リーダブル(入力⇔出力で値が変わらない)な浮動小数文字列を出力するC言語の関数(snprintf_double)

snprint_double.c

#include <float.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

int snprint_double(char* buf, int size, double f)
{
    if (isnan(f)) {
        return snprintf(buf, size, "%s", signbit(f) ? "-nan" : "+nan");
    } else if (isinf(f)) {
        return snprintf(buf, size, "%s", signbit(f) ? "-inf" : "+inf");
    } else {
        int len;
        for (int prec = fabs(f) < DBL_MIN ? 1 : DBL_DIG; ; ++prec) {
            len = snprintf(buf, size, "%.*g", prec, f);
            if (len < 0) {
                return 0;
            }
            if (16 < prec
                || (len < size && strtod(buf, NULL) == f)) {
                break;
            }
        }

        // 12.0 が 12 と表示されるのを回避する場合は以下を有効にする
#if 0
        int need_decimal_point = 1;
        for (const char* p = buf; *p; ++p) {
            if (*p == '.' || *p == 'e') {
                need_decimal_point = 0;
                break;
            }
        }
        if (need_decimal_point && len <= (int)sizeof(buf) - 2) {
            buf[len++] = '.';
            buf[len++] = '0';
        }
#endif

        return len;
    }
}

#if 1
int main(void)
{
    static const char* numbers[] = {
        "-78888.032505798281119254395134273686797671268736553338536795406087720787378465858094159870726039767164607546644",
        "-68.272904537112324",
        "-9034540.e14",
        "8113.",
        "8.4e-5",
        "-22201639700171564.74311",
        "-8954192320392299.8381203",
        "-17917743150860613.3220670",
        "-639.e19",
        "-.7110489997872715289749e17",
        "0660.",
        "29550600.1079e9",
        "61748105994541732.55867883500",
        "-68343557483.e7",
        "978021552.497e19",
        "-.8e17",
        "36932659236092295.0",
        "-91401616.e27",
        "36480499517288025.0542900284",
        "428069624325376740.86",
        "-21555778236601032.2",
        "-19955484380340809.67942279917",
        "064192082488816174.20389",
        "20883828695083711.450",
        "-29.e21",
        "25974673343120767.984",
        "56154525620347983.0704789",
        "54232355670202146.92",
        "-40.4e21",
        "23476370748988750.",
        "25320799998182632.0715",
        "-0440991669613565547.9",
        "-24009465153285070.374",
        "-20711179850745286.003484",
        "68677973679788178.433",
        "-24969950846291642195900.92e-5",
        "-21754109068434992.183",
        "34050527959032390.7765690846",
        "37488855950257261.93224",
        "-.80e24",
        "-78330960058723814.52",
        "-822.02024e18",
        "-29888056775088510.8060768",
        "-020530861637945047.436065",
        "-571586105612962456.",
        "42175547254159376.57849591399",
        "21.1642403153e17",
        "90518747605325402.26566",
        "63845556111770813.1",
        "-841492335808263887.72",
        "19593618780424409.60",
        "-133218.918423061e12",
        "-69588228482826904.23379252806",
        "-46437062846140.107706478e3",
        "60082583768504096.5903",
        "-83231118366109006.5642058255",
        "31598016798861313.0529581586",
        "-461003734889001005558274390.1e-10",
        "-59882208171768905.872394",
        "27075008032386633.6570401",
        "045158893397659345.08972",
        "-4.157065105787774e-309",
        "-3e-309",
    };

    for (int i = 0; i < (int)(sizeof(numbers)/sizeof(numbers[0])); ++i) {
        const char* number = numbers[i];

        char* endptr;
        double d0 = strtod(number, &endptr);

        char buf[2048];
        int len = snprint_double(buf, sizeof(buf), d0);

        double d1 = strtod(buf, &endptr);

        if (d0 != d1) {
            printf("Oops! %s\n", number);
            return 1;
        } else {
            printf("OK %s -> %.*s\n", number, len, buf);
        }
    }

    printf("Complete!\n");
    return 0;
}
#endif