Text Statistics & Readability — C source

/* text-stats — text statistics & readability: chars, words, sentences, syllables, Flesch. Language: C (C11). Port of src/lib/textStats.ts — chars count UTF-8 bytes & \s is ASCII-only here, the TS reference counts UTF-16 units & Unicode spaces. */
#include <ctype.h>
#include <math.h>
#include <stdio.h>
#include <string.h>

typedef struct {
    int characters, characters_no_spaces, words, sentences, paragraphs, lines, syllables;
    int reading_ms, speaking_ms;
    double flesch_e, flesch_k; /* 0 when not computable (label NULL) */
    const char *label;
} TextStats;

static double js_round(double x) { return floor(x + 0.5); } /* JS Math.round: ties toward +infinity */

/* Word byte: [A-Za-z0-9'-] plus U+2019 (UTF-8 E2 80 99) — keeps contractions whole. */
static int is_word_byte(const unsigned char *p) {
    if (isalnum(*p) || *p == '\'' || *p == '-') return 1;
    return p[0] == 0xE2 && p[1] == 0x80 && p[2] == 0x99;
}

/* Syllables in one word via the vowel-group heuristic (only ASCII a-z survive). */
static int count_syllables(const char *word) {
    char w[64]; size_t n = 0;
    for (const unsigned char *p = (const unsigned char *)word; *p && n < sizeof w; p++)
        if (islower(*p)) w[n++] = (char)*p;
        else if (isupper(*p)) w[n++] = (char)tolower(*p);
    if (n == 0) return 0;
    if (n <= 3) return 1;
    /* drop a silent trailing e/ed/es — 'l' and vowels keep their syllable ("~le") */
    if (n >= 3 && w[n-1] == 's' && w[n-2] == 'e' && !strchr("laeiouy", w[n-3])) n -= 2;
    else if (n >= 2 && w[n-1] == 'd' && w[n-2] == 'e') n -= 2;
    else if (n >= 2 && w[n-1] == 'e' && !strchr("laeiouy", w[n-2])) n -= 1;
    size_t i = w[0] == 'y' ? 1 : 0; /* drop a leading y */
    int groups = 0, in_vowel = 0;
    for (; i < n; i++) { int v = strchr("aeiouy", w[i]) != NULL; if (v && !in_vowel) groups++; in_vowel = v; }
    return groups > 1 ? groups : 1;
}

static const char *label_for(double f) {
    return f >= 80 ? "Very Easy" : f >= 70 ? "Easy" : f >= 60 ? "Standard"
        : f >= 50 ? "Fairly Hard" : f >= 30 ? "Hard" : "Very Hard";
}

static TextStats analyze(const char *text) {
    TextStats s = {0};
    size_t len = strlen(text);
    s.characters = (int)len;
    for (size_t i = 0; i < len; i++) if (!isspace((unsigned char)text[i])) s.characters_no_spaces++;
    /* words = maximal runs of word bytes; syllables sum per word */
    const unsigned char *p = (const unsigned char *)text;
    while (*p) {
        if (!is_word_byte(p)) { p++; continue; }
        char word[64]; size_t n = 0;
        while (is_word_byte(p) && n < sizeof word - 1) { word[n++] = (char)*p; p += *p == 0xE2 ? 3 : 1; }
        word[n] = '\0';
        s.words++;
        s.syllables += count_syllables(word);
    }
    /* sentences = runs of [.!?] followed by whitespace or end; min 1 when words > 0 */
    for (size_t i = 0; i < len;) {
        if (text[i] != '.' && text[i] != '!' && text[i] != '?') { i++; continue; }
        size_t j = i;
        while (j < len && (text[j] == '.' || text[j] == '!' || text[j] == '?')) j++;
        if (j == len || isspace((unsigned char)text[j])) s.sentences++;
        i = j;
    }
    if (s.words == 0) s.sentences = 0;
    else if (s.sentences == 0) s.sentences = 1;
    /* paragraphs = non-blank chunks split on runs of 2+ newlines; lines = \n count + 1 */
    for (size_t i = 0; i < len;) {
        int blank = 1;
        while (i < len && !(text[i] == '\n' && i + 1 < len && text[i + 1] == '\n')) {
            if (!isspace((unsigned char)text[i])) blank = 0;
            i++;
        }
        if (!blank) s.paragraphs++;
        while (i < len && text[i] == '\n') i++;
    }
    if (len > 0) { s.lines = 1; for (size_t i = 0; i < len; i++) if (text[i] == '\n') s.lines++; }
    s.reading_ms = (int)js_round(s.words / 200.0 * 60000.0);  /* 200 wpm */
    s.speaking_ms = (int)js_round(s.words / 130.0 * 60000.0); /* 130 wpm */
    if (s.words > 0 && s.sentences > 0) {
        double wps = (double)s.words / s.sentences, spw = (double)s.syllables / s.words;
        s.flesch_e = js_round((206.835 - 1.015 * wps - 84.6 * spw) * 10) / 10;
        s.flesch_k = js_round((0.39 * wps + 11.8 * spw - 15.59) * 10) / 10;
        s.label = label_for(s.flesch_e);
    }
    return s;
}

int main(void) {
    const char *samples[] = {
        "The quick brown fox jumps over the lazy dog.",
        "Hi.\n\nMy name is Inigo Montoya. You killed my father; prepare to die!",
    };
    for (size_t i = 0; i < sizeof samples / sizeof *samples; i++) {
        TextStats s = analyze(samples[i]);
        printf("chars=%d nospace=%d words=%d sentences=%d paragraphs=%d lines=%d syllables=%d "
               "reading=%dms speaking=%dms", s.characters, s.characters_no_spaces, s.words,
               s.sentences, s.paragraphs, s.lines, s.syllables, s.reading_ms, s.speaking_ms);
        if (s.label) printf(" flesch=%.1f (%s) grade=%.1f\n", s.flesch_e, s.label, s.flesch_k);
        else printf(" flesch=n/a\n");
    }
    return 0;
}