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BBC Micro Elite

Tactics: TACTICS (Part 7 of 7) (6502SP version)

Name: TACTICS (Part 7 of 7) [View in context] Type: Subroutine Category: Tactics Summary: Apply tactics: Set pitch, roll, and acceleration
This section looks at manoeuvring the ship. Specifically: * Work out which direction the ship should be moving, depending on the type of ship, where it is, which direction it is pointing, and how aggressive it is * Set the pitch and roll counters to head in that direction * Speed up or slow down, depending on where the ship is in relation to us Other entry points: TA151 Make the ship head towards the planet
.TA4 LDA INWK+7 \ If z_hi >= 3 then the ship is quite far away, so jump CMP #3 \ down to TA5 BCS TA5 LDA INWK+1 \ Otherwise set A = x_hi OR y_hi and extract bits 1-7 ORA INWK+4 AND #%11111110 BEQ TA15 \ If A = 0 then the ship is pretty close to us, so jump \ to TA15 so it heads away from us .TA5 \ If we get here then the ship is quite far away JSR DORND \ Set A and X to random numbers ORA #%10000000 \ Set bit 7 of A CMP INWK+32 \ If A >= byte #32 (the ship's AI flag) then jump down BCS TA15 \ to TA15 so it heads away from us \ We get here if A < byte #32, and the chances of this \ being true are greater with high values of byte #32. \ In other words, higher byte #32 values increase the \ chances of a ship changing direction to head towards \ us - or, to put it another way, ships with higher \ byte #32 values are spoiling for a fight. Thargoids \ have byte #32 set to 255, which explains an awful lot .TA20 \ If this is a missile we will have jumped straight \ here, but we also get here if the ship is either far \ away and aggressive, or not too close JSR TAS6 \ Call TAS6 to negate the vector in XX15 so it points in \ the opposite direction LDA CNT \ Change the sign of the dot product in CNT, so now it's EOR #%10000000 \ positive if the ships are facing each other, and \ negative if they are facing the same way .TA152 STA CNT \ Update CNT with the new value in A .TA15 \ If we get here, then one of the following is true: \ \ * This is a trader and XX15 is pointing towards the \ planet \ \ * The ship is pretty close to us, or it's just not \ very aggressive (though there is a random factor \ at play here too). XX15 is still pointing from our \ ship towards the enemy ship \ \ * The ship is aggressive (though again, there's an \ element of randomness here). XX15 is pointing from \ the enemy ship towards our ship \ \ * This is a missile heading for a target. XX15 is \ pointing from the missile towards the target \ \ We now want to move the ship in the direction of XX15, \ which will make aggressive ships head towards us, and \ ships that are too close turn away. Peaceful traders, \ meanwhile, head off towards the planet in search of a \ space station, and missiles home in on their targets LDY #16 \ Set (A X) = roofv . XX15 JSR TAS3 \ \ This will be positive if XX15 is pointing in the same \ direction as an arrow out of the top of the ship, in \ other words if the ship should pull up to head in the \ direction of XX15 TAX \ Copy A into X so we can retrieve it below EOR #%10000000 \ Give the ship's pitch counter the opposite sign to the AND #%10000000 \ dot product result, with a value of 0 STA INWK+30 TXA \ Retrieve the original value of A from X ASL A \ Shift A left to double it and drop the sign bit CMP RAT2 \ If A < RAT2, skip to TA11 (so if RAT2 = 0, we always BCC TA11 \ set the pitch counter to RAT) LDA RAT \ Set the magnitude of the ship's pitch counter to RAT ORA INWK+30 \ (we already set the sign above) STA INWK+30 .TA11 LDA INWK+29 \ Fetch the roll counter from byte #29 into A ASL A \ Shift A left to double it and drop the sign bit CMP #32 \ If A >= 32 then jump to TA6, as the ship is already BCS TA6 \ in the process of rolling LDY #22 \ Set (A X) = sidev . XX15 JSR TAS3 \ \ This will be positive if XX15 is pointing in the same \ direction as an arrow out of the right side of the \ ship, in other words if the ship should roll right to \ head in the direction of XX15 TAX \ Copy A into X so we can retrieve it below EOR INWK+30 \ Give the ship's roll counter a positive sign if the AND #%10000000 \ pitch counter and dot product have different signs, EOR #%10000000 \ negative if they have the same sign, with a value of 0 STA INWK+29 TXA \ Retrieve the original value of A from X ASL A \ Shift A left to double it and drop the sign bit CMP RAT2 \ If A < RAT2, skip to TA12 (so if RAT2 = 0, we always BCC TA12 \ set the roll counter to RAT) LDA RAT \ Set the magnitude of the ship's roll counter to RAT ORA INWK+29 \ (we already set the sign above) STA INWK+29 .TA12 .TA6 LDA CNT \ Fetch the dot product, and if it's negative jump to BMI TA9 \ TA9, as the ships are facing away from each other and \ the ship might want to slow down to take another shot CMP CNT2 \ The dot product is positive, so the ships are facing BCC TA9 \ each other. If A < CNT2 then the ships are not heading \ directly towards each other, so jump to TA9 to slow \ down .PH10E LDA #3 \ Otherwise set the acceleration in byte #28 to 3 STA INWK+28 RTS \ Return from the subroutine .TA9 AND #%01111111 \ Clear the sign bit of the dot product in A CMP #18 \ If A < 18 then the ship is way off the XX15 vector, so BCC TA10 \ return from the subroutine (TA10 contains an RTS) \ without slowing down, as it still has quite a bit of \ turning to do to get on course LDA #&FF \ Otherwise set A = -1 LDX TYPE \ If this is not a missile then skip the ASL instruction CPX #MSL BNE P%+3 ASL A \ This is a missile, so set A = -2, as missiles are more \ nimble and can brake more quickly STA INWK+28 \ Set the ship's acceleration to A .TA10 RTS \ Return from the subroutine .TA151 \ This is called from part 3 with the vector to the \ planet in XX15, when we want the ship to turn towards \ the planet. It does the same dot product calculation \ as part 3, but it can also change the value of RAT2 \ so that roll and pitch is always applied LDY #10 \ Set (A X) = nosev . XX15 JSR TAS3 \ \ The bigger the value of the dot product, the more \ aligned the two vectors are, with a maximum magnitude \ in A of 36 (96 * 96 >> 8). If A is positive, the \ vectors are facing in a similar direction, if it's \ negative they are facing in opposite directions CMP #&98 \ If A is positive or A <= -24, jump to ttt BCC ttt LDX #0 \ A > -24, which means the vectors are facing in STX RAT2 \ opposite directions but are quite aligned, so set \ RAT2 = 0 instead of the default value of 4, so we \ always apply roll and pitch when we turn the ship \ towards the planet .ttt JMP TA152 \ Jump to TA152 to store A in CNT and move the ship in \ the direction of XX15