feat: hit on primitives

client/world/hit_geom.go

@@ -0,0 +1,74 @@
+package world
+
+import "math/bits"
+
+// u128 is an unsigned 128-bit integer for safe squared comparisons.
+type u128 struct{ hi, lo uint64 }
+
+func u128FromMul64(a, b uint64) u128 {
+	hi, lo := bits.Mul64(a, b)
+	return u128{hi: hi, lo: lo}
+}
+
+func u128Add(a, b u128) u128 {
+	lo := a.lo + b.lo
+	hi := a.hi + b.hi
+	if lo < a.lo {
+		hi++
+	}
+	return u128{hi: hi, lo: lo}
+}
+
+func u128Cmp(a, b u128) int {
+	if a.hi < b.hi {
+		return -1
+	}
+	if a.hi > b.hi {
+		return 1
+	}
+	if a.lo < b.lo {
+		return -1
+	}
+	if a.lo > b.lo {
+		return 1
+	}
+	return 0
+}
+
+func abs64(x int64) int64 {
+	if x < 0 {
+		return -x
+	}
+	return x
+}
+
+func sqU128Int64(x int64) u128 {
+	u := uint64(abs64(x))
+	return u128FromMul64(u, u)
+}
+
+func distSqU128(dx, dy int64) u128 {
+	return u128Add(sqU128Int64(dx), sqU128Int64(dy))
+}
+
+// shortestTorusDelta returns the shortest signed delta from a->b on a torus axis of size.
+// It is deterministic in tie cases (size even, exactly half): chooses negative direction.
+func shortestTorusDelta(a, b, size int) int64 {
+	d := int64(b - a)
+	s := int64(size)
+	half := s / 2
+
+	// Normalize d into (-s, s).
+	d = d % s
+	if d <= -half {
+		d += s
+	} else if d > half {
+		d -= s
+	}
+
+	// Tie case when size even and d == +half: choose -half.
+	if s%2 == 0 && d == half {
+		d = -half
+	}
+	return d
+}