go/vec32/vec.go - buildbot - Git at Google

 // Package vec32 has some basic functions on slices of float32.
 package vec32

 import (
 	"fmt"
 	"math"
 	"sort"
 )

 const (
 	// MissingDataSentinel signifies a missing sample value.
 	//
 	// JSON doesn't support NaN or +/- Inf, so we need a valid float32 to signal
 	// missing data that also has a compact JSON representation.
 	MissingDataSentinel float32 = 1e32

 	// maxStdDevRatio is the largest magnitude value that StdDevRatio can return.
 	maxStdDevRatio = 1000
 )

 // New creates a new []float32 of the given size pre-populated
 // with MISSING_DATA_SENTINEL.
 func New(size int) []float32 {
 	ret := make([]float32, size)
 	for i := range ret {
 		ret[i] = MissingDataSentinel
 	}
 	return ret
 }

 // MeanAndStdDev returns the mean, stddev, and if an error occurred while doing
 // the calculation. MISSING_DATA_SENTINELs are ignored.
 func MeanAndStdDev(a []float32) (float32, float32, error) {
 	count := 0
 	sum := float32(0.0)
 	for _, x := range a {
 		if x != MissingDataSentinel {
 			count += 1
 			sum += x
 		}
 	}

 	if count == 0 {
 		return 0, 0, fmt.Errorf("Slice of length zero.")
 	}
 	mean := sum / float32(count)

 	vr := float32(0.0)
 	for _, x := range a {
 		if x != MissingDataSentinel {
 			vr += (x - mean) * (x - mean)
 		}
 	}
 	stddev := float32(math.Sqrt(float64(vr / float32(count))))

 	return mean, stddev, nil
 }

 // RemoveMissingDataSentinel returns a new slice with all the values that are
 // equal to the MissingDataSentinel removed.
 func RemoveMissingDataSentinel(arr []float32) []float32 {
 	ret := make([]float32, 0, len(arr))
 	for _, x := range arr {
 		if x != MissingDataSentinel {
 			ret = append(ret, x)
 		}
 	}
 	return ret
 }

 type float32Slice []float32

 func (p float32Slice) Len() int           { return len(p) }
 func (p float32Slice) Less(i, j int) bool { return p[i] < p[j] }
 func (p float32Slice) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }

 // TwoSidedStdDev returns the median, and the stddev of all the points below and
 // above the median respectively.
 //
 // That is, the vector is sorted, the median found, and then the stddev of all
 // the points below the median are returned, along with the stddev of all the
 // points above the median.
 //
 // This is useful because performance measurements are inherintly asymmetric. A
 // benchmark can always run 2x slower, or 10x slower, there's no upper bound. On
 // the other hand a performance metric can only run 100% faster, i.e. have a
 // value of 0. This implies that the distribution of samples from a benchmark
 // are skewed.
 //
 // MISSING_DATA_SENTINELs are ignored.
 //
 // The median is chosen as the midpoint instead of the mean because that ensures
 // that both sides have the same number of points (+/- 1).
 func TwoSidedStdDev(arr []float32) (float32, float32, float32, error) {
 	values := RemoveMissingDataSentinel((arr))
 	count := len(values)
 	if count < 4 {
 		return 0, 0, 0, fmt.Errorf("Insufficient number of points, at least 4 are needed: %d", len(values))
 	}
 	sort.Sort(float32Slice(values))

 	mid := len(values) / 2
 	var median float32
 	if len(values)%2 == 1 {
 		median = values[mid]
 	} else {
 		median = (values[mid-1] + values[mid]) / 2
 	}
 	return median, StdDev(values[:mid], median), StdDev(values[mid:], median), nil
 }

 // StdDevRatio returns the number of standard deviations that the last point in
 // arr is away from the median of the remaining points in arr.
 //
 // Does not presume that arr is sorted.
 //
 // In detail, this calculates a measure of how likely the last point in the
 // slice is to come from the population, as represented by the remaining
 // elements of the slice.
 //
 // We calculate TwoSidedStdDev:
 //
 //   median, lower, upper = TwoSidedStdDev(values)
 //
 // Then calculate the std dev ratio (d):
 //
 //   d = (x-median)/[lower|upper]
 //
 // The value of d is the difference between the last point in arr (x) and the
 // median, divided by the lower or upper standard deviation. If x > median then
 // we divide by upper, else we divide by lower.
 //
 // This d is a unitless dimension, the number of standard deviations the trybot
 // value is either above or below the median.
 //
 // Returns the stddevRatio, median, lower, upper, and an error if one occurred.
 func StdDevRatio(arr []float32) (float32, float32, float32, float32, error) {
 	length := len(arr)
 	if length < 5 {
 		// Last point is 'x' and TwoSidedStdDev requires four points.
 		return 0, 0, 0, 0, fmt.Errorf("Insufficient number of points.")
 	}
 	x := arr[len(arr)-1]
 	if x == MissingDataSentinel {
 		return 0, 0, 0, 0, fmt.Errorf("Can't calculate StdDevRatio for MissingDataSentinel.")
 	}
 	median, lower, upper, err := TwoSidedStdDev(arr[:length-1])
 	if err != nil {
 		return 0, median, lower, upper, err
 	}
 	var stddevRatio float32
 	if x < median {
 		stddevRatio = (median - x) / lower
 		if stddevRatio > maxStdDevRatio {
 			stddevRatio = maxStdDevRatio
 		}
 		stddevRatio *= -1
 	} else {
 		stddevRatio = (x - median) / upper
 		if stddevRatio > maxStdDevRatio {
 			stddevRatio = maxStdDevRatio
 		}
 	}

 	if math.IsNaN(float64(stddevRatio)) {
 		return 0, 0, 0, 0, fmt.Errorf("Got NaN calculating StdDevRatio")
 	}

 	return stddevRatio, median, lower, upper, nil
 }

 // ScaleBy divides each non-sentinel value in the slice by 'b', converting
 // resulting NaNs and Infs into sentinel values.
 func ScaleBy(a []float32, b float32) {
 	for i, x := range a {
 		if x != MissingDataSentinel {
 			scaled := a[i] / b
 			if math.IsNaN(float64(scaled)) || math.IsInf(float64(scaled), 0) {
 				a[i] = MissingDataSentinel
 			} else {
 				a[i] = scaled
 			}
 		}
 	}
 }

 // Norm normalizes the slice to a mean of 0 and a standard deviation of 1.0.
 // The minStdDev is the minimum standard deviation that is normalized. Slices
 // with a standard deviation less than that are not normalized for variance.
 func Norm(a []float32, minStdDev float32) {
 	mean, stddev, err := MeanAndStdDev(a)
 	if err != nil {
 		return
 	}
 	// Normalize the data to a mean of 0 and standard deviation of 1.0.
 	for i, x := range a {
 		if x != MissingDataSentinel {
 			newX := x - mean
 			if stddev > minStdDev {
 				newX = newX / stddev
 			}
 			a[i] = newX
 		}
 	}
 }

 // Fill in non-sentinel values with nearby points.
 //
 // Sentinel values are filled with points later in the array, except for the
 // end of the array where we can't do that, so we fill those points in
 // using the first non sentinel found when searching backwards from the end.
 //
 // So
 //    [1e32, 1e32,   2, 3, 1e32, 5]
 // becomes
 //    [2,    2,      2, 3, 5,    5]
 //
 // and
 //    [3, 1e32, 5, 1e32, 1e32]
 // becomes
 //    [3, 5,    5, 5,    5]
 //
 //
 // Note that a vector filled with all sentinels will be filled with 0s.
 func Fill(a []float32) {
 	// Find the first non-sentinel data point.
 	last := float32(0.0)
 	for i := len(a) - 1; i >= 0; i-- {
 		if a[i] != MissingDataSentinel {
 			last = a[i]
 			break
 		}
 	}
 	// Now fill.
 	for i := len(a) - 1; i >= 0; i-- {
 		if a[i] == MissingDataSentinel {
 			a[i] = last
 		} else {
 			last = a[i]
 		}
 	}
 }

 // FillAt returns the value at the given index of a vector, using non-sentinel
 // values with nearby points if the original is MISSING_DATA_SENTINEL.
 //
 // Note that the input vector is unchanged.
 //
 // Returns non-nil error if the given index is out of bounds.
 func FillAt(a []float32, i int) (float32, error) {
 	l := len(a)
 	if i < 0 || i >= l {
 		return 0, fmt.Errorf("FillAt index %d out of bound %d.\n", i, l)
 	}
 	b := make([]float32, l, l)
 	copy(b, a)
 	Fill(b)
 	return b[i], nil
 }

 // Dup a slice of float32.
 func Dup(a []float32) []float32 {
 	ret := make([]float32, len(a), len(a))
 	copy(ret, a)
 	return ret
 }

 // Mean calculates and returns the Mean value of the given []float32.
 //
 // Returns 0 for an array with no non-MISSING_DATA_SENTINEL values.
 func Mean(xs []float32) float32 {
 	total := float32(0.0)
 	n := 0
 	for _, v := range xs {
 		if v != MissingDataSentinel {
 			total += v
 			n++
 		}
 	}
 	if n == 0 {
 		return total
 	}
 	return total / float32(n)
 }

 // MeanMissing calculates and returns the Mean value of the given []float32.
 //
 // Returns MISSING_DATA_SENTINEL for an array with all MISSING_DATA_SENTINEL values.
 func MeanMissing(xs []float32) float32 {
 	total := float32(0.0)
 	n := 0
 	for _, v := range xs {
 		if v != MissingDataSentinel {
 			total += v
 			n++
 		}
 	}
 	if n == 0 {
 		return MissingDataSentinel
 	}
 	return total / float32(n)
 }

 // FillMeanMissing fills the slice with the mean of all the values in the slice
 // using MeanMissing.
 func FillMeanMissing(a []float32) {
 	value := MeanMissing(a)
 	// Now fill.
 	for i := range a {
 		a[i] = value
 	}
 }

 // FillStdDev fills the slice with the Standard Deviation of the values in the slice.
 //
 // If slice is filled with only MISSING_DATA_SENTINEL then the slice will be
 // filled with MISSING_DATA_SENTINEL.
 func FillStdDev(a []float32) {
 	_, stddev, err := MeanAndStdDev(a)
 	if err != nil {
 		stddev = MissingDataSentinel
 	}
 	// Now fill.
 	for i := range a {
 		a[i] = stddev
 	}
 }

 // FillCov fills the slice with the Coefficient of Variation of the values in the slice.
 //
 // If the mean is 0 or the slice is filled with only MISSING_DATA_SENTINEL then
 // the slice will be filled with MISSING_DATA_SENTINEL.
 func FillCov(a []float32) {
 	mean, stddev, err := MeanAndStdDev(a)
 	cov := MissingDataSentinel
 	if err == nil {
 		cov = stddev / mean
 	}
 	if math.IsNaN(float64(cov)) || math.IsInf(float64(cov), 0) {
 		cov = MissingDataSentinel
 	}
 	// Now fill.
 	for i := range a {
 		a[i] = cov
 	}
 }

 // ssen calculates and returns the sum squared error from the given base of []float32.
 //
 // Returns 0 for an array with no non-MISSING_DATA_SENTINEL values.
 func ssen(xs []float32, base float32) (float32, int) {
 	total := float32(0.0)
 	n := 0
 	for _, v := range xs {
 		if v != MissingDataSentinel {
 			n++
 			total += (v - base) * (v - base)
 		}
 	}
 	return total, n
 }

 // SSE calculates and returns the sum squared error from the given base of []float32.
 //
 // Returns 0 for an array with no non-MISSING_DATA_SENTINEL values.
 func SSE(xs []float32, base float32) float32 {
 	total, _ := ssen(xs, base)
 	return total
 }

 // StdDev returns the sample standard deviation.
 func StdDev(xs []float32, base float32) float32 {
 	n := len(xs)
 	if n < 2 {
 		return 0
 	}
 	sse, n := ssen(xs, base)
 	return float32(math.Sqrt(float64(sse / float32(n-1))))
 }

 // FillStep fills the slice with the step function value, i.e.  the ratio of
 // the ave of the first half of the trace values divided by the ave of the
 // second half of the trace values.
 //
 // If the second mean is 0 or the slice is filled with only MISSING_DATA_SENTINEL then
 // the slice will be filled with MISSING_DATA_SENTINEL.
 func FillStep(a []float32) {
 	mid := len(a) / 2

 	step := MissingDataSentinel
 	meanFirst := MeanMissing(a[:mid])
 	meanLast := MeanMissing(a[mid:])
 	if meanLast != MissingDataSentinel && meanFirst != MissingDataSentinel {
 		step = meanFirst / meanLast
 	}
 	if math.IsNaN(float64(step)) || math.IsInf(float64(step), 0) {
 		step = MissingDataSentinel
 	}
 	// Now fill.
 	for i := range a {
 		a[i] = step
 	}
 }
	// Package vec32 has some basic functions on slices of float32.
	package vec32

	import (
	"fmt"
	"math"
	"sort"
	)

	const (
	// MissingDataSentinel signifies a missing sample value.
	//
	// JSON doesn't support NaN or +/- Inf, so we need a valid float32 to signal
	// missing data that also has a compact JSON representation.
	MissingDataSentinel float32 = 1e32

	// maxStdDevRatio is the largest magnitude value that StdDevRatio can return.
	maxStdDevRatio = 1000
	)

	// New creates a new []float32 of the given size pre-populated
	// with MISSING_DATA_SENTINEL.
	func New(size int) []float32 {
	ret := make([]float32, size)
	for i := range ret {
	ret[i] = MissingDataSentinel
	}
	return ret
	}

	// MeanAndStdDev returns the mean, stddev, and if an error occurred while doing
	// the calculation. MISSING_DATA_SENTINELs are ignored.
	func MeanAndStdDev(a []float32) (float32, float32, error) {
	count := 0
	sum := float32(0.0)
	for _, x := range a {
	if x != MissingDataSentinel {
	count += 1
	sum += x
	}
	}

	if count == 0 {
	return 0, 0, fmt.Errorf("Slice of length zero.")
	}
	mean := sum / float32(count)

	vr := float32(0.0)
	for _, x := range a {
	if x != MissingDataSentinel {
	vr += (x - mean) * (x - mean)
	}
	}
	stddev := float32(math.Sqrt(float64(vr / float32(count))))

	return mean, stddev, nil
	}

	// RemoveMissingDataSentinel returns a new slice with all the values that are
	// equal to the MissingDataSentinel removed.
	func RemoveMissingDataSentinel(arr []float32) []float32 {
	ret := make([]float32, 0, len(arr))
	for _, x := range arr {
	if x != MissingDataSentinel {
	ret = append(ret, x)
	}
	}
	return ret
	}

	type float32Slice []float32

	func (p float32Slice) Len() int { return len(p) }
	func (p float32Slice) Less(i, j int) bool { return p[i] < p[j] }
	func (p float32Slice) Swap(i, j int) { p[i], p[j] = p[j], p[i] }

	// TwoSidedStdDev returns the median, and the stddev of all the points below and
	// above the median respectively.
	//
	// That is, the vector is sorted, the median found, and then the stddev of all
	// the points below the median are returned, along with the stddev of all the
	// points above the median.
	//
	// This is useful because performance measurements are inherintly asymmetric. A
	// benchmark can always run 2x slower, or 10x slower, there's no upper bound. On
	// the other hand a performance metric can only run 100% faster, i.e. have a
	// value of 0. This implies that the distribution of samples from a benchmark
	// are skewed.
	//
	// MISSING_DATA_SENTINELs are ignored.
	//
	// The median is chosen as the midpoint instead of the mean because that ensures
	// that both sides have the same number of points (+/- 1).
	func TwoSidedStdDev(arr []float32) (float32, float32, float32, error) {
	values := RemoveMissingDataSentinel((arr))
	count := len(values)
	if count < 4 {
	return 0, 0, 0, fmt.Errorf("Insufficient number of points, at least 4 are needed: %d", len(values))
	}
	sort.Sort(float32Slice(values))

	mid := len(values) / 2
	var median float32
	if len(values)%2 == 1 {
	median = values[mid]
	} else {
	median = (values[mid-1] + values[mid]) / 2
	}
	return median, StdDev(values[:mid], median), StdDev(values[mid:], median), nil
	}

	// StdDevRatio returns the number of standard deviations that the last point in
	// arr is away from the median of the remaining points in arr.
	//
	// Does not presume that arr is sorted.
	//
	// In detail, this calculates a measure of how likely the last point in the
	// slice is to come from the population, as represented by the remaining
	// elements of the slice.
	//
	// We calculate TwoSidedStdDev:
	//
	// median, lower, upper = TwoSidedStdDev(values)
	//
	// Then calculate the std dev ratio (d):
	//
	// d = (x-median)/[lower\|upper]
	//
	// The value of d is the difference between the last point in arr (x) and the
	// median, divided by the lower or upper standard deviation. If x > median then
	// we divide by upper, else we divide by lower.
	//
	// This d is a unitless dimension, the number of standard deviations the trybot
	// value is either above or below the median.
	//
	// Returns the stddevRatio, median, lower, upper, and an error if one occurred.
	func StdDevRatio(arr []float32) (float32, float32, float32, float32, error) {
	length := len(arr)
	if length < 5 {
	// Last point is 'x' and TwoSidedStdDev requires four points.
	return 0, 0, 0, 0, fmt.Errorf("Insufficient number of points.")
	}
	x := arr[len(arr)-1]
	if x == MissingDataSentinel {
	return 0, 0, 0, 0, fmt.Errorf("Can't calculate StdDevRatio for MissingDataSentinel.")
	}
	median, lower, upper, err := TwoSidedStdDev(arr[:length-1])
	if err != nil {
	return 0, median, lower, upper, err
	}
	var stddevRatio float32
	if x < median {
	stddevRatio = (median - x) / lower
	if stddevRatio > maxStdDevRatio {
	stddevRatio = maxStdDevRatio
	}
	stddevRatio *= -1
	} else {
	stddevRatio = (x - median) / upper
	if stddevRatio > maxStdDevRatio {
	stddevRatio = maxStdDevRatio
	}
	}

	if math.IsNaN(float64(stddevRatio)) {
	return 0, 0, 0, 0, fmt.Errorf("Got NaN calculating StdDevRatio")
	}

	return stddevRatio, median, lower, upper, nil
	}

	// ScaleBy divides each non-sentinel value in the slice by 'b', converting
	// resulting NaNs and Infs into sentinel values.
	func ScaleBy(a []float32, b float32) {
	for i, x := range a {
	if x != MissingDataSentinel {
	scaled := a[i] / b
	if math.IsNaN(float64(scaled)) \|\| math.IsInf(float64(scaled), 0) {
	a[i] = MissingDataSentinel
	} else {
	a[i] = scaled
	}
	}
	}
	}

	// Norm normalizes the slice to a mean of 0 and a standard deviation of 1.0.
	// The minStdDev is the minimum standard deviation that is normalized. Slices
	// with a standard deviation less than that are not normalized for variance.
	func Norm(a []float32, minStdDev float32) {
	mean, stddev, err := MeanAndStdDev(a)
	if err != nil {
	return
	}
	// Normalize the data to a mean of 0 and standard deviation of 1.0.
	for i, x := range a {
	if x != MissingDataSentinel {
	newX := x - mean
	if stddev > minStdDev {
	newX = newX / stddev
	}
	a[i] = newX
	}
	}
	}

	// Fill in non-sentinel values with nearby points.
	//
	// Sentinel values are filled with points later in the array, except for the
	// end of the array where we can't do that, so we fill those points in
	// using the first non sentinel found when searching backwards from the end.
	//
	// So
	// [1e32, 1e32, 2, 3, 1e32, 5]
	// becomes
	// [2, 2, 2, 3, 5, 5]
	//
	// and
	// [3, 1e32, 5, 1e32, 1e32]
	// becomes
	// [3, 5, 5, 5, 5]
	//
	//
	// Note that a vector filled with all sentinels will be filled with 0s.
	func Fill(a []float32) {
	// Find the first non-sentinel data point.
	last := float32(0.0)
	for i := len(a) - 1; i >= 0; i-- {
	if a[i] != MissingDataSentinel {
	last = a[i]
	break
	}
	}
	// Now fill.
	for i := len(a) - 1; i >= 0; i-- {
	if a[i] == MissingDataSentinel {
	a[i] = last
	} else {
	last = a[i]
	}
	}
	}

	// FillAt returns the value at the given index of a vector, using non-sentinel
	// values with nearby points if the original is MISSING_DATA_SENTINEL.
	//
	// Note that the input vector is unchanged.
	//
	// Returns non-nil error if the given index is out of bounds.
	func FillAt(a []float32, i int) (float32, error) {
	l := len(a)
	if i < 0 \|\| i >= l {
	return 0, fmt.Errorf("FillAt index %d out of bound %d.\n", i, l)
	}
	b := make([]float32, l, l)
	copy(b, a)
	Fill(b)
	return b[i], nil
	}

	// Dup a slice of float32.
	func Dup(a []float32) []float32 {
	ret := make([]float32, len(a), len(a))
	copy(ret, a)
	return ret
	}

	// Mean calculates and returns the Mean value of the given []float32.
	//
	// Returns 0 for an array with no non-MISSING_DATA_SENTINEL values.
	func Mean(xs []float32) float32 {
	total := float32(0.0)
	n := 0
	for _, v := range xs {
	if v != MissingDataSentinel {
	total += v
	n++
	}
	}
	if n == 0 {
	return total
	}
	return total / float32(n)
	}

	// MeanMissing calculates and returns the Mean value of the given []float32.
	//
	// Returns MISSING_DATA_SENTINEL for an array with all MISSING_DATA_SENTINEL values.
	func MeanMissing(xs []float32) float32 {
	total := float32(0.0)
	n := 0
	for _, v := range xs {
	if v != MissingDataSentinel {
	total += v
	n++
	}
	}
	if n == 0 {
	return MissingDataSentinel
	}
	return total / float32(n)
	}

	// FillMeanMissing fills the slice with the mean of all the values in the slice
	// using MeanMissing.
	func FillMeanMissing(a []float32) {
	value := MeanMissing(a)
	// Now fill.
	for i := range a {
	a[i] = value
	}
	}

	// FillStdDev fills the slice with the Standard Deviation of the values in the slice.
	//
	// If slice is filled with only MISSING_DATA_SENTINEL then the slice will be
	// filled with MISSING_DATA_SENTINEL.
	func FillStdDev(a []float32) {
	_, stddev, err := MeanAndStdDev(a)
	if err != nil {
	stddev = MissingDataSentinel
	}
	// Now fill.
	for i := range a {
	a[i] = stddev
	}
	}

	// FillCov fills the slice with the Coefficient of Variation of the values in the slice.
	//
	// If the mean is 0 or the slice is filled with only MISSING_DATA_SENTINEL then
	// the slice will be filled with MISSING_DATA_SENTINEL.
	func FillCov(a []float32) {
	mean, stddev, err := MeanAndStdDev(a)
	cov := MissingDataSentinel
	if err == nil {
	cov = stddev / mean
	}
	if math.IsNaN(float64(cov)) \|\| math.IsInf(float64(cov), 0) {
	cov = MissingDataSentinel
	}
	// Now fill.
	for i := range a {
	a[i] = cov
	}
	}

	// ssen calculates and returns the sum squared error from the given base of []float32.
	//
	// Returns 0 for an array with no non-MISSING_DATA_SENTINEL values.
	func ssen(xs []float32, base float32) (float32, int) {
	total := float32(0.0)
	n := 0
	for _, v := range xs {
	if v != MissingDataSentinel {
	n++
	total += (v - base) * (v - base)
	}
	}
	return total, n
	}

	// SSE calculates and returns the sum squared error from the given base of []float32.
	//
	// Returns 0 for an array with no non-MISSING_DATA_SENTINEL values.
	func SSE(xs []float32, base float32) float32 {
	total, _ := ssen(xs, base)
	return total
	}

	// StdDev returns the sample standard deviation.
	func StdDev(xs []float32, base float32) float32 {
	n := len(xs)
	if n < 2 {
	return 0
	}
	sse, n := ssen(xs, base)
	return float32(math.Sqrt(float64(sse / float32(n-1))))
	}

	// FillStep fills the slice with the step function value, i.e. the ratio of
	// the ave of the first half of the trace values divided by the ave of the
	// second half of the trace values.
	//
	// If the second mean is 0 or the slice is filled with only MISSING_DATA_SENTINEL then
	// the slice will be filled with MISSING_DATA_SENTINEL.
	func FillStep(a []float32) {
	mid := len(a) / 2

	step := MissingDataSentinel
	meanFirst := MeanMissing(a[:mid])
	meanLast := MeanMissing(a[mid:])
	if meanLast != MissingDataSentinel && meanFirst != MissingDataSentinel {
	step = meanFirst / meanLast
	}
	if math.IsNaN(float64(step)) \|\| math.IsInf(float64(step), 0) {
	step = MissingDataSentinel
	}
	// Now fill.
	for i := range a {
	a[i] = step
	}
	}