      clip(0, 0, 32, 24)
-      rectfill(0, 0, 32, 24, 0)
-
-      print(stat(0), 0, 0, 15)
-      print(stat(1), 0, 8, 15)
-      print(stat(9), 0, 16, 15)
-      print("/", 12, 16, 15)
-      print(stat(8), 20, 16, 15)
-   elseif debug == "sfx" then
-      clip(0, 0, 16, 8)
-      rectfill(0, 0, 16, 8, 0)
-
-      print(stat(24), 0, 0, 15)
-   elseif debug == "map" then
-      clip(0, 0, 32, 24)
-      rectfill(0, 0, 32, 24, 0)
-
-      print(camera_pos_x, 0, 0, 15)
-      print(camera_pos_y, 16, 0, 15)
-      print(player_pos_x, 0, 8, 15)
-      print(player_pos_y, 16, 8, 15)
-      print(get_biome_name(player_pos_x, player_pos_y), 0, 16, 15)
-   end
-
-   clip()
+function debug_sfx()
+   print("Music pattern: " .. stat(24))
+end
+
+function debug_map()
+   print("Local: " .. camera_pos_x .. " " .. camera_pos_y)
+   print("Global: " .. player_pos_x .. " " .. player_pos_y)
+   print(get_biome_name(player_pos_x, player_pos_y))
+end
+
+function debug_mod_buffer()
+   for k,v in pairs(mod_buffer) do
+      print(k .. ": " .. tostr(v))
+   end
 end
diff --git a/mapgen.lua b/mapgen.lua
index b24126f..45e018b 100644
--- a/mapgen.lua
+++ b/mapgen.lua
@@ -87,8 +87,12 @@ function get_biome_name(pos_x, pos_y)
 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.
+-- 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[tostr(pos_x) .. "+" .. tostr(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)
diff --git a/player.lua b/player.lua
index 68e96df..a4a707c 100644
--- a/player.lua
+++ b/player.lua
@@ -49,6 +49,10 @@ function handle_input()
    local new_pos_x, new_pos_y = camera_pos_x, camera_pos_y
    local new_ppos_x, new_ppos_y = player_pos_x, player_pos_y
 
+   if btnp"4" then
+      interact()
+   end
+
    if btnp"0" or btnp"1" or btnp"2" or btnp"3" then
       if btnp"0" then
          new_pos_x -= 1 -- move left
@@ -121,6 +125,22 @@ function can_interact()
    return nil, nil
 end
 
+function interact()
+   -- get the position to interact with
+   local x, y = can_interact()
+   if (x == nil or y == nil) return
+
+   local sprite = mget(x, y)
+   local data = object_interaction_map[sprite]
+   -- todo: figure out playing sound effects, animation?
+
+   -- modify the rendered tile
+   mset(x, y, data.replacement)
+
+   -- and write the change to the buffer
+   write_map_change(data.replacement, x, y)
+end
+
 -- returns x,y representing the map-local position the player is facing.
 function get_position_facing()
    return camera_pos_x+8+facing_h, camera_pos_y+8+facing_v