1 files changed, 215 insertions, 0 deletions
diff --git a/battle/magikarp_length.asm b/battle/magikarp_length.asm
new file mode 100644
index 000000000..237450928
--- /dev/null
+++ b/battle/magikarp_length.asm
@@ -0,0 +1,215 @@
+CalcMagikarpLength: ; fbbfc
+; Return Magikarp's length (in mm) at MagikarpLength (big endian).
+;
+; input:
+;   de: EnemyMonDVs
+;   bc: PlayerID
+
+; This function is poorly commented.
+
+; In short, it generates a value between 190 and 1786 using
+; a Magikarp's DVs and its trainer ID. This value is further
+; filtered in LoadEnemyMon to make longer Magikarp even rarer.
+
+; The value is generated from a lookup table.
+; The index is determined by the dv xored with the player's trainer id.
+
+; bc = rrc(dv[0]) ++ rrc(dv[1]) ^ rrc(id)
+
+; if bc < 10:     [MagikarpLength] = c + 190
+; if bc >= $ff00: [MagikarpLength] = c + 1370
+; else:           [MagikarpLength] = z * 100 + (bc - x) / y
+
+; X, Y, and Z depend on the value of b as follows:
+
+; if b = 0:        x =   310,  y =   2,  z =  3
+; if b = 1:        x =   710,  y =   4,  z =  4
+; if b = 2-9:      x =  2710,  y =  20,  z =  5
+; if b = 10-29:    x =  7710,  y =  50,  z =  6
+; if b = 30-68:    x = 17710,  y = 100,  z =  7
+; if b = 69-126:   x = 32710,  y = 150,  z =  8
+; if b = 127-185:  x = 47710,  y = 150,  z =  9
+; if b = 186-224:  x = 57710,  y = 100,  z = 10
+; if b = 225-243:  x = 62710,  y =  50,  z = 11
+; if b = 244-251:  x = 64710,  y =  20,  z = 12
+; if b = 252-253:  x = 65210,  y =   5,  z = 13
+; if b = 254:      x = 65410,  y =   2,  z = 14
+
+
+	; bc = rrc(dv[0]) ++ rrc(dv[1]) ^ rrc(id)
+
+	; id
+	ld h, b
+	ld l, c
+	ld a, [hli]
+	ld b, a
+	ld c, [hl]
+	rrc b
+	rrc c
+
+	; dv
+	ld a, [de]
+	inc de
+	rrca
+	rrca
+	xor b
+	ld b, a
+
+	ld a, [de]
+	rrca
+	rrca
+	xor c
+	ld c, a
+
+	; if bc < 10:
+	;     de = bc + 190
+	;     break
+
+	ld a, b
+	and a
+	jr nz, .no
+	ld a, c
+	cp 10
+	jr nc, .no
+
+	ld hl, 190
+	add hl, bc
+	ld d, h
+	ld e, l
+	jr .done
+
+.no
+
+	ld hl, .Lengths
+	ld a, 2
+	ld [wd265], a
+
+.read
+	ld a, [hli]
+	ld e, a
+	ld a, [hli]
+	ld d, a
+	call .BCLessThanDE
+	jr nc, .next
+
+	; c = (bc - de) / [hl]
+	call .BCMinusDE
+	ld a, b
+	ld [hDividend + 0], a
+	ld a, c
+	ld [hDividend + 1], a
+	ld a, [hl]
+	ld [hDivisor], a
+	ld b, 2
+	call Divide
+	ld a, [hQuotient + 2]
+	ld c, a
+
+	; de = c + 100 * (2 + i)
+	xor a
+	ld [hMultiplicand + 0], a
+	ld [hMultiplicand + 1], a
+	ld a, 100
+	ld [hMultiplicand + 2], a
+	ld a, [wd265]
+	ld [hMultiplier], a
+	call Multiply
+	ld b, 0
+	ld a, [hProduct + 3]
+	add c
+	ld e, a
+	ld a, [hProduct + 2]
+	adc b
+	ld d, a
+	jr .done
+
+.next
+	inc hl ; align to next triplet
+	ld a, [wd265]
+	inc a
+	ld [wd265], a
+	cp 16
+	jr c, .read
+
+	call .BCMinusDE
+	ld hl, 1600
+	add hl, bc
+	ld d, h
+	ld e, l
+
+.done
+	; hl = de * 10
+	ld h, d
+	ld l, e
+rept 2
+	add hl, hl
+endr
+	add hl, de
+	add hl, hl
+
+	; hl = hl / 254
+	ld de, -254
+	ld a, -1
+.div_254
+	inc a
+	add hl, de
+	jr c, .div_254
+
+	; d, e = hl / 12, hl % 12
+	ld d, 0
+.mod_12
+	cp 12
+	jr c, .ok
+	sub 12
+	inc d
+	jr .mod_12
+.ok
+	ld e, a
+
+	ld hl, MagikarpLength
+	ld [hl], d
+	inc hl
+	ld [hl], e
+	ret
+; fbc9a
+
+.BCLessThanDE: ; fbc9a
+; Intention: Return bc < de.
+; Reality: Return b < d.
+	ld a, b
+	cp d
+	ret c
+	ret nc ; whoops
+	ld a, c
+	cp e
+	ret
+; fbca1
+
+.BCMinusDE: ; fbca1
+; bc -= de
+	ld a, c
+	sub e
+	ld c, a
+	ld a, b
+	sbc d
+	ld b, a
+	ret
+; fbca8
+
+.Lengths: ; fbca8
+;	     ????, divisor
+	dwb   110, 1
+	dwb   310, 2
+	dwb   710, 4
+	dwb  2710, 20
+	dwb  7710, 50
+	dwb 17710, 100
+	dwb 32710, 150
+	dwb 47710, 150
+	dwb 57710, 100
+	dwb 62710, 50
+	dwb 64710, 20
+	dwb 65210, 5
+	dwb 65410, 2
+	dwb 65510, 1 ; not used
+; fbcd2