package model.asn1.parsing;

import model.asn1.ASN1Length;
import model.asn1.Tag;
import model.asn1.exceptions.ParseException;

/**
 * A mutable model represents a one-way pipe of reading the input DER bytes. It keeps track of the total input and the
 * current location, and it provides useful methods of requiring one or more bytes to present in the next.
 */
public class BytesReader {
    private final Byte[] rawInput;
    private int index;

    /**
     * EFFECTS: Initialize the reader with the given input and index set to 0.
     * REQUIRES: rawInput.length > 0
     */
    public BytesReader(Byte[] rawInput) {
        this.rawInput = rawInput;
        this.index = 0;
    }

    /**
     * EFFECTS: Calculate the number of bytes remaining to read.
     */
    public int bytesRemaining() {
        return rawInput.length - index;
    }

    /**
     * EFFECTS: Copy the given number of bytes from the [getIndex(), getIndex() + size) and optionally mark as read.
     * MODIFIES: this (if markAsRead == true)
     * REQUIRES: size <= bytesRemaining(), size > 0
     */
    public Byte[] read(int size, boolean markAsRead) {
        Byte[] result = new Byte[size];
        System.arraycopy(rawInput, index, result, 0, size);
        if (markAsRead) {
            index += size;
        }
        return result;
    }

    /**
     * EFFECTS: Copy the given number of bytes from [getIndex(), getIndex() + size) and optionally mark as read.
     *   Throws {@link ParseException} if size > bytesRemaining().
     * MODIFIES: this (if markAsRead == true)
     * REQUIRES: size > 0
     */
    public Byte[] require(int size, boolean markAsRead) throws ParseException {
        validateSize(size);
        return read(size, markAsRead);
    }

    /**
     * EFFECTS: Check if size <= bytesRemaining().
     *   Throws {@link ParseException if not}.
     * REQUIRES: size > 0
     */
    public void validateSize(int size) throws ParseException {
        if (size > bytesRemaining()) {
            throw new ParseException(String.format("%d required at location %d, but only has %d before EOF.",
                    size,
                    index,
                    bytesRemaining()));
        }
    }

    /**
     * EFFECTS: Check if the next byte has the desired tag, without changing the index.
     *   Throws {@link ParseException} if the input is illegal (not even a tag or EOF).
     */
    public boolean detectTag(Tag desired) throws ParseException {
        final int i = index;
        final Tag t = new Tag(this);
        index = i;
        return t.getCls() == desired.getCls()
                && t.isConstructive() == desired.isConstructive()
                && t.getNumber() == desired.getNumber();
    }

    /**
     * EFFECTS: Get the current tag or the tag immediately following (inner) without changing the index.
     *   Throws {@link ParseException} if the input is illegal (not even a tag or EOF).
     */
    public Tag getTag(boolean inner) throws ParseException {
        final int i = index;
        Tag t = new Tag(this);
        if (inner) {
            new ASN1Length(this);
            t = new Tag(this);
        }
        index = i;
        return t;
    }

    public Byte[] getRawInput() {
        return rawInput;
    }

    public int getIndex() {
        return index;
    }
}