optimize: convert using continued fractions fundamental recurrence formulas (qax-os#2220)

Author: shcabin

lib.go

@@ -856,28 +856,53 @@ func bstrMarshal(s string) (result string) {
 	return result
 }
 
-// newRat converts decimals to rational fractions with the required precision.
-func newRat(n float64, iterations int64, prec float64) *big.Rat {
-	x := int64(math.Floor(n))
-	y := n - float64(x)
-	rat := continuedFraction(y, 1, iterations, prec)
-	return rat.Add(rat, new(big.Rat).SetInt64(x))
-}
-
-// continuedFraction returns rational from decimal with the continued fraction
-// algorithm.
-func continuedFraction(n float64, i int64, limit int64, prec float64) *big.Rat {
-	if i >= limit || n <= prec {
-		return big.NewRat(0, 1)
-	}
-	inverted := 1 / n
-	y := int64(math.Floor(inverted))
-	x := inverted - float64(y)
-	ratY := new(big.Rat).SetInt64(y)
-	ratNext := continuedFraction(x, i+1, limit, prec)
-	res := ratY.Add(ratY, ratNext)
-	res = res.Inv(res)
-	return res
+// floatToFraction converts a float number to a fraction string representation
+// with specified placeholder widths for numerator and denominator.
+func floatToFraction(x float64, numeratorPlaceHolder, denominatorPlaceHolder int) string {
+	if denominatorPlaceHolder <= 0 {
+		return ""
+	}
+	num, den := floatToFracUseContinuedFraction(x, int64(math.Pow10(denominatorPlaceHolder)))
+	if num == 0 {
+		return strings.Repeat(" ", numeratorPlaceHolder+denominatorPlaceHolder+1)
+	}
+	numStr := strconv.FormatInt(num, 10)
+	denStr := strconv.FormatInt(den, 10)
+	numeratorPlaceHolder = max(numeratorPlaceHolder-len(numStr), 0)
+	denominatorPlaceHolder = max(denominatorPlaceHolder-len(denStr), 0)
+	return fmt.Sprintf("%s%s/%s%s", strings.Repeat(" ", numeratorPlaceHolder), numStr, denStr, strings.Repeat(" ", denominatorPlaceHolder))
+}
+
+// floatToFracUseContinuedFraction implement convert a floating-point decimal
+// to a fraction using continued fractions and recurrence relations.
+func floatToFracUseContinuedFraction(r float64, denominatorLimit int64) (num, den int64) {
+	p_1 := int64(1) // LaTex: p_{-1}
+	q_1 := int64(0) // LaTex: q_{-1}
+	p_2 := int64(0) // LaTex: p_{-2}
+	q_2 := int64(1) // LaTex: q_{-2}
+	var lasta, lastb int64
+	var curra, currb int64
+	for k := 0; ; k++ {
+		// a_{k} = \lfloor r_{k} \rfloor
+		a := int64(math.Floor(r))
+		// Fundamental recurrence formulas:  p_{k} = a_{k} \cdot p_{k-1} + p_{k-2}
+		curra, currb = a*p_1+p_2, a*q_1+q_2
+		p_2 = p_1
+		q_2 = q_1
+		p_1 = curra
+		q_1 = currb
+		frac := r - float64(a)
+		if q_1 >= denominatorLimit {
+			return lasta, lastb
+		}
+		if math.Abs(frac) < 1e-12 {
+			// use safe floating-point number comparison. If the input(r) is a real number, here is 0.
+			return curra, currb
+		}
+		lasta, lastb = curra, currb
+		// r_{k+1} = \frac{1}{r_{k} - a_{k}}
+		r = 1.0 / frac
+	}
 }
 
 // assignFieldValue assigns the value from an immutable reflect.Value to a

lib_test.go

@@ -6,6 +6,7 @@ import (
 	"encoding/xml"
 	"fmt"
 	"io"
+	"math"
 	"os"
 	"strconv"
 	"strings"
@@ -412,3 +413,10 @@ func TestUnzipToTemp(t *testing.T) {
 	_, err = f.unzipToTemp(z.File[0])
 	assert.EqualError(t, err, "EOF")
 }
+
+func TestFloat2Frac(t *testing.T) {
+	assert.Empty(t, floatToFraction(0.19, 0, 0))
+	assert.Equal(t, "1/5", floatToFraction(0.19, 1, 1))
+	assert.Equal(t, "9999/10000", strings.Trim(floatToFraction(0.9999, 10, 10), " "))
+	assert.Equal(t, "954888175898973913/351283728530932463", floatToFraction(math.E, 1, 18))
+}

numfmt.go

@@ -5439,27 +5439,8 @@ func (nf *numberFormat) printNumberLiteral(text string) string {
 // negative numeric.
 func (nf *numberFormat) fractionHandler(frac float64, token nfp.Token, numeratorPlaceHolder int) string {
 	var rat, result string
-	var lastRat *big.Rat
 	if token.TType == nfp.TokenTypeDigitalPlaceHolder {
-		denominatorPlaceHolder := len(token.TValue)
-		for i := range 5000 {
-			if r := newRat(frac, int64(i), 0); len(r.Denom().String()) <= denominatorPlaceHolder {
-				lastRat = r // record the last valid ratio, and delay conversion to string
-				continue
-			}
-			break
-		}
-		if lastRat != nil {
-			if lastRat.Num().Int64() == 0 {
-				rat = strings.Repeat(" ", numeratorPlaceHolder+denominatorPlaceHolder+1)
-			} else {
-				num := lastRat.Num().String()
-				den := lastRat.Denom().String()
-				numeratorPlaceHolder = max(numeratorPlaceHolder-len(num), 0)
-				denominatorPlaceHolder = max(denominatorPlaceHolder-len(den), 0)
-				rat = fmt.Sprintf("%s%s/%s%s", strings.Repeat(" ", numeratorPlaceHolder), num, den, strings.Repeat(" ", denominatorPlaceHolder))
-			}
-		}
+		rat = floatToFraction(frac, numeratorPlaceHolder, len(token.TValue))
 		result += rat
 	}
 	if token.TType == nfp.TokenTypeDenominator {
@@ -5539,8 +5520,9 @@ func (nf *numberFormat) numberHandler() string {
 		intLen, fracLen = nf.getNumberPartLen()
 		result          string
 	)
-	if isNum, precision, decimal := isNumeric(nf.value); isNum {
-		if precision > 15 && intLen+fracLen > 15 && !nf.useScientificNotation {
+	if intLen+fracLen > 15 && !nf.useScientificNotation {
+		isNum, precision, decimal := isNumeric(nf.value)
+		if isNum && precision > 15 {
 			return nf.printNumberLiteral(nf.printBigNumber(decimal, fracLen))
 		}
 	}

numfmt_test.go

@@ -4350,8 +4350,7 @@ func BenchmarkNumFmtPlaceHolder(b *testing.B) {
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {
 		for _, item := range items {
-			result := format(item[0], item[1], false, CellTypeNumber, nil)
-			_ = result
+			_ = format(item[0], item[1], false, CellTypeNumber, nil)
 		}
 	}
 }