fpgenerator.java

爬虫

package st.ata.util;

import java.util.Hashtable;

/**

<p> This class provides methods that construct fingerprints of strings
of bytes via operations in <i>GF[2^d]</i> for <i>0 < d <= 64</i>.
<i>GF[2^d]</i> is represented as the set of polynomials of degree
<i>d</i> with coefficients in <i>Z(2)</i>, modulo an irreducible
polynomial <i>P</i> of degree <i>d</i>.  The representation of
polynomials is as an unsigned binary number in which the least
significant exponent is kept in the most significant bit.

<p> Let S be a string of bytes and <i>g(S)</i> the string obtained by
taking the byte <code>0x01</code> followed by eight <code>0x00</code>
bytes followed by <code>S</code>.  Let <i>f(S)</i> be the polynomial
associated to the string <i>S</i> viewed as a polynomial with
coefficients in the field <i>Z(2)</i>.  The fingerprint of S is simply
the value <i>f(g(S))</i> modulo <i>P</i>.  Because polynomials are
represented with the least significant coefficient in the most
significant bit, fingerprints of degree <i>d</i> are stored in the
<code>d</code> <strong>most</code> significant bits of a long word.

<p> Fingerprints can be used as a probably unique id for the input
string.  More precisely, if <i>P</i> is chosen at random among
irreducible polynomials of degree <i>d</i>, then the probability that
any two strings <i>A</i> and <i>B</i> have the same fingerprint is
less than <i>max(|A|,|B|)/2^(d+1)</i> where <i>|A|</i> is the length
of A in bits.

<p> The routines named <code>extend[8]</code> and <code>fp[8]</code>
return reduced results, while <code>extend_[byte/char/int/long]</code>
do not.  An <em>un</em>reduced result is a number that is equal (mod
</code>polynomial</code> to the desired fingerprint but may have
degree <code>degree</code> or higher.  The method <code>reduce</code>
reduces such a result to a polynomial of degree less than
<code>degree</code>.  Obtaining reduced results takes longer than
obtaining unreduced results; thus, when fingerprinting long strings,
it's better to obtain irreduced results inside the fingerprinting loop
and use <code>reduce</code> to reduce to a fingerprint after the loop.

*/

// Tested by: TestFPGenerator

public final class FPGenerator {

    /** Return a fingerprint generator.  The fingerprints generated
        will have degree <code>degree</code> and will be generated by
        <code>polynomial</code>.  If a generator based on
        <code>polynomial</code> has already been created, it will be
        returned.  Requires that <code>polynomial</code> is an
        irreducible polynomial of degree <code>degree</code> (the
        array <code>polynomials</code> contains some irreducible
        polynomials). */
    public static FPGenerator make(long polynomial, int degree) {
        Long l = new Long(polynomial);
        FPGenerator fpgen = (FPGenerator) generators.get(l);
        if (fpgen == null) {
            fpgen = new FPGenerator(polynomial, degree);
            generators.put(l, fpgen);
        }
        return fpgen;
    }
    private static final Hashtable generators = new Hashtable(10);

    private static final long zero = 0;
    private static final long one = 0x8000000000000000L;


    /** Return a value equal (mod <code>polynomial</code>) to
        <code>fp</code> and of degree less than <code>degree</code>. */
    public long reduce(long fp) {
        int N = (8 - degree/8);
        long local = (N == 8 ? 0 : fp & (-1L << 8*N));
        long temp = zero;
        for (int i = 0; i < N; i++) {
            temp ^= ByteModTable[8+i][((int)fp) & 0xff];
            fp >>>= 8;
        };
        return local ^ temp;
    }

    /** Extends <code>f</code> with lower eight bits of <code>v</code>
        with<em>out</em> full reduction.  In other words, returns a
        polynomial that is equal (mod <code>polynomial</code>) to the
        desired fingerprint but may be of higher degree than the
        desired fingerprint. */
    public long extend_byte(long f, int v) {
        f ^= (0xff & v);
        int i = (int)f;
        long result = (f>>>8);
        result ^= ByteModTable[7][i & 0xff];
        return result;
    }

    /** Extends <code>f</code> with lower sixteen bits of <code>v</code>.
        Does not reduce. */
    public long extend_char(long f, int v) {
        f ^= (0xffff & v);
        int i = (int)f;
        long result = (f>>>16);
        result ^= ByteModTable[6][i & 0xff]; i >>>= 8;
        result ^= ByteModTable[7][i & 0xff];
        return result;
    }

    /** Extends <code>f</code> with (all bits of) <code>v</code>.
        Does not reduce. */
    public long extend_int(long f, int v) {
        f ^= (0xffffffffL & v);
        int i = (int)f;
        long result = (f>>>32);
        result ^= ByteModTable[4][i & 0xff]; i >>>= 8;
        result ^= ByteModTable[5][i & 0xff]; i >>>= 8;
        result ^= ByteModTable[6][i & 0xff]; i >>>= 8;
        result ^= ByteModTable[7][i & 0xff];
        return result;
    }

    /** Extends <code>f</code> with <code>v</code>.
        Does not reduce. */
    public long extend_long(long f, long v) {
        f ^= v;
        long result = ByteModTable[0][(int)(f & 0xff)]; f >>>= 8;
        result ^= ByteModTable[1][(int)(f & 0xff)]; f >>>= 8;
        result ^= ByteModTable[2][(int)(f & 0xff)]; f >>>= 8;
        result ^= ByteModTable[3][(int)(f & 0xff)]; f >>>= 8;
        result ^= ByteModTable[4][(int)(f & 0xff)]; f >>>= 8;
        result ^= ByteModTable[5][(int)(f & 0xff)]; f >>>= 8;
        result ^= ByteModTable[6][(int)(f & 0xff)]; f >>>= 8;
        result ^= ByteModTable[7][(int)(f & 0xff)];
        return result;
    }


    /** Compute fingerprint of "n" bytes of "buf" starting from
        "buf[start]".  Requires "[start, start+n)" is in bounds. */
    public long fp(byte[] buf, int start, int n) {
        return extend(empty, buf, start, n);
    }

    /** Compute fingerprint of (all bits of) "n" characters of "buf"
        starting from "buf[i]".  Requires "[i, i+n)" is in bounds. */
    public long fp(char[] buf, int start, int n) {
        return extend(empty, buf, start, n);
    }

// COMMENTED OUT TO REMOVE Dependency on st.ata.util.Text
//    /** Compute fingerprint of (all bits of) <code>t</code> */
//    public long fp(Text t) {
//        return extend(empty, t);
//    }
    /** Compute fingerprint of (all bits of) the characters of "s". */
    public long fp(CharSequence s) {
        return extend(empty, s);
    }

    /** Compute fingerprint of (all bits of) "n" characters of "buf"
        starting from "buf[i]".  Requires "[i, i+n)" is in bounds. */
    public long fp(int[] buf, int start, int n) {
        return extend(empty, buf, start, n);
    }

    /** Compute fingerprint of (all bits of) "n" characters of "buf"
        starting from "buf[i]".  Requires "[i, i+n)" is in bounds. */
    public long fp(long[] buf, int start, int n) {
        return extend(empty, buf, start, n);
    }

    /** Compute fingerprint of the lower eight bits of the characters
        of "s". */
    public long fp8(String s) {
        return extend8(empty, s);
    }

    /** Compute fingerprint of the lower eight bits of "n" characters
        of "buf" starting from "buf[i]".  Requires "[i, i+n)" is in
        bounds. */
    public long fp8(char[] buf, int start, int n) {
        return extend8(empty, buf, start, n);
    }


    /** Extends fingerprint <code>f</code> by adding the low eight
        bits of "b". */
    public long extend(long f, byte v) {
        return reduce(extend_byte(f, v));
    }

    /** Extends fingerprint <code>f</code> by adding (all bits of)
        "v". */
    public long extend(long f, char v) {
        return reduce(extend_char(f, v));
    }

    /** Extends fingerprint <code>f</code> by adding (all bits of)
        "v". */
    public long extend(long f, int v) {
        return reduce(extend_int(f, v));
    }

    /** Extends fingerprint <code>f</code> by adding (all bits of)
        "v". */
    public long extend(long f, long v) {
        return reduce(extend_long(f, v));
    }

    /** Extends fingerprint <code>f</code> by adding "n" bytes of
        "buf" starting from "buf[start]".
        Result is reduced.
        Requires "[i,&nbsp;i+n)" is in bounds. */
    public long extend(long f, byte[] buf, int start, int n) {
        for (int i = 0; i < n; i++) {
            f = extend_byte(f, buf[start+i]);
        }
        return reduce(f);
    }

    /** Extends fingerprint <code>f</code> by adding (all bits of) "n"
        characters of "buf" starting from "buf[i]".
        Result is reduced.
        Requires "[i,&nbsp;i+n)" is in bounds. */
    public long extend(long f, char[] buf, int start, int n) {