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 * The Universal Permissive License (UPL), Version 1.0
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 * (a) the Software, and
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 * (b) any piece of software and/or hardware listed in the lrgrwrks.txt file if
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package com.oracle.graal.python.builtins.objects.cext.common;

import java.util.Arrays;
import java.util.Iterator;

import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;

public final class ReferenceStack<T> implements Iterable<T> {
    private static final int INITIAL_CAPACITY = 64;

    private final IntegerStack freeList;
    private Object[] nativeObjectWrapperList;

    @TruffleBoundary
    public ReferenceStack() {
        nativeObjectWrapperList = new Object[INITIAL_CAPACITY];
        freeList = new IntegerStack(INITIAL_CAPACITY);
        freeList.addToFreeList(0, INITIAL_CAPACITY);
    }

    @TruffleBoundary
    private void enlargeNativeReferenceList() {
        int oldSize = nativeObjectWrapperList.length;
        int newSize = oldSize * 2;
        nativeObjectWrapperList = Arrays.copyOf(nativeObjectWrapperList, newSize);
        freeList.addToFreeList(oldSize, newSize);
    }

    @SuppressWarnings("unchecked")
    public T get(int idx) {
        if (0 <= idx && idx < nativeObjectWrapperList.length) {
            return (T) nativeObjectWrapperList[idx];
        } else {
            assert false : "incorrect reference ID";
        }
        return null;
    }

    @SuppressWarnings("unchecked")
    public T remove(int idx) {
        assert 0 <= idx && idx < nativeObjectWrapperList.length;
        T ref = (T) nativeObjectWrapperList[idx];
        nativeObjectWrapperList[idx] = null;
        freeList.push(idx);
        return ref;
    }

    public int reserve() {
        int nativeRefID = freeList.pop();
        if (nativeRefID == -1) {
            enlargeNativeReferenceList();
            nativeRefID = freeList.pop();
        }
        assert nativeRefID != -1;
        return nativeRefID;
    }

    public void commit(int idx, T nativeObjectReference) {
        assert 0 <= idx && idx < nativeObjectWrapperList.length;
        assert nativeObjectWrapperList[idx] == null : "cannot overwrite an allocated native object reference slot";
        nativeObjectWrapperList[idx] = nativeObjectReference;
    }

    @SuppressWarnings("unchecked")
    public T resurrect(int idx, T nativeObjectReference) {
        assert 0 <= idx && idx < nativeObjectWrapperList.length;
        T old = (T) nativeObjectWrapperList[idx];
        nativeObjectWrapperList[idx] = nativeObjectReference;
        return old;
    }

    @Override
    @SuppressWarnings("unchecked")
    public Iterator<T> iterator() {
        return (Iterator<T>) Arrays.asList(nativeObjectWrapperList).iterator();
    }

    static final class IntegerStack {
        private int[] stack;
        private int top = 0;

        public IntegerStack(int initialCapacity) {
            stack = new int[initialCapacity];
        }

        void push(int i) {
            if (top >= stack.length) {
                stack = Arrays.copyOf(stack, stack.length * 2);
            }
            stack[top++] = i;
        }

        int pop() {
            if (top <= 0) {
                return -1;
            }
            return stack[--top];
        }

        void addToFreeList(int start, int end) {
            for (int i = end - 1; i >= start; i--) {
                push(i);
            }
        }
    }
}
