TNT/src/utf8.c

#include "utf8.h"

/* Get the number of bytes in a UTF-8 character from its first byte */
int utf8_byte_length(unsigned char first_byte) {
    if ((first_byte & 0x80) == 0) return 1;      /* 0xxxxxxx */
    if ((first_byte & 0xE0) == 0xC0) return 2;   /* 110xxxxx */
    if ((first_byte & 0xF0) == 0xE0) return 3;   /* 1110xxxx */
    if ((first_byte & 0xF8) == 0xF0) return 4;   /* 11110xxx */
    return 1; /* Invalid UTF-8, treat as single byte */
}

/* Decode a UTF-8 character and return its codepoint */
uint32_t utf8_decode(const char *str, int *bytes_read) {
    const unsigned char *s = (const unsigned char *)str;
    uint32_t codepoint = 0;
    int len = utf8_byte_length(s[0]);

    if (len < 1 || len > 4) {
        len = 1;
    }

    for (int i = 1; i < len; i++) {
        if (s[i] == '\0' || (s[i] & 0xC0) != 0x80) {
            /* Truncated or invalid continuation byte — treat as single byte */
            *bytes_read = 1;
            return s[0];
        }
    }

    *bytes_read = len;

    switch (len) {
        case 1:
            codepoint = s[0];
            break;
        case 2:
            codepoint = ((s[0] & 0x1F) << 6) | (s[1] & 0x3F);
            break;
        case 3:
            codepoint = ((s[0] & 0x0F) << 12) | ((s[1] & 0x3F) << 6) | (s[2] & 0x3F);
            break;
        case 4:
            codepoint = ((s[0] & 0x07) << 18) | ((s[1] & 0x3F) << 12) |
                       ((s[2] & 0x3F) << 6) | (s[3] & 0x3F);
            break;
    }

    return codepoint;
}

/* UTF-8 character width calculation for CJK and other wide characters */
int utf8_char_width(uint32_t codepoint) {
    /* ASCII */
    if (codepoint < 0x80) return 1;

    /* CJK Unified Ideographs */
    if ((codepoint >= 0x4E00 && codepoint <= 0x9FFF) ||    /* CJK Unified */
        (codepoint >= 0x3400 && codepoint <= 0x4DBF) ||    /* CJK Extension A */
        (codepoint >= 0x20000 && codepoint <= 0x2A6DF) ||  /* CJK Extension B */
        (codepoint >= 0x2A700 && codepoint <= 0x2B73F) ||  /* CJK Extension C */
        (codepoint >= 0x2B740 && codepoint <= 0x2B81F) ||  /* CJK Extension D */
        (codepoint >= 0x2B820 && codepoint <= 0x2CEAF) ||  /* CJK Extension E */
        (codepoint >= 0xF900 && codepoint <= 0xFAFF) ||    /* CJK Compatibility */
        (codepoint >= 0x2F800 && codepoint <= 0x2FA1F)) {  /* CJK Compat Suppl */
        return 2;
    }

    /* Hangul Syllables (Korean) */
    if (codepoint >= 0xAC00 && codepoint <= 0xD7AF) return 2;

    /* Hiragana and Katakana (Japanese) */
    if ((codepoint >= 0x3040 && codepoint <= 0x309F) ||    /* Hiragana */
        (codepoint >= 0x30A0 && codepoint <= 0x30FF)) {    /* Katakana */
        return 2;
    }

    /* Fullwidth forms */
    if (codepoint >= 0xFF00 && codepoint <= 0xFFEF) return 2;

    /* Default to single width */
    return 1;
}

/* Calculate display width of a UTF-8 string */
int utf8_string_width(const char *str) {
    int width = 0;
    int bytes_read;
    const char *p = str;

    while (*p != '\0') {
        uint32_t codepoint = utf8_decode(p, &bytes_read);
        width += utf8_char_width(codepoint);
        p += bytes_read;
    }

    return width;
}

/* Count the number of UTF-8 characters in a string */
int utf8_strlen(const char *str) {
    int count = 0;
    int bytes_read;
    const char *p = str;

    while (*p != '\0') {
        utf8_decode(p, &bytes_read);
        count++;
        p += bytes_read;
    }

    return count;
}

/* Truncate string to fit within max_width display characters */
void utf8_truncate(char *str, int max_width) {
    int width = 0;
    int bytes_read;
    char *p = str;
    char *last_valid = str;

    while (*p != '\0') {
        uint32_t codepoint = utf8_decode(p, &bytes_read);
        int char_width = utf8_char_width(codepoint);

        if (width + char_width > max_width) {
            break;
        }

        width += char_width;
        p += bytes_read;
        last_valid = p;
    }

    *last_valid = '\0';
}

/* Remove last UTF-8 character from string */
void utf8_remove_last_char(char *str) {
    int len = strlen(str);
    if (len == 0) return;

    /* Find the start of the last character by walking backwards */
    int i = len - 1;
    while (i > 0 && (str[i] & 0xC0) == 0x80) {
        i--;  /* Continue byte of multi-byte sequence */
    }

    str[i] = '\0';
}

/* Remove last word from string (mimic Ctrl+W) */
void utf8_remove_last_word(char *str) {
    int len = strlen(str);
    if (len == 0) return;

    int i = len;

    /* Skip trailing spaces */
    while (i > 0 && str[i - 1] == ' ') {
        i--;
    }

    /* Skip non-spaces (the word) */
    while (i > 0 && str[i - 1] != ' ') {
        i--;
    }

    str[i] = '\0';
}

/* Validate a UTF-8 byte sequence */
bool utf8_is_valid_sequence(const char *bytes, int len) {
    if (len <= 0 || len > 4 || !bytes) {
        return false;
    }

    const unsigned char *b = (const unsigned char *)bytes;

    /* Check first byte matches the expected length */
    int expected_len = utf8_byte_length(b[0]);
    if (expected_len != len) {
        return false;
    }

    /* Validate continuation bytes (must be 10xxxxxx) */
    for (int i = 1; i < len; i++) {
        if ((b[i] & 0xC0) != 0x80) {
            return false;
        }
    }

    /* Validate codepoint ranges to prevent overlong encodings */
    uint32_t codepoint = 0;
    switch (len) {
        case 1:
            /* 0xxxxxxx - valid range: 0x01-0x7F (reject NUL) */
            codepoint = b[0];
            if (codepoint == 0 || codepoint > 0x7F) return false;
            break;
        case 2:
            /* 110xxxxx 10xxxxxx - valid range: 0x80-0x7FF */
            codepoint = ((b[0] & 0x1F) << 6) | (b[1] & 0x3F);
            if (codepoint < 0x80 || codepoint > 0x7FF) return false;
            break;
        case 3:
            /* 1110xxxx 10xxxxxx 10xxxxxx - valid range: 0x800-0xFFFF */
            codepoint = ((b[0] & 0x0F) << 12) | ((b[1] & 0x3F) << 6) | (b[2] & 0x3F);
            if (codepoint < 0x800 || codepoint > 0xFFFF) return false;
            /* Reject UTF-16 surrogates (0xD800-0xDFFF) */
            if (codepoint >= 0xD800 && codepoint <= 0xDFFF) return false;
            break;
        case 4:
            /* 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx - valid range: 0x10000-0x10FFFF */
            codepoint = ((b[0] & 0x07) << 18) | ((b[1] & 0x3F) << 12) |
                       ((b[2] & 0x3F) << 6) | (b[3] & 0x3F);
            if (codepoint < 0x10000 || codepoint > 0x10FFFF) return false;
            break;
    }

    return true;
}

bool utf8_is_valid_string(const char *str) {
    const unsigned char *p = (const unsigned char *)str;

    if (!str) {
        return false;
    }

    while (*p != '\0') {
        int len = utf8_byte_length(*p);
        if (len < 1 || len > 4) {
            return false;
        }

        for (int i = 1; i < len; i++) {
            if (p[i] == '\0') {
                return false;
            }
        }

        if (!utf8_is_valid_sequence((const char *)p, len)) {
            return false;
        }

        p += len;
    }

    return true;
}