/*
******************************************************************************
*
*   Copyright (C) 2002-2003, International Business Machines
*   Corporation and others.  All Rights Reserved.
*
******************************************************************************
*   file name:  bocu1.c
*   encoding:   US-ASCII
*   tab size:   8 (not used)
*   indentation:4
*
*   created on: 2002jan24
*   created by: Markus W. Scherer
*
*   This is a sample implementation of encoder and decoder functions for BOCU-1,
*   a MIME-compatible Binary Ordered Compression for Unicode.
*/

#include <stdio.h>
#include <string.h>

/*
 * Standard ICU header.
 * - Includes inttypes.h or defines its types.
 * - Defines UChar for UTF-16 as an unsigned 16-bit type (wchar_t or uint16_t).
 * - Defines UTF* macros to handle reading and writing
 *   of in-process UTF-8/16 strings.
 */
#include "unicode/utypes.h"

#include "bocu1.h"

/* BOCU-1 implementation functions ------------------------------------------ */

/**
 * Compute the next "previous" value for differencing
 * from the current code point.
 *
 * @param c current code point, 0..0x10ffff
 * @return "previous code point" state value
 */
U_INLINE int32_t
bocu1Prev(int32_t c) {
    /* compute new prev */
    if(0x3040<=c && c<=0x309f) {
        /* Hiragana is not 128-aligned */
        return 0x3070;
    } else if(0x4e00<=c && c<=0x9fa5) {
        /* CJK Unihan */
        return 0x4e00-BOCU1_REACH_NEG_2;
    } else if(0xac00<=c && c<=0xd7a3) {
        /* Korean Hangul (cast to int32_t to avoid wraparound on 16-bit compilers) */
        return ((int32_t)0xd7a3+(int32_t)0xac00)/2;
    } else {
        /* mostly small scripts */
        return (c&~0x7f)+BOCU1_ASCII_PREV;
    }
}

/**
 * Encode a difference -0x10ffff..0x10ffff in 1..4 bytes
 * and return a packed integer with them.
 *
 * The encoding favors small absolut differences with short encodings
 * to compress runs of same-script characters.
 *
 * @param diff difference value -0x10ffff..0x10ffff
 * @return
 *      0x010000zz for 1-byte sequence zz
 *      0x0200yyzz for 2-byte sequence yy zz
 *      0x03xxyyzz for 3-byte sequence xx yy zz
 *      0xwwxxyyzz for 4-byte sequence ww xx yy zz (ww>0x03)
 */
U_CFUNC int32_t
packDiff(int32_t diff) {
    int32_t result, m, lead, count, shift;

    if(diff>=BOCU1_REACH_NEG_1) {
        /* mostly positive differences, and single-byte negative ones */
        if(diff<=BOCU1_REACH_POS_1) {
            /* single byte */
            return 0x01000000|(BOCU1_MIDDLE+diff);
        } else if(diff<=BOCU1_REACH_POS_2) {
            /* two bytes */
            diff-=BOCU1_REACH_POS_1+1;
            lead=BOCU1_START_POS_2;
            count=1;
        } else if(diff<=BOCU1_REACH_POS_3) {
            /* three bytes */
            diff-=BOCU1_REACH_POS_2+1;
            lead=BOCU1_START_POS_3;
            count=2;
        } else {
            /* four bytes */
            diff-=BOCU1_REACH_POS_3+1;
            lead=BOCU1_START_POS_4;
            count=3;
        }
    } else {
        /* two- and four-byte negative differences */
        if(diff>=BOCU1_REACH_NEG_2) {
            /* two bytes */
            diff-=BOCU1_REACH_NEG_1;
            lead=BOCU1_START_NEG_2;
            count=1;
        } else if(diff>=BOCU1_REACH_NEG_3) {
            /* three bytes */
            diff-=BOCU1_REACH_NEG_2;
            lead=BOCU1_START_NEG_3;
            count=2;
        } else {
            /* four bytes */
            diff-=BOCU1_REACH_NEG_3;
            lead=BOCU1_START_NEG_4;
            count=3;
        }
    }

    /* encode the length of the packed result */
    if(count<3) {
        result=(count+1)<<24;
    } else /* count==3, MSB used for the lead byte */ {
        result=0;
    }

    /* calculate trail bytes like digits in itoa() */
    shift=0;
    do {
        NEGDIVMOD(diff, BOCU1_TRAIL_COUNT, m);
        result|=BOCU1_TRAIL_TO_BYTE(m)<<shift;
        shift+=8;
    } while(--count>0);

    /* add lead byte */
    result|=(lead+diff)<<shift;

    return result;
}

/**
 * BOCU-1 encoder function.
 *
 * @param pPrev pointer to the integer that holds
 *        the "previous code point" state;
 *        the initial value should be 0 which
 *        encodeBocu1 will set to the actual BOCU-1 initial state value
 * @param c the code point to encode
 * @return the packed 1/2/3/4-byte encoding, see packDiff(),
 *         or 0 if an error occurs
 *
 * @see packDiff
 */
U_CFUNC int32_t
encodeBocu1(int32_t *pPrev, int32_t c) {
    int32_t prev;

    if(pPrev==NULL || c<0 || c>0x10ffff) {
        /* illegal argument */
        return 0;
    }

    prev=*pPrev;
    if(prev==0) {
        /* lenient handling of initial value 0 */
        prev=*pPrev=BOCU1_ASCII_PREV;
    }

    if(c<=0x20) {
        /*
         * ISO C0 control & space:
         * Encode directly for MIME compatibility,
         * and reset state except for space, to not disrupt compression.
         */
        if(c!=0x20) {
            *pPrev=BOCU1_ASCII_PREV;
        }
        return 0x01000000|c;
    }

    /*
     * all other Unicode code points c==U+0021..U+10ffff
     * are encoded with the difference c-prev
     *
     * a new prev is computed from c,
     * placed in the middle of a 0x80-block (for most small scripts) or
     * in the middle of the Unihan and Hangul blocks
     * to statistically minimize the following difference
     */
    *pPrev=bocu1Prev(c);
    return packDiff(c-prev);
}

/**
 * Function for BOCU-1 decoder; handles multi-byte lead bytes.
 *
 * @param pRx pointer to the decoder state structure
 * @param b lead byte;
 *          BOCU1_MIN<=b<BOCU1_START_NEG_2 or BOCU1_START_POS_2<=b<=BOCU1_MAX_LEAD
 * @return -1 (state change only)
 *
 * @see decodeBocu1
 */
static int32_t
decodeBocu1LeadByte(Bocu1Rx *pRx, uint8_t b) {
    int32_t c, count;

    if(b>=BOCU1_START_NEG_2) {
        /* positive difference */
        if(b<BOCU1_START_POS_3) {
            /* two bytes */
            c=((int32_t)b-BOCU1_START_POS_2)*BOCU1_TRAIL_COUNT+BOCU1_REACH_POS_1+1;
            count=1;
        } else if(b<BOCU1_START_POS_4) {
            /* three bytes */
            c=((int32_t)b-BOCU1_START_POS_3)*BOCU1_TRAIL_COUNT*BOCU1_TRAIL_COUNT+BOCU1_REACH_POS_2+1;
            count=2;
        } else {
            /* four bytes */
            c=BOCU1_REACH_POS_3+1;
            count=3;
        }
    } else {
        /* negative difference */
        if(b>=BOCU1_START_NEG_3) {
            /* two bytes */
            c=((int32_t)b-BOCU1_START_NEG_2)*BOCU1_TRAIL_COUNT+BOCU1_REACH_NEG_1;
            count=1;
        } else if(b>BOCU1_MIN) {
            /* three bytes */
            c=((int32_t)b-BOCU1_START_NEG_3)*BOCU1_TRAIL_COUNT*BOCU1_TRAIL_COUNT+BOCU1_REACH_NEG_2;
            count=2;
        } else {
            /* four bytes */
            c=-BOCU1_TRAIL_COUNT*BOCU1_TRAIL_COUNT*BOCU1_TRAIL_COUNT+BOCU1_REACH_NEG_3;
            count=3;
        }
    }

    /* set the state for decoding the trail byte(s) */
    pRx->diff=c;
    pRx->count=count;
    return -1;
}

/**
 * Function for BOCU-1 decoder; handles multi-byte trail bytes.
 *
 * @param pRx pointer to the decoder state structure
 * @param b trail byte
 * @return result value, same as decodeBocu1
 *
 * @see decodeBocu1
 */
static int32_t
decodeBocu1TrailByte(Bocu1Rx *pRx, uint8_t b) {
    int32_t t, c, count;

    if(b<=0x20) {
        /* skip some C0 controls and make the trail byte range contiguous */
        t=bocu1ByteToTrail[b];
        if(t<0) {
            /* illegal trail byte value */
            pRx->prev=BOCU1_ASCII_PREV;
            pRx->count=0;
            return -99;
        }
#if BOCU1_MAX_TRAIL<0xff
    } else if(b>BOCU1_MAX_TRAIL) {
        return -99;
#endif
    } else {
        t=(int32_t)b-BOCU1_TRAIL_BYTE_OFFSET;
    }

    /* add trail byte into difference and decrement count */
    c=pRx->diff;
    count=pRx->count;

    if(count==1) {
        /* final trail byte, deliver a code point */
        c=pRx->prev+c+t;
        if(0<=c && c<=0x10ffff) {
            /* valid code point result */
            pRx->prev=bocu1Prev(c);
            pRx->count=0;
            return c;
        } else {
            /* illegal code point result */
            pRx->prev=BOCU1_ASCII_PREV;
            pRx->count=0;
            return -99;
        }
    }

    /* intermediate trail byte */
    if(count==2) {
        pRx->diff=c+t*BOCU1_TRAIL_COUNT;
    } else /* count==3 */ {
        pRx->diff=c+t*BOCU1_TRAIL_COUNT*BOCU1_TRAIL_COUNT;
    }
    pRx->count=count-1;
    return -1;
}

/**
 * BOCU-1 decoder function.
 *
 * @param pRx pointer to the decoder state structure;
 *        the initial values should be 0 which
 *        decodeBocu1 will set to actual initial state values
 * @param b an input byte
 * @return
 *      0..0x10ffff for a result code point
 *      -1 if only the state changed without code point output
 *     <-1 if an error occurs
 */
U_CFUNC int32_t
decodeBocu1(Bocu1Rx *pRx, uint8_t b) {
    int32_t prev, c, count;

    if(pRx==NULL) {
        /* illegal argument */
        return -99;
    }

    prev=pRx->prev;
    if(prev==0) {
        /* lenient handling of initial 0 values */
        prev=pRx->prev=BOCU1_ASCII_PREV;
        count=pRx->count=0;
    } else {
        count=pRx->count;
    }

    if(count==0) {
        /* byte in lead position */
        if(b<=0x20) {
            /*
             * Direct-encoded C0 control code or space.
             * Reset prev for C0 control codes but not for space.
             */
            if(b!=0x20) {
                pRx->prev=BOCU1_ASCII_PREV;
            }
            return b;
        }

        /*
         * b is a difference lead byte.
         *
         * Return a code point directly from a single-byte difference.
         *
         * For multi-byte difference lead bytes, set the decoder state
         * with the partial difference value from the lead byte and
         * with the number of trail bytes.
         *
         * For four-byte differences, the signedness also affects the
         * first trail byte, which has special handling farther below.
         */
        if(b>=BOCU1_START_NEG_2 && b<BOCU1_START_POS_2) {
            /* single-byte difference */
            c=prev+((int32_t)b-BOCU1_MIDDLE);
            pRx->prev=bocu1Prev(c);
            return c;
        } else if(b==BOCU1_RESET) {
            /* only reset the state, no code point */
            pRx->prev=BOCU1_ASCII_PREV;
            return -1;
        } else {
            return decodeBocu1LeadByte(pRx, b);
        }
    } else {
        /* trail byte in any position */
        return decodeBocu1TrailByte(pRx, b);
    }
}