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// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This file is adapted from https://github.com/robpike/ivy/blob/master/value/loop.go.
package apd
import (
"math"
"github.com/pkg/errors"
)
type loop struct {
c *Context
name string // The name of the function we are evaluating.
i uint64 // Loop count.
precision int32
maxIterations uint64 // When to give up.
arg *Decimal // original argument to function; only used for diagnostic.
prevZ *Decimal // Result from the previous iteration.
delta *Decimal // |Change| from previous iteration.
}
const digitsToBitsRatio = math.Ln10 / math.Ln2
// newLoop returns a new loop checker. Arguments:
// - name: name of the function being calculated (for error messages)
// - arg: argument to the function (for error messages)
// - precision: desired precision; the loop ends when consecutive estimates
// differ less than the desired precision. Note that typically
// the inner computations in an iteration need higher precision,
// so this is normally lower than the precision in the context.
// - maxItersPerDigit: after this many iterations per digit of precision, the
// loop ends in error.
func (c *Context) newLoop(name string, arg *Decimal, precision uint32, maxItersPerDigit int) *loop {
return &loop{
c: c,
name: name,
arg: new(Decimal).Set(arg),
precision: int32(precision),
maxIterations: 10 + uint64(maxItersPerDigit*int(precision)),
prevZ: new(Decimal),
delta: new(Decimal),
}
}
// done reports whether the loop is done. If it does not converge
// after the maximum number of iterations, it returns an error.
func (l *loop) done(z *Decimal) (bool, error) {
if _, err := l.c.Sub(l.delta, l.prevZ, z); err != nil {
return false, err
}
sign := l.delta.Sign()
if sign == 0 {
return true, nil
}
if sign < 0 {
// Convergence can oscillate when the calculation is nearly
// done and we're running out of bits. This stops that.
// See next comment.
l.delta.Neg(l.delta)
}
// We stop if the delta is smaller than a change of 1 in the
// (l.precision)-th digit of z. Examples:
//
// p = 4
// z = 12345.678 = 12345678 * 10^-3
// eps = 10.000 = 10^(-4+8-3)
//
// p = 3
// z = 0.001234 = 1234 * 10^-6
// eps = 0.00001 = 10^(-3+4-6)
eps := Decimal{Coeff: *bigOne, Exponent: -l.precision + int32(z.NumDigits()) + z.Exponent}
if l.delta.Cmp(&eps) <= 0 {
return true, nil
}
l.i++
if l.i == l.maxIterations {
return false, errors.Errorf(
"%s %s: did not converge after %d iterations; prev,last result %s,%s delta %s precision: %d",
l.name, l.arg.String(), l.maxIterations, z, l.prevZ, l.delta, l.precision,
)
}
l.prevZ.Set(z)
return false, nil
}