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float_to_digits.d: add some comments

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Marius Gerbershagen 2022-09-10 19:12:39 +02:00
parent d55d31df3b
commit 763369ee84
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42
src/c/printer/float_to_digits.d
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@ -2,6 +2,7 @@
/* vim: set filetype=c tabstop=2 shiftwidth=2 expandtab: */
/*
* float_to_digits.d: floating point printer functions
*
* Copyright (c) 2010 Juan Jose Garcia Ripoll
*
@ -14,6 +15,11 @@
#include <ecl/ecl.h>
#include <ecl/ecl-inl.h>
/*
* The following algorithm is based on Burger, Dybvig (1996)
* (available at http://www.cs.indiana.edu/~dyb/pubs/FP-Printing-PLDI96.pdf)
*/
#define PRINT_BASE ecl_make_fixnum(10)
#define EXPT_RADIX(x) ecl_ash(ecl_make_fixnum(1),x)
@ -196,6 +202,42 @@ change_precision(float_approx *approx, cl_object position, cl_object relativep)
}
}
/*
* si_float_to_digits: create a correctly rounded approximation for a
* floating point number in the form of a string of (base 10) digits.
* Zero digits at the end or beginning of the string are omitted.
*
* Arguments:
*
* - digits: an adjustable base-string to store the digits in or NIL
* to allocate a new base-string
* - number: the floating point number to approximate
* - position: specifies the number of digits to generate. If position
* is NIL create as many digits as necessary to reconstruct the
* number without error; otherwise:
* - if relativep is false, position specifies the position relative
* to the decimal point up to which to generate digits. Example: a
* number of the form (d_n)...(d_0).(d_{-1})...*10^0 is translated
* into a string (d_n)...(d_{position}) and then rounded. Here,
* (d_i) is the digit at position i relative to the decimal point.
* - if replativep is true, the number is normalized such that the
* decimal point lies behind the first digit and position
* specifies the number of digits to generate after this shifted
* decimal point. Example: for k >= 0, a number of the form
* (d_1).(d_2)...(d_n)*10^(-k) is translated into a string
* (d_1)(d_2)...(d_{position+1}) and then rounded irrespective of
* the value of k. For k < 0, the result is a string of the form
* (d_1)(d_2)..(d_{position-k+1}) (k leading zeros are omitted).
* - relativep: a generalized boolean
*
* Return values:
*
* - k: a scale factor such that the string of digits multiplied with
* 10^k is (up to rounding) equal to number. To be precise, for
* return values k and (d_1)...(d_n), number ≅ 10^(k-n)*int((d_1)...(d_n)).
* - digits: the created digits
*/
cl_object
si_float_to_digits(cl_object digits, cl_object number, cl_object position,
cl_object relativep)