private static MethodHandle buildFiller(int nargs) {
if (nargs <= LEFT_ARGS) return ARRAY_IDENTITY; // no args to fill; return the array unchanged
// we need room for both mh and a in mh.invoke(a, arg*[nargs])
final int CHUNK = LEFT_ARGS;
int rightLen = nargs % CHUNK;
int midLen = nargs - rightLen;
if (rightLen == 0) {
midLen = nargs - (rightLen = CHUNK);
if (FILL_ARRAY_TO_RIGHT[midLen] == null) {
// build some precursors from left to right
for (int j = LEFT_ARGS % CHUNK; j < midLen; j += CHUNK) if (j > LEFT_ARGS) fillToRight(j);
}
}
if (midLen < LEFT_ARGS) rightLen = nargs - (midLen = LEFT_ARGS);
assert (rightLen > 0);
MethodHandle midFill = fillToRight(midLen); // recursive fill
MethodHandle rightFill = FILL_ARRAYS[rightLen].bindTo(midLen); // [midLen..nargs-1]
assert (midFill.type().parameterCount() == 1 + midLen - LEFT_ARGS);
assert (rightFill.type().parameterCount() == 1 + rightLen);
// Combine the two fills:
// right(mid(a, x10..x19), x20..x23)
// The final product will look like this:
// right(mid(newArrayLeft(24, x0..x9), x10..x19), x20..x23)
if (midLen == LEFT_ARGS) return rightFill;
else return MethodHandles.collectArguments(rightFill, 0, midFill);
}
private static MethodHandle buildVarargsArray(
MethodHandle newArray, MethodHandle finisher, int nargs) {
// Build up the result mh as a sequence of fills like this:
// finisher(fill(fill(newArrayWA(23,x1..x10),10,x11..x20),20,x21..x23))
// The various fill(_,10*I,___*[J]) are reusable.
int leftLen = Math.min(nargs, LEFT_ARGS); // absorb some arguments immediately
int rightLen = nargs - leftLen;
MethodHandle leftCollector = newArray.bindTo(nargs);
leftCollector = leftCollector.asCollector(Object[].class, leftLen);
MethodHandle mh = finisher;
if (rightLen > 0) {
MethodHandle rightFiller = fillToRight(LEFT_ARGS + rightLen);
if (mh == ARRAY_IDENTITY) mh = rightFiller;
else mh = MethodHandles.collectArguments(mh, 0, rightFiller);
}
if (mh == ARRAY_IDENTITY) mh = leftCollector;
else mh = MethodHandles.collectArguments(mh, 0, leftCollector);
return mh;
}