galaxy-game/client/world/hit_primitives.go

package world

import (
	"image/color"
	"math/bits"
)

func effectiveHitSlopPx(hitSlopPx int, def int) int {
	if hitSlopPx > 0 {
		return hitSlopPx
	}
	return def
}

func alphaNonZero(c color.Color) bool {
	if c == nil {
		return false
	}
	_, _, _, a := c.RGBA()
	return a != 0
}

func hitPoint(p Point, cx, cy int, zoomFp int, allowWrap bool, worldW, worldH int) (Hit, bool) {
	slopPx := effectiveHitSlopPx(p.HitSlopPx, DefaultHitSlopPointPx)
	slopW := PixelSpanToWorldFixed(slopPx, zoomFp)

	var dx, dy int64
	if allowWrap {
		dx = shortestTorusDelta(p.X, cx, worldW)
		dy = shortestTorusDelta(p.Y, cy, worldH)
	} else {
		dx = int64(cx - p.X)
		dy = int64(cy - p.Y)
	}

	// Point is treated as a small disc: dist <= slop.
	ds := distSqU128(dx, dy)
	rs := sqU128Int64(int64(slopW))

	if u128Cmp(ds, rs) <= 0 {
		return Hit{
			ID:         p.Id,
			Kind:       KindPoint,
			Priority:   p.Priority,
			StyleID:    p.StyleID,
			DistanceSq: ds,
			X:          p.X,
			Y:          p.Y,
		}, true
	}
	return Hit{}, false
}

func hitCircle(c Circle, effRadiusFp int, style Style, cx, cy int, zoomFp int, allowWrap bool, worldW, worldH int) (Hit, bool) {
	slopPx := effectiveHitSlopPx(c.HitSlopPx, DefaultHitSlopCirclePx)
	slopW := PixelSpanToWorldFixed(slopPx, zoomFp)

	fillVisible := alphaNonZero(style.FillColor)

	// Determine if circle is point-like at current zoom.
	// IMPORTANT: point-like disc behavior applies only for filled circles.
	rPx := worldSpanFixedToCanvasPx(effRadiusFp, zoomFp)
	pointLike := fillVisible && rPx < CirclePointLikeMinRadiusPx

	var dx, dy int64
	if allowWrap {
		dx = shortestTorusDelta(c.X, cx, worldW)
		dy = shortestTorusDelta(c.Y, cy, worldH)
	} else {
		dx = int64(cx - c.X)
		dy = int64(cy - c.Y)
	}

	ds := distSqU128(dx, dy)

	// Filled + point-like: treat as a disc with minimum visible radius + slop.
	if pointLike {
		// Treat as a disc with minimum visible radius in px.
		minRW := PixelSpanToWorldFixed(CirclePointLikeMinRadiusPx, zoomFp)
		effR := minRW
		if effRadiusFp > effR {
			effR = effRadiusFp
		}
		r := effR + slopW
		if u128Cmp(ds, sqU128Int64(int64(r))) <= 0 {
			return Hit{
				ID:         c.Id,
				Kind:       KindCircle,
				Priority:   c.Priority,
				StyleID:    c.StyleID,
				DistanceSq: ds,
				X:          c.X,
				Y:          c.Y,
				Radius:     effRadiusFp,
			}, true
		}
		return Hit{}, false
	}

	// Filled circle: hit-test by disc (surface).
	if fillVisible {
		r := effRadiusFp + slopW
		if u128Cmp(ds, sqU128Int64(int64(r))) <= 0 {
			return Hit{
				ID:         c.Id,
				Kind:       KindCircle,
				Priority:   c.Priority,
				StyleID:    c.StyleID,
				DistanceSq: ds,
				X:          c.X,
				Y:          c.Y,
				Radius:     effRadiusFp,
			}, true
		}
		return Hit{}, false
	}

	// Stroke-only circle: ring hit, but NEVER at exact center.
	// For very small circles, expand the effective radius to a minimum visible size
	// so that ring selection remains practical, while still excluding center.
	effR := effRadiusFp
	if rPx < CirclePointLikeMinRadiusPx {
		minRW := PixelSpanToWorldFixed(CirclePointLikeMinRadiusPx, zoomFp)
		if minRW > effR {
			effR = minRW
		}
	}

	low := effR - slopW
	// IMPORTANT: center must not hit for stroke-only circles.
	if low < 1 {
		low = 1
	}
	high := effR + slopW

	lowSq := sqU128Int64(int64(low))
	highSq := sqU128Int64(int64(high))

	if u128Cmp(ds, lowSq) >= 0 && u128Cmp(ds, highSq) <= 0 {
		return Hit{
			ID:         c.Id,
			Kind:       KindCircle,
			Priority:   c.Priority,
			StyleID:    c.StyleID,
			DistanceSq: ds,
			X:          c.X,
			Y:          c.Y,
			Radius:     effRadiusFp,
		}, true
	}
	return Hit{}, false
}

func hitLine(l Line, cx, cy int, zoomFp int, allowWrap bool, worldW, worldH int) (Hit, bool) {
	slopPx := effectiveHitSlopPx(l.HitSlopPx, DefaultHitSlopLinePx)
	slopW := PixelSpanToWorldFixed(slopPx, zoomFp)

	// For wrap: compare against torus-shortest representation (same as rendering).
	// We test all segments produced by torusShortestLineSegments and take the best (min distance).
	segs := []lineSeg{{x1: l.X1, y1: l.Y1, x2: l.X2, y2: l.Y2}}
	if allowWrap {
		segs = torusShortestLineSegments(l, worldW, worldH)
	}

	best := Hit{}
	found := false

	for _, s := range segs {
		ds := distSqPointToSegmentU128(int64(cx), int64(cy), int64(s.x1), int64(s.y1), int64(s.x2), int64(s.y2))

		// Check ds <= slopW^2
		if u128Cmp(ds, sqU128Int64(int64(slopW))) <= 0 {
			h := Hit{
				ID:         l.Id,
				Kind:       KindLine,
				Priority:   l.Priority,
				StyleID:    l.StyleID,
				DistanceSq: ds,
				X1:         l.X1,
				Y1:         l.Y1,
				X2:         l.X2,
				Y2:         l.Y2,
			}
			if !found || hitLess(h, best) {
				best = h
				found = true
			}
		}
	}

	return best, found
}

// distSqPointToSegmentU128 computes squared distance from point P to segment AB using safe 128-bit comparisons.
func distSqPointToSegmentU128(px, py, ax, ay, bx, by int64) u128 {
	abx := bx - ax
	aby := by - ay
	apx := px - ax
	apy := py - ay

	// Degenerate segment => distance to point A.
	if abx == 0 && aby == 0 {
		return distSqU128(apx, apy)
	}

	dot := apx*abx + apy*aby
	if dot <= 0 {
		return distSqU128(apx, apy)
	}

	abLen2 := abx*abx + aby*aby
	if dot >= abLen2 {
		bpx := px - bx
		bpy := py - by
		return distSqU128(bpx, bpy)
	}

	// Perpendicular distance: dist^2 = cross^2 / |AB|^2, compare in 128 if needed by callers.
	// Here we actually return an exact rational? We return floor(cross^2 / abLen2) in integer domain
	// would lose precision. Instead, for HitTest we only compare dist^2 <= slop^2, but we also use
	// dist^2 for tie-breaking. We'll compute an approximate using integer division in 128/64.
	//
	// cross = AP x AB
	cross := apx*aby - apy*abx

	// cross^2 fits in u128, abLen2 fits in int64.
	c2 := sqU128Int64(cross)
	return u128DivByU64(c2, uint64(abLen2))
}

// u128DivByU64 returns floor(a / d) where d>0, producing u128 result.
// Here we only need it for tie-breaking (monotonic).
func u128DivByU64(a u128, d uint64) u128 {
	if d == 0 {
		panic("u128DivByU64: divide by zero")
	}
	// Simple long division for 128/64 -> 128 quotient (but high part will be small here).
	// We compute using two-step: divide high then combine.
	qHi := a.hi / d
	rHi := a.hi % d

	// Combine remainder with low as 128-bit number (rHi<<64 + lo) divided by d.
	// Use bits.Div64 for (hi, lo)/d.
	qLo, _ := bits.Div64(rHi, a.lo, d)

	return u128{hi: qHi, lo: qLo}
}