Lufia & the Fortress of Doom's battle and field rules (USA ROM, our own disassembly of its code)

The game's random numbers come from a table of 55 bytes that it steps through and, each time it wraps, mixes with itself (each byte XORed with another of the table): random(n) is n times the next byte, divided by 256 and rounded down, so it runs from 0 to n − 1 and takes only n's low byte. lufia/random

About: Lufia & the Fortress of Doom Random number generator

Random battles start from a roll on each step: on a map that has them, outside some map states that skip the roll (not identified here), a step taken while the party's step count is 4 or more starts a battle when random(128) is below 10 × danger ÷ 128, halved off the world map, where danger is 128 unless something changes it (see the danger claim). So with danger 128 a step after the fourth starts a battle with chance 10 in 128 (1 in 12.8) on the world map and 5 in 128 (1 in 25.6) elsewhere, computed. In one state the code tests (two flags not identified here) the roll is out of 255 instead. While a step count that an item or spell effect sets is above 0 and danger is 192 or more, every step starts a battle, the fourth-step wait included; that count goes down by one a step in some map states. lufia/encounter-roll

About: Lufia & the Fortress of Doom Random encounter

Value: threshold 10 · range 128 · wait 4

Lufia Forge reads $077F = 1 as the world map, and the rate as the constant 10 in the LDA at $01:8876; the chance per step here is the code's arithmetic, not a measured rate.

Cited by: Random encounter

The Monster Ring raises the chance of a random battle on the world map from 10 to 15 in 128 a step, and the Clear Silk to 11; off the world map the Monster Ring raises it from 5 to 7 in 128, and the Clear Silk leaves it at 5. The world-map figures are the game's threshold routine given the world-map flag, since no run reached the world map, and every figure assumes the roll is out of 128: in one movement state not followed here it is out of 255 (see the roll claim). The roll scales by a danger number, 128 by default. Each party member's encounter number is the average of the nonzero encounter numbers on the five pieces they wear besides the weapon, the party's is the average of its members' nonzero ones, and danger is the party's number when it has one. Of the 256 items only two have an encounter number: the Monster Ring (192) and the Clear Silk (147). The code also has a second danger source, set by an effect handler that no known item or spell has been traced to; by the routine's arithmetic, with both set danger is their mean, or 192 when they add past 255, and with that one alone it is that one. Measured in MesenCE on the prologue's first map, with the items put on the party: danger 128 with the prologue's own equipment, 147 with the Clear Silk on one member or on all four, 192 with the Monster Ring, and 169 with both, on one member or on two; with the second source written directly, 123 (147 and 100), 192 (192 and 200) and 200 alone. A walk on that map rolled against thresholds of 5, 5 and 7 in 128 with neither item, the Clear Silk and the Monster Ring. lufia/encounter-danger

About: Lufia & the Fortress of Doom Random encounter

{
 "default": 128,
 "overflow": 192,
 "chance": {
  "none": [
   10,
   5
  ],
  "clearSilk": [
   11,
   5
  ],
  "monsterRing": [
   15,
   7
  ]
 }
}

Lufia Forge names property 23 "Encounter rate (?)"; in the item table only Clear Silk (item 56) and Monster Ring (item 138) carry it. The probe puts the items in the party's equipment slots in RAM and calls the game's equip routine $08:F857 for each member and the threshold routine $01:8876, from the field's main loop at $01:8083; the world-map call sets the current-map byte $077F to 1, the value Lufia Forge reads as the world map. The second source is $16B6, set by the handler at $08:F113; the probe writes it directly.

Cited by: Random encounter

A random battle's eight monster slots are filled by draws from the encounter group, each draw putting 1-3 of one monster in consecutive slots. A roll of k + random(k + 1), with k a number set for the battle, then decides how many of the first slots stay filled; fixed formations skip it. Then the slots are kept in order while three limits hold: at most 3 different monsters (a run of the same monster counts once), a total of one picture-size value per different monster below 30, and a total of another below 64; the first slot that breaks a limit is emptied with every later slot not part of a kept run. lufia/battle-size

About: Lufia & the Fortress of Doom Random encounter

Cited by: Random encounter

Each monster enters battle with its table HP plus random(HP ÷ 8 + 1), or random(255) when the HP is 2040 or more: up to an eighth more below 2040 HP, at most 254 more from 2040 up (computed); a monster whose HP is 65535 gets no bonus. lufia/monster-hp

A defeated monster adds its table experience to the battle's total; its gold plus a random bonus (each byte of gold ÷ 8 rolled separately, so up to about an eighth more, computed); and, if it has a drop item, the item when random(255) is below its rate byte, a chance of (rate + 1) in 256 for rates 1 to 255 (computed). One drop is kept per battle: a later winning roll replaces it. lufia/rewards

About: Lufia & the Fortress of Doom Item drop table

Cited by: Item drop table

A physical attack hits when random(255) is below the attacker's hit byte: a monster's is 255, so a monster's attack always passes this roll (computed), and a party member's is the second byte of their weapon's property 14 (255 if it is 0): 255 on 40 of the 42 weapons, 127 on the Luck Blade, 51 on the Gloom Pick. Before the roll, when a party member attacks a party member, the target's two status flags of value $0C are cleared (not identified here). lufia/physical-hit

About: Lufia & the Fortress of Doom Hit chance

No agility or evade test was found on this path; the target's checks that end an attack early (a target no longer there, an attack of 0) are separate.

A physical hit's damage starts from ATP − DFP: (ATP − DFP + 1) ÷ 2 + 1 when ATP is higher, 2 when they are within 16 of each other the other way (or equal), and 0 when DFP is 16 or more higher. A critical hit doubles that base, and makes a base of 0 into 2. The base is then multiplied by a factor of 96 plus the mean of (base ÷ 16 + 2) rolls of random(64), over 128, so the larger the base the closer the factor stays to about 127 (computed), and the mean, rounded up, of two rolls of random(r) is added, where r is ATP ÷ 8 rounded to the nearest, plus 1. DFP counts one and a half times against a target in one state the code tests (not identified here), and an attack with ATP 0 misses. Steps for slayer types, elements, weapon specials, a stone target (no damage) and drain follow and aren't covered here. lufia/physical-damage

About: Lufia & the Fortress of Doom Damage formula

The stat codes $10 and $18 are named ATP and DFP by Lufia Forge's stat list for items; for a monster the lookup's offset table ($08:F30C) sends them to the record's words at +20 and +22, which the monsters table carries as ATP and DFP. The rolls use random's low-byte rule, so an ATP of 2040 or more wraps the second term's range.

Cited by: Damage formula

A physical attack between the two sides is a critical hit when random(255) is 1 to 5, a chance of 5 in 256 (computed), unless the attacker is a party member whose weapon has a special effect or the target is a party member with one status flag (value 2, not identified here); a critical doubles the damage's base. lufia/critical-hit

About: Lufia & the Fortress of Doom Critical hit

The party's Run command never escapes a fixed formation (the bosses'), always escapes in one battle state the code tests (not identified here), and otherwise escapes when random(A ÷ 4) is at least random(M ÷ 2), where A is the highest AGL among the party members present and M the highest among the monsters still in the battle, each counting its base plus a second stat word that the lookup adds to every stat (cleared to 0 when the monsters load and for the party at a step of the battle code; what raises it wasn't traced). So with equal agility the party's roll is out of half the monsters' range, and an escape succeeds about 1 time in 4 (26% at AGL 120, 28% at 40, computed for an evenly spread random byte). lufia/run

About: Lufia & the Fortress of Doom Fleeing

The prologue's party when its first map loads is Maxim at level 78, Selan 77, Guy 77 and Artea 75, with the stats and equipment stored in their character records, and the prologue's script has given them 10 items (87 Miracle, 62 Ex Potion, 28 Ex Magic, 50 Revive, 92 Awaken, 88 Stone Cure, 95 Mystery Pin, 91 Shriek, 36 Ex Arrows, 29 Ex Bomb) and taught Maxim 10, Selan 8 and Artea 12 spells. Measured in MesenCE from power-on, pressing A every 10 frames with Start on every fourth press, about 60 frames (60 to 69) after the prologue's first map is entered. lufia/prologue-party

{
 "character": [
  "Maxim",
  "Selan",
  "Guy",
  "Artea"
 ],
 "level": [
  78,
  77,
  77,
  75
 ],
 "hp": [
  611,
  478,
  717,
  622
 ],
 "mp": [
  370,
  629,
  0,
  514
 ],
 "str": [
  234,
  145,
  255,
  211
 ],
 "int": [
  127,
  250,
  88,
  249
 ],
 "agl": [
  122,
  129,
  103,
  164
 ],
 "mgr": [
  222,
  254,
  153,
  217
 ],
 "weapon": [
  "Dual Blade",
  "Silver Rod",
  "Zircon Axe",
  "Artea's Bow"
 ],
 "armor": [
  "Zircon Armor",
  "Silver Robe",
  "Zircon Armor",
  "Zircon Robe"
 ],
 "shield": [
  "Zircon Plate",
  "Silver Wrist",
  "Silver Plate",
  "Zircon Wrist"
 ],
 "helmet": [
  "Zircon Helm",
  "Sakret",
  "Silver Helm",
  "Zircon Beret"
 ],
 "shoes": [
  "Sword Shoes",
  "Cat Heels",
  "Mach Shoes",
  "Cat Heels"
 ],
 "ring": [
  "Wave Ring",
  "Flash Ring",
  "White Ring",
  "Hi Daze Ring"
 ],
 "item": [
  "Miracle",
  "Ex Potion",
  "Ex Magic",
  "Revive",
  "Awaken",
  "Stone Cure",
  "Mystery Pin",
  "Shriek",
  "Ex Arrows",
  "Ex Bomb"
 ],
 "count": [
  87,
  62,
  28,
  50,
  92,
  88,
  95,
  91,
  36,
  29
 ],
 "spells": {
  "0": "Valor, Boost, Champion, Stronger, Poison, Stun, Dead, Stone, Waken, Protect",
  "1": "Thunder, Bolt, Vulcan, Glacier, Blizzard, Succumb, Fake, Mirror",
  "3": "Flood, Sunder, Dread, Mirror, Bounce, Trick, Valor, Champion, Rally, Courage, Waken, Warp"
 }
}

Slots 0-3 are the Hero's, Lufia's, Aguro's and Jerin's in the story proper.