formatfloat.c 9.34 KB
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/*
 * This file is part of the Micro Python project, http://micropython.org/
 *
 * The MIT License (MIT)
 *
 * Copyright (c) 2013, 2014 Damien P. George
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * 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 AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

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/***********************************************************************

  formatfloat.c  - Ruutine for converting a single-precision floating
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                    point number into a string.
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  The code in this funcion was inspired from Fred Bayer's pdouble.c.
  Since pdouble.c was released as Public Domain, I'm releasing this
  code as public domain as well.
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  The original code can be found in https://github.com/dhylands/format-float
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  Dave Hylands
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***********************************************************************/

#include <stdlib.h>
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#include <stdint.h>
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#include "mpconfig.h"

#if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT
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#include "formatfloat.h"
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// 1 sign bit, 8 exponent bits, and 23 mantissa bits.
// exponent values 0 and 255 are reserved, exponent can be 1 to 254.
// exponent is stored with a bias of 127.
// The min and max floats are on the order of 1x10^37 and 1x10^-37

#define FLT_SIGN_MASK   0x80000000
#define FLT_EXP_MASK    0x7F800000
#define FLT_MAN_MASK    0x007FFFFF

static const float g_pos_pow[] = {
    1e32, 1e16, 1e8, 1e4, 1e2, 1e1
};
static const float g_neg_pow[] = {
    1e-32, 1e-16, 1e-8, 1e-4, 1e-2, 1e-1
};

int format_float(float f, char *buf, size_t buf_size, char fmt, int prec, char sign) {

    char *s = buf;
    int buf_remaining = buf_size - 1;

    union {
        float f;
        uint32_t u;
    } num = {f};

    if (buf_size < 7) {
        // Smallest exp notion is -9e+99 which is 6 chars plus terminating
        // nulll.

        if (buf_size >= 2) {
            *s++ = '?';
        }
        if (buf_size >= 1) {
            *s++ = '\0';
        }
        return buf_size >= 2;
    }
    if (num.u & FLT_SIGN_MASK) {
        *s++ = '-';
        num.u &= ~FLT_SIGN_MASK;
    } else {
        if (sign) {
            *s++ = sign;
        }
    }
    buf_remaining -= (s - buf); // Adjust for sign

    if ((num.u & FLT_EXP_MASK) == FLT_EXP_MASK) {
        char uc = fmt & 0x20;
        if ((num.u & FLT_MAN_MASK) == 0) {
            *s++ = 'I' ^ uc;
            *s++ = 'N' ^ uc;
            *s++ = 'F' ^ uc;
        } else {
            *s++ = 'N' ^ uc;
            *s++ = 'A' ^ uc;
            *s++ = 'N' ^ uc;
        }
        *s = '\0';
        return s - buf;
    }

    if (prec < 0) {
        prec = 6;
    }
    char e_char = 'E' | (fmt & 0x20);   // e_char will match case of fmt
    fmt |= 0x20; // Force fmt to be lowercase
    char org_fmt = fmt;
    if (fmt == 'g' && prec == 0) {
        prec = 1;
    }
    int e, e1; 
    int dec = 0;
    char e_sign = '\0';
    int num_digits = 0;
    const float *pos_pow = g_pos_pow;
    const float *neg_pow = g_neg_pow;

    if (num.u == 0) {
        e = 0;
        if (fmt == 'e') {
            e_sign = '+';
        } else if (fmt == 'f') {
            num_digits = prec + 1;
        }
    } else if (num.u < 0x3f800000) { // f < 1.0
        // Build negative exponent
        for (e = 0, e1 = 32; e1; e1 >>= 1, pos_pow++, neg_pow++) {
            if (*neg_pow > num.f) {
                e += e1;
                num.f *= *pos_pow;
            }
        }
        if (num.f < 1.0F && num.f >= 0.9999995F) {
            num.f = 1.0F;
        } else {
            e++; 
            num.f *= 10.0F;
        }

        // If the user specified 'g' format, and e is <= 4, then we'll switch
        // to the fixed format ('f')

        if (fmt == 'f' || (fmt == 'g' && e <= 4)) {
            fmt = 'f';
            dec = -1;
            *s++ = '0';

            if (prec + e + 1 > buf_remaining) {
                prec = buf_remaining - e - 1;
            }

            if (org_fmt == 'g') {
                prec += (e - 1);
            }
            num_digits = prec;
            if (num_digits) {
                *s++ = '.'; 
                while (--e && num_digits) {
                    *s++ = '0';
                    num_digits--;
                }
            }
        } else {
            // For e & g formats, we'll be printing the exponent, so set the
            // sign.
            e_sign = '-';
            dec = 0;

            if (prec > (buf_remaining - 6)) {
                prec = buf_remaining - 6;
                if (fmt == 'g') {
                    prec++;
                }
            }
        }
    } else {
        // Build positive exponent
        for (e = 0, e1 = 32; e1; e1 >>= 1, pos_pow++, neg_pow++) {
            if (*pos_pow <= num.f) {
                e += e1;
                num.f *= *neg_pow;
            }
        }

        // If the user specified fixed format (fmt == 'f') and e makes the 
        // number too big to fit into the available buffer, then we'll
        // switch to the 'e' format.

        if (fmt == 'f') {
            if (e >= buf_remaining) {
                fmt = 'e';
            } else if ((e + prec + 2) > buf_remaining) {
                prec = buf_remaining - e - 2;
                if (prec < 0) {
                    // This means no decimal point, so we can add one back
                    // for the decimal.
                    prec++;
                }
            }
        }
        if (fmt == 'e' && prec > (buf_remaining - 6)) {
            prec = buf_remaining - 6;
        }
        // If the user specified 'g' format, and e is < prec, then we'll switch
        // to the fixed format.

        if (fmt == 'g' && e < prec) {
            fmt = 'f';
            prec -= (e + 1);
        }
        if (fmt == 'f') {
            dec = e;
            num_digits = prec + e + 1;
        } else {
            e_sign = '+';
        }
    }
    if (prec < 0) {
        // This can happen when the prec is trimmed to prevent buffer overflow
        prec = 0;
    }

    // We now have num.f as a floating point number between >= 1 and < 10
    // (or equal to zero), and e contains the absolute value of the power of
    // 10 exponent. and (dec + 1) == the number of dgits before the decimal.

    // For e, prec is # digits after the decimal
    // For f, prec is # digits after the decimal
    // For g, prec is the max number of significant digits
    //
    // For e & g there will be a single digit before the decimal
    // for f there will be e digits before the decimal

    if (fmt == 'e') {
        num_digits = prec + 1;
    } else if (fmt == 'g') {
        if (prec == 0) {
            prec = 1;
        }
        num_digits = prec; 
    }

    // Print the digits of the mantissa
    for (int i = 0; i < num_digits; ++i, --dec) {
        int32_t d = num.f;
        *s++ = '0' + d;
        if (dec == 0 && prec > 0) {
            *s++ = '.';
        }
        num.f -= (float)d;
        num.f *= 10.0F;
    }

    // Round
    if (num.f >= 5.0F) {
        char *rs = s;
        rs--;
        while (1) {
            if (*rs == '.') {
                rs--;
                continue;
            }
            if (*rs < '0' || *rs > '9') {
                // + or -
                rs++; // So we sit on the digit to the right of the sign
                break;
            }
            if (*rs < '9') {
                (*rs)++;
                break;
            }
            *rs = '0';
            if (rs == buf) {
                break;
            }
            rs--; 
        }
        if (*rs == '0') {
            // We need to insert a 1
            if (rs[1] == '.' && fmt != 'f') {
                // We're going to round 9.99 to 10.00
                // Move the decimal point
                rs[0] = '.';
                rs[1] = '0';
                if (e_sign == '-') {
                    e--;
                } else {
                    e++; 
                }
            }
            s++;
            char *ss = s; 
            while (ss > rs) {
                *ss = ss[-1];
                ss--;
            }
            *rs = '1';
        }
        if (num.u < 0x3f800000 && fmt == 'f') {
            // We rounded up to 1.0
            prec--;
        }
    }

    if (org_fmt == 'g' && prec > 0) {
        // Remove trailing zeros and a trailing decimal point
        while (s[-1] == '0') {
            s--;
        }
        if (s[-1] == '.') {
            s--;
        }
    }
    // Append the exponent
    if (e_sign) {
        *s++ = e_char;
        *s++ = e_sign;
        *s++ = '0' + (e / 10);
        *s++ = '0' + (e % 10);
    }
    *s = '\0';

    return s - buf;
}

#endif