Ilia Denisov
d63fe44618
The pre-existing `Deltas` helper used the height to wrap the x-axis, which silently produced wrong values on any rectangular galaxy (`w != h`). Square galaxies — the only configuration the engine ships today — masked the bug, so it stayed in tree. `Deltas` is now a thin wrapper around the new `ShortestDelta(a, b, size)`, which returns the signed per-axis shortest delta on a 1-D circle (range `(-size/2, size/2]`). The signed flavour is what the Phase 19 ship-group renderer needs to draw an IncomingGroup trajectory across the torus seam; `Deltas` continues to return the pair of absolute deltas for distance computation. Adds `pkg/calc/map_test.go` with table-driven coverage for both helpers, including a regression that exercises the rectangular case the bug was hiding behind, and the half-circumference tie-break. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
40 lines
1.3 KiB
Go
40 lines
1.3 KiB
Go
package calc
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import "math"
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// ShortDistance returns the shortest Euclidean distance between two
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// points on a torus of size w×h.
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func ShortDistance(w, h uint32, x1, y1, x2, y2 float64) float64 {
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return math.Hypot(Deltas(w, h, x1, y1, x2, y2))
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}
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// Deltas returns the per-axis absolute distance between two points on
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// a torus of size w×h. Each axis wraps independently: the x-axis
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// against width w, the y-axis against height h. The returned values
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// are always non-negative; combine via math.Hypot for the Euclidean
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// torus distance, or use [ShortestDelta] when the signed direction
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// matters (for example when drawing a wrap-aware line).
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func Deltas(w, h uint32, x1, y1, x2, y2 float64) (float64, float64) {
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return math.Abs(ShortestDelta(x1, x2, w)), math.Abs(ShortestDelta(y1, y2, h))
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}
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// ShortestDelta returns the signed delta (b - a) on a 1-D circle of
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// circumference size, picking whichever direction has the shorter
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// absolute distance. The result lies in (-size/2, size/2]: at exactly
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// half the circumference the function returns +size/2 so the tie-case
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// is deterministic regardless of input order.
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func ShortestDelta(a, b float64, size uint32) float64 {
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if size == 0 {
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return b - a
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}
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s := float64(size)
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d := math.Mod(b-a, s)
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half := s / 2
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if d > half {
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d -= s
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} else if d <= -half {
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d += s
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}
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return d
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}
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