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An Execution Environment for Java SCOREs
There is a newer version: 0.9.6
package foundation.icon.ee.io;

import java.math.BigInteger;
import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.Arrays;

public class RLPDataReader implements DataReader {
    private static class ListFrame {
        int endPos;
    }

    private final ByteBuffer bb;
    private final byte[] arr;
    private final ArrayList frames = new ArrayList<>();
    private ListFrame topFrame;
    private int o;
    private int l;

    public RLPDataReader(byte[] data) {
        this(ByteBuffer.wrap(data));
    }

    private RLPDataReader(ByteBuffer bb) {
        this.bb = bb;
        this.arr = bb.array();
        this.topFrame = new ListFrame();
        this.frames.add(topFrame);
        this.topFrame.endPos = bb.limit();
    }

    private void readRLPString() {
        var b = peek();
        if (b >= 0xc0) {
            throw new IllegalStateException();
        }
        peekRLPString(b);
        bb.position(o + l);
    }

    private void peekRLPString(int b) {
        var p = bb.arrayOffset() + bb.position();
        if (b <= 0x7f) {
            o = bb.position();
            l = 1;
        } else if (b <= 0xb7) {
            o = 1 + bb.position();
            l = b - 0x80;
        } else if (b == 0xb8) {
            o = 2 + bb.position();
            l = arr[p + 1] & 0xff;
        } else if (b == 0xb9) {
            o = 3 + bb.position();
            l = ((arr[p + 1] & 0xff) << 8) |
                    (arr[p + 2] & 0xff);
        } else if (b == 0xba) {
            o = 4 + bb.position();
            l = ((arr[p + 1] & 0xff) << 16) |
                    ((arr[p + 2] & 0xff) << 8) |
                    (arr[p + 3] & 0xff);
        } else if (b == 0xbb) {
            o = 5 + bb.position();
            l = ((arr[p + 1] & 0xff) << 24) |
                    ((arr[p + 2] & 0xff) << 16) |
                    ((arr[p + 3] & 0xff) << 8) |
                    (arr[p + 4] & 0xff);
        } else {
            throw new UnsupportedOperationException();
        }
    }

    private void peekRLPListHeader(int b) {
        var p = bb.arrayOffset() + bb.position();
        if (b <= 0xf7) {
            o = 1 + bb.position();
            l = b - 0xc0;
        } else if (b == 0xf8) {
            o = 2 + bb.position();
            l = arr[p + 1] & 0xff;
        } else if (b == 0xf9) {
            o = 3 + bb.position();
            l = ((arr[p + 1] & 0xff) << 8) |
                    (arr[p + 2] & 0xff);
        } else if (b == 0xfa) {
            o = 4 + bb.position();
            l = ((arr[p + 1] & 0xff) << 16) |
                    ((arr[p + 2] & 0xff) << 8) |
                    (arr[p + 3] & 0xff);
        } else if (b == 0xfb) {
            o = 5 + bb.position();
            l = ((arr[p + 1] & 0xff) << 24) |
                    ((arr[p + 2] & 0xff) << 16) |
                    ((arr[p + 3] & 0xff) << 8) |
                    (arr[p + 4] & 0xff);
        } else {
            throw new UnsupportedOperationException();
        }
    }

    private boolean peekRLPNull(int b) {
        if (b != 0xf8) {
            return false;
        }
        var p = bb.arrayOffset() + bb.position();
        if (arr[p + 1] == 0) {
            o = 2 + bb.position();
            l = 0;
            return true;
        }
        return false;
    }

    private int peek() {
        return (bb.get(bb.position()) & 0xff);
    }

    public boolean readBoolean() {
        return readBigInteger().signum() != 0;
    }

    public byte readByte() {
        return readBigInteger().byteValue();
    }

    public short readShort() {
        return readBigInteger().shortValue();
    }

    public char readChar() {
        return (char) readBigInteger().intValue();
    }

    public int readInt() {
        return readBigInteger().intValue();
    }

    public float readFloat() {
        readRLPString();
        if (l != 4) {
            throw new IllegalStateException();
        }
        return bb.getFloat(o);
    }

    public long readLong() {
        return readBigInteger().longValue();
    }

    public double readDouble() {
        readRLPString();
        if (l != 8) {
            throw new IllegalStateException();
        }
        return bb.getDouble(o);
    }

    public BigInteger readBigInteger() {
        readRLPString();
        var offset = bb.arrayOffset() + o;
        return new BigInteger(arr, offset, l);
    }

    public String readString() {
        readRLPString();
        var offset = bb.arrayOffset() + o;
        return new String(arr, offset, l, StandardCharsets.UTF_8);
    }

    public byte[] readByteArray() {
        readRLPString();
        var offset = bb.arrayOffset() + o;
        return Arrays.copyOfRange(arr, offset, offset + l);
    }

    public boolean readNullity() {
        return this.tryReadNull();
    }

    public void skip(int count) {
        for (int i = 0; i < count; i++) {
            var b = peek();
            if (!peekRLPNull(b)) {
                if (b < 0xc0) {
                    peekRLPString(b);
                } else {
                    peekRLPListHeader(b);
                }
            }
            bb.position(o + l);
        }
    }

    private void _readRLPListHeader() {
        var b = peek();
        peekRLPListHeader(b);
        bb.position(o);
        topFrame = new ListFrame();
        topFrame.endPos = bb.position() + l;
        frames.add(topFrame);
    }

    private void _readRLPListFooter() {
        frames.remove(frames.size() - 1);
        topFrame = frames.get(frames.size() - 1);
    }

    public void readListHeader() {
        _readRLPListHeader();
    }

    public void readMapHeader() {
        readListHeader();
    }

    public boolean hasNext() {
        return bb.position() < topFrame.endPos;
    }

    public void readFooter() {
        _readRLPListFooter();
    }

    private boolean tryReadNull() {
        var b = peek();
        if (!peekRLPNull(b)) {
            return false;
        }
        bb.position(o + l);
        return true;
    }

    public long getTotalReadBytes() {
        return bb.position();
    }
}