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BBC Micro Elite

Universe: TT111

Name: TT111 [View in context] Type: Subroutine Category: Universe Summary: Set the current system to the nearest system to a point
Given a set of galactic coordinates in (QQ9, QQ10), find the nearest system to this point in the galaxy, and set this as the currently selected system. Arguments: QQ9 The x-coordinate near which we want to find a system QQ10 The y-coordinate near which we want to find a system Returns: QQ8(1 0) The distance from the current system to the nearest system to the original coordinates QQ9 The x-coordinate of the nearest system to the original coordinates QQ10 The y-coordinate of the nearest system to the original coordinates QQ15 to QQ15+5 The three 16-bit seeds of the nearest system to the original coordinates Other entry points: TT111-1 Contains an RTS
.TT111 { JSR TT81 \ Set the seeds in QQ15 to those of system 0 in the \ current galaxy (i.e. copy the seeds from QQ21 to QQ15) \ We now loop through every single system in the galaxy \ and check the distance from (QQ9, QQ10). We get the \ galactic coordinates of each system from the system's \ seeds, like this: \ \ x = w1_hi (which is stored in QQ15+3) \ y = w0_hi (which is stored in QQ15+1) \ \ so the following loops through each system in the \ galaxy in turn and calculates the distance between \ (QQ9, QQ10) and (w1_hi, w0_hi) to find the closest one LDY #127 \ Set Y = T = 127 to hold the shortest distance we've STY T \ found so far, which we initially set to half the \ distance across the galaxy, or 127, as our coordinate \ system ranges from (0,0) to (255, 255) LDA #0 \ Set A = U = 0 to act as a counter for each system in STA U \ the current galaxy, which we start at system 0 and \ loop through to 255, the last system .TT130 LDA QQ15+3 \ Set A = w1_hi - QQ9, the horizontal distance between SEC \ (w1_hi, w0_hi) and (QQ9, QQ10) SBC QQ9 BCS TT132 \ If a borrow didn't occur, i.e. w1_hi >= QQ9, then the \ result is positive, so jump to TT132 and skip the \ following two instructions EOR #&FF \ Otherwise negate the result in A, so A is always ADC #1 \ positive (i.e. A = |w1_hi - QQ9|) .TT132 LSR A \ Set S = A / 2 STA S \ = |w1_hi - QQ9| / 2 LDA QQ15+1 \ Set A = w0_hi - QQ10, the vertical distance between SEC \ (w1_hi, w0_hi) and (QQ9, QQ10) SBC QQ10 BCS TT134 \ If a borrow didn't occur, i.e. w0_hi >= QQ10, then the \ result is positive, so jump to TT134 and skip the \ following two instructions EOR #&FF \ Otherwise negate the result in A, so A is always ADC #1 \ positive (i.e. A = |w0_hi - QQ10|) .TT134 LSR A \ Set A = S + A / 2 CLC \ = |w1_hi - QQ9| / 2 + |w0_hi - QQ10| / 2 ADC S \ \ So A now contains the sum of the horizontal and \ vertical distances, both divided by 2 so the result \ fits into one byte, and although this doesn't contain \ the actual distance between the systems, it's a good \ enough approximation to use for comparing distances CMP T \ If A >= T, then this system's distance is bigger than BCS TT135 \ our "minimum distance so far" stored in T, so it's no \ closer than the systems we have already found, so \ skip to TT135 to move on to the next system STA T \ This system is the closest to (QQ9, QQ10) so far, so \ update T with the new "distance" approximation LDX #5 \ As this system is the closest we have found yet, we \ want to store the system's seeds in case it ends up \ being the closest of all, so we set up a counter in X \ to copy six bytes (for three 16-bit numbers) .TT136 LDA QQ15,X \ Copy the X-th byte in QQ15 to the X-th byte in QQ19, STA QQ19,X \ where QQ15 contains the seeds for the system we just \ found to be the closest so far, and QQ19 is temporary \ storage DEX \ Decrement the counter BPL TT136 \ Loop back to TT136 if we still have more bytes to \ copy .TT135 JSR TT20 \ We want to move on to the next system, so call TT20 \ to twist the three 16-bit seeds in QQ15 INC U \ Increment the system counter in U BNE TT130 \ If U > 0 then we haven't done all 256 systems yet, so \ loop back up to TT130 \ We have now finished checking all the systems in the \ galaxy, and the seeds for the closest system are in \ QQ19, so now we want to copy these seeds to QQ15, \ to set the selected system to this closest system LDX #5 \ So we set up a counter in X to copy six bytes (for \ three 16-bit numbers) .TT137 LDA QQ19,X \ Copy the X-th byte in QQ19 to the X-th byte in QQ15, STA QQ15,X DEX \ Decrement the counter BPL TT137 \ Loop back to TT137 if we still have more bytes to \ copy LDA QQ15+1 \ The y-coordinate of the system described by the seeds STA QQ10 \ in QQ15 is in QQ15+1 (w0_hi), so we copy this to QQ10 \ as this is where we store the selected system's \ y-coordinate LDA QQ15+3 \ The x-coordinate of the system described by the seeds STA QQ9 \ in QQ15 is in QQ15+3 (w1_hi), so we copy this to QQ9 \ as this is where we store the selected system's \ x-coordinate \ We have now found the closest system to (QQ9, QQ10) \ and have set it as the selected system, so now we \ need to work out the distance between the selected \ system and the current system SEC \ Set A = QQ9 - QQ0, the horizontal distance between SBC QQ0 \ the selected system's x-coordinate (QQ9) and the \ current system's x-coordinate (QQ0) BCS TT139 \ If a borrow didn't occur, i.e. QQ9 >= QQ0, then the \ result is positive, so jump to TT139 and skip the \ following two instructions EOR #&FF \ Otherwise negate the result in A, so A is always ADC #1 \ positive (i.e. A = |QQ9 - QQ0|) \ A now contains the difference between the two \ systems' x-coordinates, with the sign removed. We \ will refer to this as the x-delta ("delta" means \ change or difference in maths) .TT139 JSR SQUA2 \ Set (A P) = A * A \ = |QQ9 - QQ0| ^ 2 \ = x_delta ^ 2 STA K+1 \ Store (A P) in K(1 0) LDA P STA K LDA QQ10 \ Set A = QQ10 - QQ1, the vertical distance between the SEC \ selected system's y-coordinate (QQ10) and the current SBC QQ1 \ system's y-coordinate (QQ1) BCS TT141 \ If a borrow didn't occur, i.e. QQ10 >= QQ1, then the \ result is positive, so jump to TT141 and skip the \ following two instructions EOR #&FF \ Otherwise negate the result in A, so A is always ADC #1 \ positive (i.e. A = |QQ10 - QQ1|) .TT141 LSR A \ Set A = A / 2 \ A now contains the difference between the two \ systems' y-coordinates, with the sign removed, and \ halved. We halve the value because the galaxy in \ in Elite is rectangular rather than square, and is \ twice as wide (x-axis) as it is high (y-axis), so to \ get a distance that matches the shape of the \ long-range galaxy chart, we need to halve the \ distance between the vertical y-coordinates. We will \ refer to this as the y-delta JSR SQUA2 \ Set (A P) = A * A \ = (|QQ10 - QQ1| / 2) ^ 2 \ = y_delta ^ 2 \ By this point we have the following results: \ \ K(1 0) = x_delta ^ 2 \ (A P) = y_delta ^ 2 \ \ so to find the distance between the two points, we \ can use Pythagoras - so first we need to add the two \ results together, and then take the square root PHA \ Store the high byte of the y-axis value on the stack, \ so we can use A for another purpose LDA P \ Set Q = P + K, which adds the low bytes of the two CLC \ calculated values ADC K STA Q PLA \ Restore the high byte of the y-axis value from the \ stack into A again ADC K+1 \ Set R = A + K+1, which adds the high bytes of the two STA R \ calculated values, so we now have: \ \ (R Q) = K(1 0) + (A P) \ = (x_delta ^ 2) + (y_delta ^ 2) JSR LL5 \ Set Q = SQRT(R Q), so Q now contains the distance \ between the two systems, in terms of coordinates \ We now store the distance to the selected system * 4 \ in the two-byte location QQ8, by taking (0 Q) and \ shifting it left twice, storing it in (QQ8+1 QQ8) LDA Q \ First we shift the low byte left by setting ASL A \ A = Q * 2, with bit 7 of A going into the C flag LDX #0 \ Now we set the high byte in QQ8+1 to 0 and rotate STX QQ8+1 \ the C flag into bit 0 of QQ8+1 ROL QQ8+1 ASL A \ And then we repeat the shift left of (QQ8+1 A) ROL QQ8+1 STA QQ8 \ And store A in the low byte, QQ8, so QQ8(1 0) now \ contains Q * 4. Given that the width of the galaxy is \ 256 in coordinate terms, the width of the galaxy \ would be 1024 in the units we store in QQ8 JMP TT24 \ Call TT24 to calculate system data from the seeds in \ QQ15 and store them in the relevant locations, so our \ new selected system is fully set up, and return from \ the subroutine using a tail call }