// Copyright 2013, Andreas Samoljuk
// Copyright 2023, Julius Pfrommer
//
// Boost Software License - Version 1.0 - August 17th, 2003
//
// Permission is hereby granted, free of charge, to any person or organization
// obtaining a copy of the software and accompanying documentation covered by
// this license (the "Software") to use, reproduce, display, distribute,
// execute, and transmit the Software, and to prepare derivative works of the
// Software, and to permit third-parties to whom the Software is furnished to
// do so, all subject to the following:
//
// The copyright notices in the Software and this entire statement, including
// the above license grant, this restriction and the following disclaimer,
// must be included in all copies of the Software, in whole or in part, and
// all derivative works of the Software, unless such copies or derivative
// works are solely in the form of machine-executable object code generated by
// a source language processor.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
// SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
// FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

#include "dtoa.h"
#include <stdint.h>
#include <stdbool.h>
#include <string.h>

#define mantissa_bits 52
#define exponent_bits 11
#define fracmask  0x000FFFFFFFFFFFFFU
#define expmask   0x7FF0000000000000U
#define hiddenbit 0x0010000000000000U
#define signmask  0x8000000000000000U
#define expbias   (1023 + 52)

#define absv(n) ((n) < 0 ? -(n) : (n))
#define minv(a, b) ((a) < (b) ? (a) : (b))

static uint64_t tens[] = {
    10000000000000000000U, 1000000000000000000U, 100000000000000000U,
    10000000000000000U, 1000000000000000U, 100000000000000U,
    10000000000000U, 1000000000000U, 100000000000U,
    10000000000U, 1000000000U, 100000000U,
    10000000U, 1000000U, 100000U,
    10000U, 1000U, 100U,
    10U, 1U
};

#define npowers     87
#define steppowers  8
#define firstpower -348 /* 10 ^ -348 */
#define expmax     -32
#define expmin     -60

typedef struct Fp {
    uint64_t frac;
    int exp;
} Fp;

static Fp powers_ten[] = {
    { 18054884314459144840U, -1220 }, { 13451937075301367670U, -1193 },
    { 10022474136428063862U, -1166 }, { 14934650266808366570U, -1140 },
    { 11127181549972568877U, -1113 }, { 16580792590934885855U, -1087 },
    { 12353653155963782858U, -1060 }, { 18408377700990114895U, -1034 },
    { 13715310171984221708U, -1007 }, { 10218702384817765436U, -980 },
    { 15227053142812498563U, -954 },  { 11345038669416679861U, -927 },
    { 16905424996341287883U, -901 },  { 12595523146049147757U, -874 },
    { 9384396036005875287U,  -847 },  { 13983839803942852151U, -821 },
    { 10418772551374772303U, -794 },  { 15525180923007089351U, -768 },
    { 11567161174868858868U, -741 },  { 17236413322193710309U, -715 },
    { 12842128665889583758U, -688 },  { 9568131466127621947U,  -661 },
    { 14257626930069360058U, -635 },  { 10622759856335341974U, -608 },
    { 15829145694278690180U, -582 },  { 11793632577567316726U, -555 },
    { 17573882009934360870U, -529 },  { 13093562431584567480U, -502 },
    { 9755464219737475723U,  -475 },  { 14536774485912137811U, -449 },
    { 10830740992659433045U, -422 },  { 16139061738043178685U, -396 },
    { 12024538023802026127U, -369 },  { 17917957937422433684U, -343 },
    { 13349918974505688015U, -316 },  { 9946464728195732843U,  -289 },
    { 14821387422376473014U, -263 },  { 11042794154864902060U, -236 },
    { 16455045573212060422U, -210 },  { 12259964326927110867U, -183 },
    { 18268770466636286478U, -157 },  { 13611294676837538539U, -130 },
    { 10141204801825835212U, -103 },  { 15111572745182864684U, -77 },
    { 11258999068426240000U, -50 },   { 16777216000000000000U, -24 },
    { 12500000000000000000U,   3 },   { 9313225746154785156U,   30 },
    { 13877787807814456755U,  56 },   { 10339757656912845936U,  83 },
    { 15407439555097886824U, 109 },   { 11479437019748901445U, 136 },
    { 17105694144590052135U, 162 },   { 12744735289059618216U, 189 },
    { 9495567745759798747U,  216 },   { 14149498560666738074U, 242 },
    { 10542197943230523224U, 269 },   { 15709099088952724970U, 295 },
    { 11704190886730495818U, 322 },   { 17440603504673385349U, 348 },
    { 12994262207056124023U, 375 },   { 9681479787123295682U,  402 },
    { 14426529090290212157U, 428 },   { 10748601772107342003U, 455 },
    { 16016664761464807395U, 481 },   { 11933345169920330789U, 508 },
    { 17782069995880619868U, 534 },   { 13248674568444952270U, 561 },
    { 9871031767461413346U,  588 },   { 14708983551653345445U, 614 },
    { 10959046745042015199U, 641 },   { 16330252207878254650U, 667 },
    { 12166986024289022870U, 694 },   { 18130221999122236476U, 720 },
    { 13508068024458167312U, 747 },   { 10064294952495520794U, 774 },
    { 14996968138956309548U, 800 },   { 11173611982879273257U, 827 },
    { 16649979327439178909U, 853 },   { 12405201291620119593U, 880 },
    { 9242595204427927429U,  907 },   { 13772540099066387757U, 933 },
    { 10261342003245940623U, 960 },   { 15290591125556738113U, 986 },
    { 11392378155556871081U, 1013 },  { 16975966327722178521U, 1039 },
    { 12648080533535911531U, 1066 }
};

static Fp
find_cachedpow10(int exp, int* k) {
    const double one_log_ten = 0.30102999566398114;
    int approx = (int)(-(exp + npowers) * one_log_ten);
    int idx = (approx - firstpower) / steppowers;
    while(1) {
        int current = exp + powers_ten[idx].exp + 64;
        if(current < expmin) {
            idx++;
            continue;
        }
        if(current > expmax) {
            idx--;
            continue;
        }
        *k = (firstpower + idx * steppowers);
        return powers_ten[idx];
    }
}

static Fp build_fp(uint64_t bits) {
    Fp fp;
    fp.frac = bits & fracmask;
    fp.exp = (bits & expmask) >> 52;
    if(fp.exp) {
        fp.frac += hiddenbit;
        fp.exp -= expbias;
    } else {
        fp.exp = -expbias + 1;
    }
    return fp;
}

static void normalize(Fp* fp) {
    while((fp->frac & hiddenbit) == 0) {
        fp->frac <<= 1;
        fp->exp--;
    }
    int shift = 64 - 52 - 1;
    fp->frac <<= shift;
    fp->exp -= shift;
}

static void get_normalized_boundaries(Fp* fp, Fp* lower, Fp* upper) {
    upper->frac = (fp->frac << 1) + 1;
    upper->exp  = fp->exp - 1;
    while ((upper->frac & (hiddenbit << 1)) == 0) {
        upper->frac <<= 1;
        upper->exp--;
    }

    int u_shift = 64 - 52 - 2;
    upper->frac <<= u_shift;
    upper->exp = upper->exp - u_shift;

    int l_shift = fp->frac == hiddenbit ? 2 : 1;
    lower->frac = (fp->frac << l_shift) - 1;
    lower->exp = fp->exp - l_shift;
    lower->frac <<= lower->exp - upper->exp;
    lower->exp = upper->exp;
}

static Fp multiply(Fp* a, Fp* b) {
    const uint64_t lomask = 0x00000000FFFFFFFF;
    uint64_t ah_bl = (a->frac >> 32)    * (b->frac & lomask);
    uint64_t al_bh = (a->frac & lomask) * (b->frac >> 32);
    uint64_t al_bl = (a->frac & lomask) * (b->frac & lomask);
    uint64_t ah_bh = (a->frac >> 32)    * (b->frac >> 32);
    uint64_t tmp = (ah_bl & lomask) + (al_bh & lomask) + (al_bl >> 32); 
    /* round up */
    tmp += 1U << 31;
    Fp fp;
    fp.frac = ah_bh + (ah_bl >> 32) + (al_bh >> 32) + (tmp >> 32);
    fp.exp = a->exp + b->exp + 64;
    return fp;
}

static void round_digit(char* digits, unsigned ndigits, uint64_t delta,
                        uint64_t rem, uint64_t kappa, uint64_t frac) {
    while(rem < frac && delta - rem >= kappa &&
          (rem + kappa < frac || frac - rem > rem + kappa - frac)) {
        digits[ndigits - 1]--;
        rem += kappa;
    }
}

static unsigned generate_digits(Fp* fp, Fp* upper, Fp* lower, char* digits, int* K) {
    uint64_t wfrac = upper->frac - fp->frac;
    uint64_t delta = upper->frac - lower->frac;

    Fp one;
    one.frac = 1ULL << -upper->exp;
    one.exp  = upper->exp;

    uint64_t part1 = upper->frac >> -one.exp;
    uint64_t part2 = upper->frac & (one.frac - 1);

    unsigned idx = 0;
    int kappa = 10;
    uint64_t* divp;

    /* 1000000000 */
    for(divp = tens + 10; kappa > 0; divp++) {
        uint64_t div = *divp;
        uint64_t digit = part1 / div;
        if(digit || idx) {
            digits[idx++] = (char)(digit + '0');
        }

        part1 -= digit * div;
        kappa--;

        uint64_t tmp = (part1 <<-one.exp) + part2;
        if(tmp <= delta) {
            *K += kappa;
            round_digit(digits, idx, delta, tmp, div << -one.exp, wfrac);
            return idx;
        }
    }

    /* 10 */
    uint64_t* unit = tens + 18;
    while(true) {
        part2 *= 10;
        delta *= 10;
        kappa--;

        uint64_t digit = part2 >> -one.exp;
        if(digit || idx) {
            digits[idx++] = (char)(digit + '0');
        }

        part2 &= one.frac - 1;
        if(part2 < delta) {
            *K += kappa;
            round_digit(digits, idx, delta, part2, one.frac, wfrac * *unit);
            break;
        }
        unit--;
    }
    return idx;
}

static unsigned grisu2(uint64_t bits, char* digits, int* K) {
    Fp w = build_fp(bits);
    Fp lower, upper;
    get_normalized_boundaries(&w, &lower, &upper);
    normalize(&w);
    int k;
    Fp cp = find_cachedpow10(upper.exp, &k);
    w     = multiply(&w,     &cp);
    upper = multiply(&upper, &cp);
    lower = multiply(&lower, &cp);
    lower.frac++;
    upper.frac--;
    *K = -k;
    return generate_digits(&w, &upper, &lower, digits, K);
}

static unsigned
emit_digits(char* digits, unsigned ndigits, char* dest, int K, bool neg) {
    int exp = absv(K + (int)ndigits - 1);

    /* write plain integer */
    if(K >= 0 && (exp < (int)ndigits + 7)) {
        memcpy(dest, digits, ndigits);
        memset(dest + ndigits, '0', (unsigned)K);
        memcpy(dest + ndigits + (unsigned)K, ".0", 2); /* always append .0 for naked integers */
        return (unsigned)(ndigits + (unsigned)K + 2);
    }

    /* write decimal w/o scientific notation */
    if(K < 0 && (K > -7 || exp < 4)) {
        int offset = (int)ndigits - absv(K);
        if(offset <= 0) {
            /* fp < 1.0 -> write leading zero */
            offset = -offset;
            dest[0] = '0';
            dest[1] = '.';
            memset(dest + 2, '0', (size_t)offset);
            memcpy(dest + offset + 2, digits, ndigits);
            return ndigits + 2 + (unsigned)offset;
        } else {
            /* fp > 1.0 */
            memcpy(dest, digits, (size_t)offset);
            dest[offset] = '.';
            memcpy(dest + offset + 1, digits + offset, ndigits - (unsigned)offset);
            return ndigits + 1;
        }
    }

    /* write decimal w/ scientific notation */
    ndigits = minv(ndigits, (unsigned)(18 - neg));
    unsigned idx = 0;
    dest[idx++] = digits[0];
    if(ndigits > 1) {
        dest[idx++] = '.';
        memcpy(dest + idx, digits + 1, ndigits - 1);
        idx += ndigits - 1;
    }

    dest[idx++] = 'e';

    char sign = K + (int)ndigits - 1 < 0 ? '-' : '+';
    dest[idx++] = sign;

    int cent = 0;
    if(exp > 99) {
        cent = exp / 100;
        dest[idx++] = (char)(cent + '0');
        exp -= cent * 100;
    }
    if(exp > 9) {
        int dec = exp / 10;
        dest[idx++] = (char)(dec + '0');
        exp -= dec * 10;

    } else if(cent) {
        dest[idx++] = '0';
    }
    dest[idx++] = (char)(exp % 10 + '0');
    return idx;
}

unsigned dtoa(double d, char* buffer) {
    uint64_t bits = 0;
    memcpy(&bits, &d, sizeof(double));

    uint64_t mantissa = bits & ((1ull << mantissa_bits) - 1);
    uint32_t exponent = (uint32_t)
        ((bits >> mantissa_bits) & ((1u << exponent_bits) - 1));

    if(exponent == 0 && mantissa == 0) {
        memcpy(buffer, "0.0", 3);
        return 3;
    }

    bool sign = ((bits >> (mantissa_bits + exponent_bits)) & 1) != 0;
    unsigned pos = 0;
    if(sign) {
        buffer[0] = '-';
        pos++;
        buffer++;
    }

    if(exponent == ((1u << exponent_bits) - 1u)) {
        if(mantissa != 0) {
            memcpy(buffer, "nan", 3);
            return 3;
        } else {
            memcpy(&buffer[pos], "inf", 3);
            return pos + 3;
        }
    }

    int K = 0;
    char digits[18];
    memset(digits, 0, 18);
    unsigned ndigits = grisu2(bits, digits, &K);
    return pos + emit_digits(digits, ndigits, buffer, K, sign);
}