/*
******************************************************************************

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******************************************************************************
*   file name:  scsumini.c
*   encoding:   US-ASCII
*   tab size:   8 (not used)
*   indentation:4
*
*   created on: 2003jun24
*   created by: Markus W. Scherer
*
*   Minimal SCSU encoder.
*   This SCSU encoder is sufficient and small but not very efficient.
*   While it could be used as is, it is intended to show that a simple, small
*   yet functioning SCSU encoder is feasible.
*
*   This file will by itself compile on many platforms into a standalone
*   command-line program. A user can type Unicode code points and the program
*   will output the SCSU bytes from the encoder.
*
*   Features:
*   - Encoder state: 4 bits (boolean mode and 3-bit window number)
*   - Uses very small set of SCSU features; does not define dynamic windows
*     but uses predefined ones.
*   - Generates conformant SCSU output.
*   - Generates XML-suitable output.
*   - Encodes U+FEFF as recommended for SCSU signatures.
*   - Encodes US-ASCII and ISO-8859-1 text as itself.
*   - Encodes scripts in compact form for which SCSU has predefined dynamic windows.
*   - Encodes many texts with no more bytes than either UTF-8 or UTF-16.
*   - Encodes some texts with more bytes than UTF-8/16, especially texts in
*     many small scripts:
*     This encoder will use Unicode mode for them (2 bytes per character)
*     and for each space character switch to single byte mode, emit the space,
*     and switch back to Unicode mode. (3 bytes per space)
*/

#include <stdio.h>

/* Type definitions, usually from a system header. */

/** 32-bit signed integer (need at least a 22-bit signed integer here) */
typedef long int32_t;


/** Forward declaration of a byte output function. (Takes an int to avoid casts.) */
extern void out(int byte);


/* SCSU command byte values */
enum {
    SQ0=0x01, /* Quote from window pair 0 */
    SQU=0x0E, /* Quote a single Unicode character */
    SCU=0x0F, /* Change to Unicode mode */
    SC0=0x10, /* Select window 0 */

    UC0=0xE0, /* Select window 0 */
    UQU=0xF0  /* Quote a single Unicode character */
};

/* constant offsets for the 8 static and 8 dynamic windows */
static const int32_t offsets[16]={
    /* initial offsets for the 8 dynamic (sliding) windows */
    0x0080, /* Latin-1 */
    0x00C0, /* Latin Extended A */
    0x0400, /* Cyrillic */
    0x0600, /* Arabic */
    0x0900, /* Devanagari */
    0x3040, /* Hiragana */
    0x30A0, /* Katakana */
    0xFF00, /* Fullwidth ASCII */

    /* offsets for the 8 static windows */
    0x0000, /* ASCII for quoted tags */
    0x0080, /* Latin - 1 Supplement (for access to punctuation) */
    0x0100, /* Latin Extended-A */
    0x0300, /* Combining Diacritical Marks */
    0x2000, /* General Punctuation */
    0x2080, /* Currency Symbols */
    0x2100, /* Letterlike Symbols and Number Forms */
    0x3000  /* CJK Symbols and punctuation */
};

static /* inline */ char isInWindow(int32_t offset, int32_t c) {
    return (char)(offset<=c && c<=(offset+0x7f));
}

/* get the index of the static/dynamic window that contains c; -1 if none */
static int getWindow(int32_t c) {
    int i;

    for(i=0; i<16; ++i) {
        if(isInWindow(offsets[i], c)) {
            return i;
        }
    }
    return -1;
}

struct SCSUMiniState {
    char isUnicodeMode;
    char window;
};
typedef struct SCSUMiniState SCSUMiniState;

/**
 * Function for minimal SCSU encoding.
 * Calls out(byte) to output SCSU bytes.
 *
 * @param c Input Unicode code point or UTF-16 code unit.
 *        The output will be the same whether the function is called with
 *        a sequence of code points or an equivalent sequence of UTF-16 code
 *        units, or an equivalent mixture of them.
 *        It must be 0<=c<=0x10ffff.
 * @param pState Pointer to SCSUMiniState struct.
 *        Indicates whether the encoder is currently in Unicode mode,
 *        and which is the current dynamic window for single-byte mode.
 *        All fields must initially be set to 0 (single-byte mode, window 0).
 */
extern void SCSUMiniEncode(int32_t c, SCSUMiniState *pState) {
    char window; /* dynamic window 0..7 */
    int w;       /* result of getWindow(), -1..7 */

    /*
     * Check arguments:
     * - Is c a valid code point? (0<=c<=0x10ffff)
     * - Is pState a valid pointer to a state struct with valid values?
     *   (isUnicodeMode=0..1  and window=0..7)
     */
    if( c<0 || c>0x10ffff ||
        pState==NULL || (pState->isUnicodeMode&~1)!=0 || (pState->window&~7)!=0
    ) {
        /* illegal argument error */
        return;
    }

    if(c>0xffff) {
        /* encode a supplementary code point as a surrogate pair */
        SCSUMiniEncode(0xd7c0+(c>>10), pState);
        SCSUMiniEncode(0xdc00+(c&0x3ff), pState);
        return;
    }

    window=pState->window;
    if(!pState->isUnicodeMode) {
        /* single-byte mode */
        if(c<0x20) {
            /*
             * Encode C0 control code:
             * Check the code point against the bit mask 0010 0110 0000 0001
             * which contains 1-bits at the bit positions corresponding to
             * code points 0D 0A 09 00 (CR LF TAB NUL)
             * which are encoded directly.
             * All other C0 control codes are quoted with SQ0.
             */
            if(c<=0xf && ((1<<c)&0x2601)==0) {
                out(SQ0);
            }
            out(c);
        } else if(c<=0x7f) {
            /* encode US-ASCII directly */
            out(c);
        } else if(isInWindow(offsets[window], c)) {
            /* use the current dynamic window */
            out(0x80+(c-offsets[window]));
        } else if((w=getWindow(c))>=0) {
            if(w<=7) {
                /* switch to a dynamic window */
                out(SC0+w);
                out(0x80+(c-offsets[w]));
                pState->window=window=(char)w;
            } else {
                /* quote from a static window */
                out(SQ0+(w-8));
                out(c-offsets[w]);
            }
        } else if(c==0xfeff) {
            /* encode the signature character U+FEFF with SQU */
            out(SQU);
            out(0xfe);
            out(0xff);
        } else {
            /* switch to Unicode mode */
            out(SCU);
            pState->isUnicodeMode=1;
            SCSUMiniEncode(c, pState);
        }
    } else {
        /* Unicode mode */
        if(c<=0x7f) {
            /* US-ASCII: switch to single-byte mode with the previous dynamic window */
            pState->isUnicodeMode=0;
            out(UC0+window);
            SCSUMiniEncode(c, pState);
        } else if((w=getWindow(c))>=0 && w<=7) {
            /* switch to single-byte mode with a matching dynamic window */
            out(UC0+w);
            pState->window=window=(char)w;
            pState->isUnicodeMode=0;
            SCSUMiniEncode(c, pState);
        } else {
            if(0xe000<=c && c<=0xf2ff) {
                out(UQU);
            }
            out(c>>8);
            out(c);
        }
    }
}

/* driver for sample code */
extern void out(int byte) {
    printf("--> %02X\n", byte&0xff);
}

extern int main(int argc, const char *argv[]) {
    int32_t c;
    SCSUMiniState state={ 0, 0 }; /* initial state */

    puts("Type hexadecimal Unicode code point values, one per line, for SCSU encoding:");

    while(1==scanf("%lX", &c)) {
        SCSUMiniEncode(c, &state);
    }
    return 0;
}

.