/*      $OpenBSD: mbrtoc16.c,v 1.1 2023/08/20 15:02:51 schwarze Exp $ */
/*
 * Copyright (c) 2022 Ingo Schwarze <schwarze@openbsd.org>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <stdint.h>
#include <uchar.h>
#include <wchar.h>

/*
 * Keep this structure compatible with
 * struct _utf8_state in the file citrus/citrus_utf8.c.
 * In particular, only use values for the "want" field
 * that do not collide with values used by the function
 * _citrus_utf8_ctype_mbrtowc().
 */
struct _utf16_state {
        wchar_t ch;
        int     want;
};

size_t
mbrtoc16(char16_t *pc16, const char *s, size_t n, mbstate_t *ps)
{
        static mbstate_t         mbs;
        struct _utf16_state     *us;
        size_t                   rv;
        wchar_t                  wc;

        /*
         * Fall back to a state object local to this function
         * and do not use the fallback object in mbrtowc(3)
         * because an application program might mix calls to mbrtowc(3)
         * and mbrtoc16(3) decoding different strings, and they must
         * not clobber each other's state.
         */
        if (ps == NULL)
                ps = &mbs;

        us = (struct _utf16_state *)ps;

        /*
         * Handle the special case of NULL input first such that
         * a low surrogate left over from a previous call does not
         * clobber an object pointed to by the pc16 argument.
         */
        if (s == NULL) {
                s = "";
                n = 1;
                pc16 = NULL;
        }

        /*
         * If the previous call stored a high surrogate,
         * store the corresponding low surrogate now
         * and do not inspect any further input yet.
         */
        if (us->want == (size_t)-3) {
                if (pc16 != NULL)
                        *pc16 = 0xdc00 + (us->ch & 0x3ff);
                us->ch = 0;
                us->want = 0;
                return -3;
        }

        /*
         * Decode the multibyte character.
         * All the mbrtowc(3) use cases can be reached from here,
         * including continuing an imcomplete character started earlier,
         * decoding a NUL character, a valid complete character,
         * an incomplete character to be continued later,
         * or a decoding error.
         */
        rv = mbrtowc(&wc, s, n, ps);

        if (rv < (size_t)-2) {
                /* A new character that is valid and complete. */
                if (wc > UINT16_MAX) {
                        /* Store a high surrogate. */
                        if (pc16 != NULL)
                                *pc16 = 0xd7c0 + (wc >> 10);
                        /* Remember that the low surrogate is pending. */
                        us->ch = wc;
                        us->want = -3;
                } else if (pc16 != NULL)
                        /* Store a basic multilingual plane codepoint. */
                        *pc16 = wc;
        }
        return rv;
}