Get Existent Battle Unit Pointer

#   ROUTINE: GET EXISTENT BATTLE UNIT POINTER
#   Parameters:
#       r4 = (unitIndex) In-battle unit index
#   Returns:
#       r2 = (unit) In-battle unit pointer of unit with specified unit index, if index is valid (0 to 20) and unit exists; null, otherwise

801810d4: 2c820015 sltiu r2,r4,0x0015
801810d8: 1040000c beq r2,r0,0x8018110c             #   if (unitIndex >= 21)
801810dc: 00002821 addu r5,r0,r0                    #       return null;
801810e0: 000410c0 sll r2,r4,0x03                   #   unitIndex * 8
801810e4: 00441023 subu r2,r2,r4                    #   unitIndex * 7
801810e8: 00021180 sll r2,r2,0x06                   #   unitIndex * 442                 // 442 == sizeof(unit)
801810ec: 3c038019 lui r3,0x8019
801810f0: 246308cc addiu r3,r3,0x08cc               #   In-battle Unit array base pointer (units)
801810f4: 00432821 addu r5,r2,r3                    #   unit = units[unitIndex]
801810f8: 90a30001 lbu r3,0x0001(r5)                #   unit.UnitID
801810fc: 340200ff ori r2,r0,0x00ff
80181100: 14620002 bne r3,r2,0x8018110c             #   if (unit.UnitID != 0xff)        // Unit doesn't exist
80181104: 00000000 nop                              #       return unit;
80181108: 00002821 addu r5,r0,r0
8018110c: 03e00008 jr r31                           #   return null;
80181110: 00a01021 addu r2,r5,r0