.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 = s1_hi (which is stored in QQ15+3) ; y = s0_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 (s1_hi, s0_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 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure ; the PPU to use nametable 0 and pattern table 0 LDA QQ15+3 ; Set A = s1_hi - QQ9, the horizontal distance between SEC ; (s1_hi, s0_hi) and (QQ9, QQ10) SBC QQ9 BCS TT132 ; If a borrow didn't occur, i.e. s1_hi >= QQ9, then the ; result is positive, so jump to TT132 and skip the ; following two instructions EOR #$FF ; Otherwise negate the result in A, so A is always ADC #1 ; positive (i.e. A = |s1_hi - QQ9|) .TT132 LSR A ; Set S = A / 2 STA S ; = |s1_hi - QQ9| / 2 LDA QQ15+1 ; Set A = s0_hi - QQ10, the vertical distance between SEC ; (s1_hi, s0_hi) and (QQ9, QQ10) SBC QQ10 BCS TT134 ; If a borrow didn't occur, i.e. s0_hi >= QQ10, then the ; result is positive, so jump to TT134 and skip the ; following two instructions EOR #$FF ; Otherwise negate the result in A, so A is always ADC #1 ; positive (i.e. A = |s0_hi - QQ10|) .TT134 LSR A ; Set A = S + A / 2 CLC ; = |s1_hi - QQ9| / 2 + |s0_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 LDA U ; Store the system number U in systemNumber, so when we STA systemNumber ; are done looping through all the candidates, the ; winner's number will be in systemNumber .TT135 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure ; the PPU to use nametable 0 and pattern table 0 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 STA selectedSystem,X ; selectedSystem so we can easily change the selected ; system back to the system we just found STA QQ15,X ; Copy the X-th byte in QQ19 to the X-th byte in QQ15 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 (s0_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 (s1_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 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure ; the PPU to use nametable 0 and pattern table 0 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 A = A + K+1, which adds the high bytes of the two ; calculated values BCC P%+4 ; If the above addition overflowed, set A = 255 LDA #255 STA R ; Store A in R, so we now have R = A + K+1, and: ; ; (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 0) 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_b6 ; 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 callName: TT111 [Show more] Type: Subroutine Category: Universe Summary: Set the current system to the nearest system to a pointContext: See this subroutine in context in the source code References: This subroutine is called as follows: * BR1 calls TT111 * Ghy calls TT111 * HME2 calls TT111 * SelectNearbySystem calls TT111 * SetSelectedSystem calls TT111 * SetupAfterLoad calls TT111 * TT25 calls TT111
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 systemNumber The system number of the nearest system
Other entry points: TT111-1 Contains an RTS
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Subroutine LL5 (category: Maths (Arithmetic))
Calculate Q = SQRT(R Q)
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Macro SETUP_PPU_FOR_ICON_BAR (category: PPU)
If the PPU has started drawing the icon bar, configure the PPU to use nametable 0 and pattern table 0
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Subroutine SQUA2 (category: Maths (Arithmetic))
Calculate (A P) = A * A
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Label TT130 is local to this routine
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Label TT132 is local to this routine
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Label TT134 is local to this routine
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Label TT135 is local to this routine
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Label TT136 is local to this routine
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Label TT137 is local to this routine
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Label TT139 is local to this routine
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Label TT141 is local to this routine
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Subroutine TT20 (category: Universe)
Twist the selected system's seeds four times
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Subroutine TT24_b6 (category: Universe)
Call the TT24 routine in ROM bank 6
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Subroutine TT81 (category: Universe)
Set the selected system's seeds to those of system 0
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Variable selectedSystem in workspace WP
The three 16-bit seeds for the selected system, i.e. the one we most recently snapped the crosshairs to in a chart view
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Variable systemNumber in workspace WP
The current system number, as calculated in TT111 when finding the nearest system in the galaxy