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Elite on the BBC Micro

Tactics: TACTICS (Part 7 of 7) [Cassette version]

Name: TACTICS (Part 7 of 7) [Show more] Type: Subroutine Category: Tactics Summary: Apply tactics: Set pitch, roll, and acceleration Deep dive: Program flow of the tactics routine
Context: See this subroutine in context in the source code Variations: See code variations for this subroutine in the different versions References: No direct references to this subroutine in this source file

This section looks at manoeuvring the ship. Specifically: * Work out which direction the ship should be moving, depending on whether it's an escape pod, 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
.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 LDA XX15 \ Reverse the signs of XX15 and the dot product in CNT, EOR #%10000000 \ starting with the x-coordinate STA XX15 LDA XX15+1 \ Then reverse the sign of the y-coordinate EOR #%10000000 STA XX15+1 LDA XX15+2 \ And then the z-coordinate, so now the XX15 vector goes EOR #%10000000 \ from the enemy ship to our ship (it was previously the STA XX15+2 \ other way round) LDA CNT \ And finally change the sign of the dot product in CNT, EOR #%10000000 \ so now it's positive if the ships are facing each STA CNT \ other, and negative if they are facing the same way .TA15 \ If we get here, then one of the following is true: \ \ * This is an escape pod 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. Escape pods, \ 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 EOR #%10000000 \ Set the ship's pitch counter to 3, with the opposite AND #%10000000 \ sign to the dot product result, which gently pitches ORA #%00000011 \ the ship towards the direction of the XX15 vector STA INWK+30 LDA INWK+29 \ Fetch the roll counter from byte #29 into A and clear AND #%01111111 \ the sign bit CMP #16 \ If A >= 16 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 EOR INWK+30 \ Set the ship's roll counter to 5, with the sign set to AND #%10000000 \ positive if the pitch counter and dot product have EOR #%10000101 \ different signs, negative if they have the same sign STA INWK+29 .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 #22 \ The dot product is positive, so the ships are facing BCC TA9 \ each other. If A < 22 then the ships are not heading \ directly towards each other, so jump to TA9 to slow \ down 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