Avoid allocations while interpolating cube lines (#8773)

src/main/cpp/net/eagle0/shardok/library/util/HexCubeUtils.cpp

@@ -78,7 +78,8 @@ auto FixupCube(
 // Always rounds half up, instead of away from zero.
 auto RoundHalfUp(const double d) -> double { return std::floor(d + 0.5); }
 
-auto CubeLerp(const Cube &c1, const Cube &c2, const double t) -> std::vector<Cube> {
+auto AppendCubeLerp(std::vector<Cube> &cubes, const Cube &c1, const Cube &c2, const double t)
+        -> void {
     const auto floatX = std::lerp(c1.x, c2.x, t);
     const auto newX = RoundHalfUp(floatX);
     const auto xDiff = abs(newX - floatX);
@@ -91,8 +92,6 @@ auto CubeLerp(const Cube &c1, const Cube &c2, const double t) -> std::vector<Cub
     const auto newZ = RoundHalfUp(floatZ);
     const auto zDiff = abs(newZ - floatZ);
 
-    std::vector<Cube> cubes{};
-    cubes.reserve(2);
     cubes.push_back(FixupCube(newX, xDiff, newY, yDiff, newZ, zDiff));
 
     // The 0.5 case represents a situation where the line passes exactly between two hexes. In that
@@ -103,8 +102,6 @@ auto CubeLerp(const Cube &c1, const Cube &c2, const double t) -> std::vector<Cub
     } else if (yDiff == 0.5) {
         cubes.push_back(FixupCube(newX, xDiff, floatY - 0.5, yDiff, newZ, zDiff));
     }
-
-    return cubes;
 }
 
 // Slight variation of the method described in
@@ -119,9 +116,7 @@ auto CubeLineInclusive(const Cube &c1, const Cube &c2) -> std::vector<Cube> {
     cubes.reserve(2 * (dist + 1));
     const double reciprocal = 1.0 / dist;
 
-    for (int i = 0; i <= dist; ++i) {
-        for (const auto &c : CubeLerp(c1, c2, i * reciprocal)) { cubes.push_back(c); }
-    }
+    for (int i = 0; i <= dist; ++i) { AppendCubeLerp(cubes, c1, c2, i * reciprocal); }
 
     return cubes;
 }