368 lines
9.1 KiB
V
368 lines
9.1 KiB
V
/**********************************************************************
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*
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* f32 to string
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*
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* Copyright (c) 2019-2020 Dario Deledda. All rights reserved.
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* Use of this source code is governed by an MIT license
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* that can be found in the LICENSE file.
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*
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* This file contains the f64 to string functions
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*
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* These functions are based on the work of:
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* Publication:PLDI 2018: Proceedings of the 39th ACM SIGPLAN
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* Conference on Programming Language Design and ImplementationJune 2018
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* Pages 270–282 https://doi.org/10.1145/3192366.3192369
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*
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* inspired by the Go version here:
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* https://github.com/cespare/ryu/tree/ba56a33f39e3bbbfa409095d0f9ae168a595feea
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*
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**********************************************************************/
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module ftoa
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struct Uint128 {
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mut:
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lo u64 = u64(0)
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hi u64 = u64(0)
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}
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// dec64 is a floating decimal type representing m * 10^e.
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struct Dec64 {
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mut:
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m u64 = u64(0)
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e int = 0
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}
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// support union for convert f64 to u64
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union Uf64 {
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mut:
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f f64 = f64(0)
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u u64
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}
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/******************************************************************************
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*
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* Conversion Functions
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*
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******************************************************************************/
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const(
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mantbits64 = u32(52)
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expbits64 = u32(11)
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bias64 = u32(1023) // f64 exponent bias
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maxexp64 = 2047
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)
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fn (d Dec64) get_string_64(neg bool, n_digit int) string {
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mut out := d.m
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mut out_len := decimal_len_64(out)
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mut buf := [byte(0)].repeat(out_len + 6 + 1 +1) // sign + mant_len + . + e + e_sign + exp_len(2) + \0
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mut i := 0
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if n_digit > 0 && out_len > n_digit {
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out_len = n_digit+1
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}
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if neg {
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buf[i]=`-`
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i++
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}
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mut disp := 0
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if out_len <= 1 {
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disp = 1
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}
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y := i + out_len
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mut x := 0
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for x < (out_len-disp-1) {
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buf[y - x] = `0` + byte(out%10)
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out /= 10
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i++
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x++
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}
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if out_len >= 1 {
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buf[y - x] = `.`
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x++
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i++
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}
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if y-x >= 0 {
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buf[y - x] = `0` + byte(out%10)
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i++
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}
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/*
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x=0
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for x<buf.len {
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C.printf("d:%c\n",buf[x])
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x++
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}
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C.printf("\n")
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*/
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buf[i]=`e`
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i++
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mut exp := d.e + out_len - 1
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if exp < 0 {
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buf[i]=`-`
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i++
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exp = -exp
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} else {
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buf[i]=`+`
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i++
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}
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// Always print two digits to match strconv's formatting.
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/* d1 := exp % 10
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d0 := exp / 10
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buf[i]=`0` + byte(d0)
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i++
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buf[i]=`0` + byte(d1)
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i++
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buf[i]=0
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*/
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// Always print at least two digits to match strconv's formatting.
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d2 := exp % 10
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exp /= 10
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d1 := exp % 10
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d0 := exp / 10
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if d0 > 0 {
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buf[i]=`0` + byte(d0)
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i++
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}
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buf[i]=`0` + byte(d1)
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i++
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buf[i]=`0` + byte(d2)
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i++
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buf[i]=0
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/*
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x=0
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for x<buf.len {
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C.printf("d:%c\n",buf[x])
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x++
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}
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*/
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return tos(byteptr(&buf[0]), i)
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}
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fn f64_to_decimal_exact_int(i_mant u64, exp u64) (Dec64, bool) {
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mut d := Dec64{}
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e := exp - bias64
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if e > mantbits64 {
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return d, false
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}
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shift := mantbits64 - e
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mant := i_mant | 0x0010_0000_0000_0000 // implicit 1
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//mant := i_mant | (1 << mantbits64) // implicit 1
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d.m = mant >> shift
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if (d.m << shift) != mant {
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return d, false
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}
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for (d.m % 10) == 0 {
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d.m /= 10
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d.e++
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}
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return d, true
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}
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fn f64_to_decimal(mant u64, exp u64) Dec64 {
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mut e2 := 0
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mut m2 := u64(0)
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if exp == 0 {
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// We subtract 2 so that the bounds computation has
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// 2 additional bits.
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e2 = 1 - bias64 - mantbits64 - 2
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m2 = mant
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} else {
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e2 = int(exp) - bias64 - mantbits64 - 2
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m2 = (u64(1)<<mantbits64) | mant
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}
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even := (m2 & 1) == 0
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accept_bounds := even
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// Step 2: Determine the interval of valid decimal representations.
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mv := u64(4 * m2)
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mm_shift := bool_to_u64(mant != 0 || exp <= 1)
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// Step 3: Convert to a decimal power base uing 128-bit arithmetic.
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mut vr := u64(0)
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mut vp := u64(0)
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mut vm := u64(0)
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mut e10 := 0
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mut vm_is_trailing_zeros := false
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mut vr_is_trailing_zeros := false
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if e2 >= 0 {
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// This expression is slightly faster than max(0, log10Pow2(e2) - 1).
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q := log10_pow2(e2) - bool_to_u32(e2 > 3)
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e10 = int(q)
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k := pow5_inv_num_bits_64 + pow5_bits(int(q)) - 1
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i := -e2 + int(q) + k
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mul := pow5_inv_split_64[q]
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vr = mul_shift_64(u64(4) * m2 , mul, i)
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vp = mul_shift_64(u64(4) * m2 + u64(2) , mul, i)
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vm = mul_shift_64(u64(4) * m2 - u64(1) - mm_shift, mul, i)
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if q <= 21 {
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// This should use q <= 22, but I think 21 is also safe.
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// Smaller values may still be safe, but it's more
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// difficult to reason about them. Only one of mp, mv,
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// and mm can be a multiple of 5, if any.
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if mv%5 == 0 {
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vr_is_trailing_zeros = multiple_of_power_of_five_64(mv, q)
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} else if accept_bounds {
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// Same as min(e2 + (^mm & 1), pow5Factor64(mm)) >= q
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// <=> e2 + (^mm & 1) >= q && pow5Factor64(mm) >= q
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// <=> true && pow5Factor64(mm) >= q, since e2 >= q.
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vm_is_trailing_zeros = multiple_of_power_of_five_64(mv-1-mm_shift, q)
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} else if multiple_of_power_of_five_64(mv+2, q) {
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vp--
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}
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}
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} else {
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// This expression is slightly faster than max(0, log10Pow5(-e2) - 1).
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q := log10_pow5(-e2) - bool_to_u32(-e2 > 1)
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e10 = int(q) + e2
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i := -e2 - int(q)
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k := pow5_bits(i) - pow5_num_bits_64
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mut j := int(q) - k
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mul := pow5_split_64[i]
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vr = mul_shift_64(u64(4) * m2 , mul, j)
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vp = mul_shift_64(u64(4) * m2 + u64(2) , mul, j)
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vm = mul_shift_64(u64(4) * m2 - u64(1) - mm_shift, mul, j)
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if q <= 1 {
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// {vr,vp,vm} is trailing zeros if {mv,mp,mm} has at least q trailing 0 bits.
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// mv = 4 * m2, so it always has at least two trailing 0 bits.
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vr_is_trailing_zeros = true
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if accept_bounds {
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// mm = mv - 1 - mmShift, so it has 1 trailing 0 bit iff mmShift == 1.
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vm_is_trailing_zeros = (mm_shift == 1)
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} else {
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// mp = mv + 2, so it always has at least one trailing 0 bit.
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vp--
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}
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} else if q < 63 { // TODO(ulfjack/cespare): Use a tighter bound here.
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// We need to compute min(ntz(mv), pow5Factor64(mv) - e2) >= q - 1
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// <=> ntz(mv) >= q - 1 && pow5Factor64(mv) - e2 >= q - 1
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// <=> ntz(mv) >= q - 1 (e2 is negative and -e2 >= q)
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// <=> (mv & ((1 << (q - 1)) - 1)) == 0
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// We also need to make sure that the left shift does not overflow.
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vr_is_trailing_zeros = multiple_of_power_of_two_64(mv, q - 1)
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}
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}
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// Step 4: Find the shortest decimal representation
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// in the interval of valid representations.
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mut removed := 0
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mut last_removed_digit := byte(0)
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mut out := u64(0)
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// On average, we remove ~2 digits.
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if vm_is_trailing_zeros || vr_is_trailing_zeros {
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// General case, which happens rarely (~0.7%).
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for {
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vp_div_10 := vp / 10
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vm_div_10 := vm / 10
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if vp_div_10 <= vm_div_10 {
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break
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}
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vm_mod_10 := vm % 10
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vr_div_10 := vr / 10
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vr_mod_10 := vr % 10
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vm_is_trailing_zeros = vm_is_trailing_zeros && vm_mod_10 == 0
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vr_is_trailing_zeros = vr_is_trailing_zeros && (last_removed_digit == 0)
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last_removed_digit = byte(vr_mod_10)
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vr = vr_div_10
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vp = vp_div_10
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vm = vm_div_10
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removed++
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}
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if vm_is_trailing_zeros {
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for {
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vm_div_10 := vm / 10
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vm_mod_10 := vm % 10
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if vm_mod_10 != 0 {
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break
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}
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vp_div_10 := vp / 10
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vr_div_10 := vr / 10
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vr_mod_10 := vr % 10
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vr_is_trailing_zeros = vr_is_trailing_zeros && (last_removed_digit == 0)
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last_removed_digit = byte(vr_mod_10)
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vr = vr_div_10
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vp = vp_div_10
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vm = vm_div_10
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removed++
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}
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}
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if vr_is_trailing_zeros && (last_removed_digit == 5) && (vr % 2) == 0 {
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// Round even if the exact number is .....50..0.
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last_removed_digit = 4
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}
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out = vr
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// We need to take vr + 1 if vr is outside bounds
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// or we need to round up.
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if (vr == vm && (!accept_bounds || !vm_is_trailing_zeros)) || last_removed_digit >= 5 {
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out++
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}
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} else {
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// Specialized for the common case (~99.3%).
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// Percentages below are relative to this.
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mut round_up := false
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for vp / 100 > vm / 100 {
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// Optimization: remove two digits at a time (~86.2%).
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round_up = (vr % 100) >= 50
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vr /= 100
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vp /= 100
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vm /= 100
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removed += 2
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}
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// Loop iterations below (approximately), without optimization above:
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// 0: 0.03%, 1: 13.8%, 2: 70.6%, 3: 14.0%, 4: 1.40%, 5: 0.14%, 6+: 0.02%
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// Loop iterations below (approximately), with optimization above:
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// 0: 70.6%, 1: 27.8%, 2: 1.40%, 3: 0.14%, 4+: 0.02%
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for vp / 10 > vm / 10 {
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round_up = (vr % 10) >= 5
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vr /= 10
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vp /= 10
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vm /= 10
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removed++
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}
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// We need to take vr + 1 if vr is outside bounds
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// or we need to round up.
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out = vr + bool_to_u64(vr == vm || round_up)
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}
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return Dec64{m: out, e: e10 + removed}
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}
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// f64_to_str return a string in scientific notation with max n_digit after the dot
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pub fn f64_to_str(f f64, n_digit int) string {
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mut u1 := Uf64{}
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u1.f = f
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u := u1.u
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neg := (u>>(mantbits64+expbits64)) != 0
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mant := u & ((u64(1)<<mantbits64) - u64(1))
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exp := (u >> mantbits64) & ((u64(1)<<expbits64) - u64(1))
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//println("s:${neg} mant:${mant} exp:${exp} float:${f} byte:${u1.u:016lx}")
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// Exit early for easy cases.
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if (exp == maxexp64) || (exp == 0 && mant == 0) {
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return get_string_special(neg, exp == 0, mant == 0)
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}
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mut d, ok := f64_to_decimal_exact_int(mant, exp)
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if !ok {
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//println("to_decimal")
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d = f64_to_decimal(mant, exp)
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}
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//println("${d.m} ${d.e}")
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return d.get_string_64(neg, n_digit)
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}
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