Enemy spawning

References

1. ^While Lakitu's stretch of a level runs, the game checks every 128 frames whether a Lakitu is on screen. When none is, it counts the check, and on the third it sends a new Lakitu if an enemy slot is free (if none is, it tries again at the next check); the count starts over when a Lakitu is set up. The new one flies high in worlds 1 to 6 ($20) and lower in worlds 7 to 9 and A to D ($60). When one is on screen, the same check leads to the Spiny throw, not covered here.Codedoppelganger's SMB2J disassembly
   How we know

   7251LakituAndSpinyHandler:
   7252          lda FrenzyEnemyTimer    ;if timer here not expired, leave
   7253          bne ExLSHand
   7254          cpx #$05                ;if we are on the special use slot, leave
   7255          bcs ExLSHand
   7256          lda #$80                ;set timer
   7257          sta FrenzyEnemyTimer
   7258          ldy #$04                ;start with the last enemy slot
   7259ChkLak:   lda Enemy_ID,y          ;check all enemy slots to see
   7260          cmp #Lakitu             ;if lakitu is on one of them
   7261          beq CreateSpiny         ;if so, branch out of this loop
   7262          dey                     ;otherwise check another slot
   ⋮
   

   7234SetupLakitu:
   7235      lda #$00                   ;erase counter for lakitu's reappearance
   7236      sta LakituReappearTimer
   

   928   ldx #$14                  ;set offset to decrement only frame timers
   929   dec IntervalTimerControl  ;if interval timer control not expired, branch
   930   bpl DecrTheTimers         ;to skip and thus decrement only frame timers
   931   lda #$14
   932   sta IntervalTimerControl  ;otherwise reset interval timer control to 20 frames
   933   ldx #$23                  ;and load offset to decrement frame and interval timers
   

   Checked against the evidence by its author, an AI model, 2026-10-26.

   Checked by a second AI model, 2026-10-26.

   In the claim ledger

2. ^A spawner absorbs pollution at 20 units plus 1% of its chunk's pollution per update. Each unit has a pollution cost to join an attack: a small biter 4, a medium 20, a big 80, a behemoth 400, and spitters 4, 12, 30 and 200. An attack group gathers for 1 to 10 minutes and holds at most 200 units.Codefactorio-data 2.1.23
   How we know

   174    absorptions_per_second = { pollution = { absolute = 20, proportional = 0.01 } },
   

   103    absorptions_to_join_attack = { pollution = 4 },
   

   346    absorptions_to_join_attack = { pollution = 20 },
   

   406    absorptions_to_join_attack = { pollution = 80 },
   

   467    absorptions_to_join_attack = { pollution = 400 },
   

   574    absorptions_to_join_attack = { pollution = 12 },
   

   631    absorptions_to_join_attack = { pollution = 30 },
   

   688    absorptions_to_join_attack = { pollution = 200 },
   

   131      -- manual groups created through the API are unaffected.
   132      max_unit_group_size = 200
   

   106      -- pollution triggered group waiting time is a random time between min and max gathering time
   107      min_group_gathering_time = 3600,
   108      max_group_gathering_time = 10 * 3600,
   

   Checked against the evidence by its author, an AI model, 2026-10-26.

   Checked by a second AI model, 2026-10-26.

   In the claim ledger

3. ^Spawners, not factories, turn pollution into attacks: each update a spawner absorbs 20 plus 1% of its chunk's pollution (the wiki gives two versions of when it stops or what it takes instead, below), and after absorbing a certain amount it sends one of its units to a gathering point; the gathered group attacks after 1 to 10 minutes. The absorbed pollution pays for sending units, not for spawning them.WikiFactorio Wiki: Pollutionrevised
   How we know

   If a chunk's pollution is greater than 20, each enemy spawner absorbs 20 + 0.01 * [chunk's pollution] every 64 ticks, otherwise it absorbs 3 times the amount of pollution needed for the most expensive unit it can spawn for the current evolution factor. … After a certain amount of pollution is absorbed the spawner sends one of its biters/spitters to a rendezvous point. … Every 1 to 10 minutes (random) the mustered biters launch an attack.

   Every 64 ticks, spawners absorb pollution until they have absorbed more than 3 times the amount needed to send the most expensive unit at the current evolution level.

   Note that absorbing pollution is not for spawning biters, but for sending biters to an attack group.

   Checked against the evidence by its author, an AI model, 2026-10-26.

   Revised after a check by a second AI model, 2026-10-26.

   In the claim ledger

4. ^A spawner keeps up to 7 units of its own and spawns one every 6 seconds at evolution 0, quickening to every 2.5 seconds at evolution 1.Codefactorio-data 2.1.23 · Factorio Wiki: Enemies
   How we know

   177    max_count_of_owned_units = 7,
   

   203    -- With zero evolution the spawn rate is 6 seconds, with max evolution it is 2.5 seconds
   204    spawning_cooldown = {360, 150},
   

   This interval (spawning_cooldown in the enemy-spawner definition) is interpolated between 360 (0 evolution) and 150 (1 evolution) game ticks (= 6 to 2.5 seconds).

   Checked against the evidence by its author, an AI model, 2026-10-26.

   Checked by a second AI model, 2026-10-26.

   In the claim ledger

5. ^Enemies expand: between 10 and 60 minutes apart, shorter as evolution rises (60 minutes at evolution 0, 10 at 1, on the curve −e² + 2e), a group of 5 to 10 units times 8 to the power of evolution (so 40 to 80 at evolution 1, at the default enemy base settings; 2.1 also scales it with the map's base-size setting) sets out to a chunk 3 to 5 chunks out from the enemy bases already there, where its members turn into new spawners and worms, one every 30 seconds.Codefactorio-data 2.1.23 · Friday Facts #445 · Factorio Wiki: Enemiesrevised
   How we know

   48      -- Distance in chunks from the furthest base around.
   49      -- This prevents expansions from reaching too far into the
   50      -- player's territory
   51      max_expansion_distance = 5,
   52      min_expansion_distance = 3,
   

   83      -- Size of the group that goes to build new base.
   84      settler_group_min_size = 5,
   85      settler_group_max_size = 10,
   86      -- Exponential factor used to determine the size of the settler group based on the evolution factor.
   87      -- The size is calculated as:
   88      -- size = random(min_size, max_size) * (evolution_group_size_factor ^ evolution_factor)
   89      evolution_group_size_factor = 8.0,
   ⋮
   91      -- Ticks to expand to a single
   92      -- position for a base is used.
   ⋮
   94      -- cooldown is calculated as follows:
   95      --   cooldown = lerp(max_expansion_cooldown, min_expansion_cooldown, -e^2 + 2 * e),
   96      -- where lerp is the linear interpolation function, and e is the current evolution factor.
   ⋮
   

   So for 2.1.13 we have increased the cooldowns: 30 seconds by default, and 10 seconds for Deathworld.

   Modify group size dependent on the map generator biter base size.

   336    - Increased expansion group build base cooldown from 5 seconds to 30 seconds.
   337    - Reduced early game max enemy expansion group size from 20 to 10.
   

   When the expansion group reaches its target, each unit will begin dying in sequence to randomly create a spawner or a worm after some delay.

   Checked against the evidence by its author, an AI model, 2026-10-26.

   Revised after a check by a second AI model, 2026-10-26.

   In the claim ledger