/* fpconv_powers.h -- floating point conversion utilities.
 *
 * Fast and accurate double to string conversion based on Florian Loitsch's
 * Grisu-algorithm[1].
 *
 * [1] https://www.cs.tufts.edu/~nr/cs257/archive/florian-loitsch/printf.pdf
 * ----------------------------------------------------------------------------
 *
 * Copyright (c) 2021, Redis Labs
 * Copyright (c) 2013-2019, night-shift <as.smljk at gmail dot com>
 * Copyright (c) 2009, Florian Loitsch < florian.loitsch at inria dot fr >
 * All rights reserved.
 *
 * 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 <stdint.h>

#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 }
};

/**
 *  Grisu needs a cache of precomputed powers-of-ten.
 *  This function, given an exponent and an integer k
 *  return the normalized floating-point approximation of the power of 10.
 * @param exp
 * @param k
 * @return
 */
static Fp find_cachedpow10(int exp, int* k)
{
    const double one_log_ten = 0.30102999566398114;

    const int approx = -(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];
    }
}