modp_numtoa.c

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/* -*- mode: c; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 4 -*- */
/* vi: set expandtab shiftwidth=4 tabstop=4: */

#include "modp_numtoa.h"

#include <stdint.h>
#include <stdio.h>
#include <math.h>

// other interesting references on num to string convesion
// http://www.jb.man.ac.uk/~slowe/cpp/itoa.html
// and http://www.ddj.com/dept/cpp/184401596?pgno=6

/**
 * Powers of 10
 * 10^0 to 10^9
 */
static const double pow10[] = {0, 10, 100, 1000, 10000, 100000, 1000000,
                               10000000, 100000000, 1000000000};

static void strreverse(char* begin, char* end)
{
    char aux;
    while (end > begin)
        aux = *end, *end-- = *begin, *begin++ = aux;
}

void modp_itoa10(int32_t value, char* str)
{
    char* wstr=str;
    int sign;
    // Take care of sign
    if ((sign=value) < 0) value = -value;
    // Conversion. Number is reversed.
    do *wstr++ = (48 + (value % 10)); while( value /= 10);
    if (sign < 0) *wstr++ = '-';
    *wstr='\0';

    // Reverse string
    strreverse(str,wstr-1);
}

void modp_uitoa10(uint32_t value, char* str)
{
    char* wstr=str;
    // Conversion. Number is reversed.
    do *wstr++ = 48 + (value % 10); while (value /= 10);
    *wstr='\0';
    // Reverse string
    strreverse(str, wstr-1);
}

void modp_dtoa(double value, char* str, int prec)
{
    const double thres_min = 1.0/2048.0;
    const double thres_max = (double)(0x7FFFFFFF);
    char* wstr = str;

    if (prec < 0) {
        prec = 0;
    } else if (prec > 9) {
        prec = 9;
    }

    int neg = 0;
    if (value < 0) {
        neg = 1;
        value = -value;
    }

    int whole = (int) value;
    double tmp = (value - whole) * pow10[prec];
    uint32_t frac = (uint32_t)(tmp);
    if (tmp - frac > 0.5) {
        ++frac;
    }

    if (value > thres_max || (frac > 0 && frac < thres_min)) {
        sprintf(str, "%e", neg ? -value : value);
        return;
    }

    int count = prec;
    // now do fractional part, as an unsigned number
    do {
        --count;
        *wstr++ = 48 + (frac % 10);
    } while (frac /= 10);
    // add extra 0s
    while (count-- > 0) *wstr++ = '0';
    // add decimal
    *wstr++ = '.';

    // do whole part
    // Take care of sign
    // Conversion. Number is reversed.
    do *wstr++ = 48 + (whole % 10); while (whole /= 10);
    if (neg) {
        *wstr++ = '-';
    }
    *wstr='\0';
    strreverse(str, wstr-1);
}