A bar for the drift, and a lighter touch sideways
A moon has no air, so a sideways drift never stops by itself and stopping one means cancelling the velocity EXACTLY. That is not hard to do; it is hard to do blind, which is what it was - a number nothing on the screen said anything about. So sprite one is a bar whose width is the drift. It runs right from the middle of the screen for a rightward one and left for a leftward one, so which way is as plain as how fast, and stopped is the one state with nothing drawn at all. The whole of it is a target width written once a frame; the device stretches one tile into it and the program draws nothing. Sideways thrust is one a tick rather than two. At two, the smallest correction available was twice the size it needed to be and overshooting was the normal outcome. WHICH ZERO MEANS NOTHING TURNED OUT TO MATTER. A target width of nought is the NATURAL width, not an empty sprite - so a bar with no drift in it came out eight pixels wide, sitting at the middle of the screen, saying "stopped" in the same shape it says "drifting slightly". What draws nothing is a SIZE of nought, which is the other zero in the other byte. Both meanings are deliberate and documented and it still caught me out inside a week of writing them down. The check for it earned its place by failing on that before it was found, which is the best evidence a check can offer. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
This commit is contained in:
Anachronaut
co-authored by
Claude Opus 5
Claude Opus 5
parent
b1dde7908c
commit
db0c26e13f
@@ -78,6 +78,7 @@ everyFrame:
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CALL move
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CALL follow
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CALL showLander
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CALL showDrift
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SETD.0 Flying
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LDA.0
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BNA everyFrame
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@@ -529,6 +530,137 @@ showLander:
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OUTA 0xE9
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RET
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; ---- How fast sideways, as a bar ----
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;
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; A moon has no air, so a drift never stops by itself and stopping one means cancelling the
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; velocity EXACTLY. That is not hard to do; it is hard to do BLIND, which is what it was - a
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; number nothing on the screen said anything about.
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;
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; So sprite one is a bar whose width is the drift and which disappears at nought. It grows
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; right from the middle of the screen for a rightward drift and left for a leftward one, so
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; which way is as plain as how fast, and "stopped" is the one state with nothing drawn.
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;
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; The whole of it is the sprite's target width, which the device stretches one tile into. The
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; program does no drawing at all: it works out one number a frame and writes it.
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showDrift:
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; The magnitude, and which way. A two's complement pair is inverted and stepped, and the
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; step's carry is what runs into the high half.
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SETD.0 SpeedAcross
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LDA.0
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SETD.1 DriftLow
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STA.1
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INCD.0
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LDA.0
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SETD.1 DriftHigh
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STA.1
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INIB 0x80
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AND
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SETD.1 DriftLeftward
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STQ.1 ; The top bit of the high half, which is the sign.
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BRQ driftSized
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SETD.0 DriftLow
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LDA.0
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NOTA
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MVQA
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INCA
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STA.0
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SETD.0 DriftHigh
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LDA.0
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NOTA
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MVQA
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RSTB
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ADD ; Plus the carry the step above left, which is the other half.
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STQ.0
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driftSized:
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; ---- Clamped, because a bar wider than the screen says nothing a full one does not ----
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;
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; And because a target width is sixteen bits: an unclamped one would be asking the device to
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; draw a bar sixty thousand pixels wide, which it clips, but only after being asked.
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SETD.0 DriftHigh
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LDA.0
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BNA driftWide ; Anything in the high half is already past the limit.
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SETD.0 DriftLow
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LDA.0
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INIB 0d120
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CCF
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SUB
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BRC driftReady ; Borrowed, so it is under the limit and stands.
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driftWide:
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INIA 0d120
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SETD.0 DriftLow
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STA.0
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driftReady:
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; ---- Two zeros, meaning two different things ----
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;
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; A TARGET WIDTH OF NOUGHT IS THE NATURAL WIDTH, not an empty sprite. So a bar with no drift
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; in it came out eight pixels wide - one whole tile, sitting at the middle of the screen,
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; saying "stopped" in the same shape it says "drifting slightly". What draws nothing is a
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; SIZE of nought, which is the other zero and the other byte.
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SETD.0 DriftLow
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LDA.0
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BRA driftEmpty
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INIA 0x11 ; One tile by one, however wide it ends up drawn.
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BRI driftSized2
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driftEmpty:
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RSTA ; No tiles at all, which is the off switch.
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driftSized2:
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SETD.1 DriftSize
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STA.1
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; Where it starts: the middle for a rightward drift, and that much back for a leftward one.
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INIA 0d160
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SETD.0 DriftLeftward
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LDB.0
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BRB driftAt ; Nought is rightward, so the bar starts at the middle.
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SETD.0 DriftLow
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LDB.0
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CCF
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SUB
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MVQA
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driftAt:
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SETD.1 DriftAt
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STA.1
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INIA 0d4
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OUTA 0xE3
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INIA 0xC0
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OUTA 0xE4
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INIA 0x10
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OUTA 0xE5 ; Sprite one begins sixteen bytes in.
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INIA 0xC8
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OUTA 0xE9 ; The ground block, which is a solid tile.
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INIA 0x01
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OUTA 0xE9 ; Attribute one, so the bar is not the colour of the moon.
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SETD.0 DriftAt
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LDA.0
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OUTA 0xE9
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RSTA
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OUTA 0xE9 ; X.
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INIA 0d188
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OUTA 0xE9
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RSTA
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OUTA 0xE9 ; Y, near the bottom and clear of the ground most places.
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SETD.0 DriftSize
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LDA.0
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OUTA 0xE9 ; One tile by one, or nothing at all when there is no drift.
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RSTA
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OUTA 0xE9 ; No flags.
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SETD.0 DriftLow
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LDA.0
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OUTA 0xE9
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RSTA
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OUTA 0xE9 ; The width, which is the drift. NOUGHT DRAWS NOTHING.
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INIA 0d3
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OUTA 0xE9
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RSTA
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OUTA 0xE9 ; Three pixels tall.
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OUTA 0xE9 ; And no depth.
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RET
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; ---- Sums ----
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;
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; DP0 names the two byte value being changed, low half first, and DP1 the one being added to
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@@ -666,14 +798,29 @@ ThrustRight:
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; it is one byte each.
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PadUp:
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0xFE 0xFF ; Minus two.
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; ---- Sideways, at half the vertical ----
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;
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; ONE, not two. There is no air on a moon, so nothing slows a drift but the opposite thruster
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; and stopping means cancelling the velocity exactly. At two a tick the smallest correction
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; was twice as big as it needed to be and overshooting was the normal outcome.
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PadLeft:
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0xFE 0xFF ; Minus two.
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0xFF 0xFF ; Minus one.
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PadRight:
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0x02 0x00
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0x01 0x00
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Held:
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0x00
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HasPad:
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0x00
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DriftLow:
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0x00
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DriftHigh:
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0x00
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DriftLeftward:
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0x00
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DriftAt:
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0x00
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DriftSize:
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0x00
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HalfScreen:
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0xA0 0x00 ; 160 pixels, which is half of a forty column screen.
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