copy float compare dependency

Per discussion in the pull request, we'd like to avoid having an extra
dependency on a float comparison package. Instead, we copy the float compare
functions from the float comparison package.

The float comparison package we're choosing is this. The author of this
package has commented in the pull request and it looks like we have consensus
that this is the best option.
github.com/beorn7/floats

prometheus/histogram_test.go

@@ -28,6 +28,8 @@ import (
 	"github.com/golang/protobuf/proto"
 	"google.golang.org/protobuf/types/known/timestamppb"
 
+	"github.com/prometheus/client_golang/prometheus/internal"
+
 	dto "github.com/prometheus/client_model/go"
 )
 
@@ -358,9 +360,9 @@ func TestBuckets(t *testing.T) {
 		1.0, 1.6681, 2.7825, 4.6415, 7.7426, 12.9154, 21.5443,
 		35.9381, 59.9484, 100.0000,
 	}
-	const tolerance = 0.0001
-	if !equalFloat64s(got, want, tolerance) {
-		t.Errorf("exponential buckets range: got %v, want %v (tolerance %f)", got, want, tolerance)
+	const epsilon = 0.0001
+	if !internal.AlmostEqualFloat64s(got, want, epsilon) {
+		t.Errorf("exponential buckets range: got %v, want %v (epsilon %f)", got, want, epsilon)
 	}
 }
 
@@ -466,23 +468,3 @@ func TestHistogramExemplar(t *testing.T) {
 		}
 	}
 }
-
-// equalFloat64 returns true if a and b are within the tolerance. We have this
-// because float comparison varies on different architectures. For example,
-// architectures which do FMA yield slightly different results.
-// https://github.com/prometheus/client_golang/pull/899#issuecomment-1244506390
-func equalFloat64(a, b, tolerance float64) bool {
-	return math.Abs(a-b) < tolerance
-}
-
-func equalFloat64s(a, b []float64, tolerance float64) bool {
-	if len(a) != len(b) {
-		return false
-	}
-	for i := range a {
-		if !equalFloat64(a[i], b[i], tolerance) {
-			return false
-		}
-	}
-	return true
-}

prometheus/internal/difflib.go

@@ -22,6 +22,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"math"
 	"strings"
 )
 
@@ -46,6 +47,97 @@ func calculateRatio(matches, length int) float64 {
 	return 1.0
 }
 
+var (
+	// minNormalFloat64 is the smallest positive normal value of type float64.
+	minNormalFloat64 = math.Float64frombits(0x0010000000000000)
+
+	// minNormalFloat32 is the smallest positive normal value of type float32.
+	minNormalFloat32 = math.Float32frombits(0x00800000)
+)
+
+// AlmostEqualFloat64 returns true if a and b are equal within a relative error
+// of epsilon. See http://floating-point-gui.de/errors/comparison/ for the
+// details of the applied method.
+//
+// This function is copy/paste to avoid a dependency.
+// https://github.com/beorn7/floats
+func AlmostEqualFloat64(a, b, epsilon float64) bool {
+	if a == b {
+		return true
+	}
+	absA := math.Abs(a)
+	absB := math.Abs(b)
+	diff := math.Abs(a - b)
+	if a == 0 || b == 0 || absA+absB < minNormalFloat64 {
+		return diff < epsilon*minNormalFloat64
+	}
+	return diff/math.Min(absA+absB, math.MaxFloat64) < epsilon
+}
+
+// AlmostEqualFloat64s is the slice form of AlmostEqualFloat64.
+func AlmostEqualFloat64s(a, b []float64, epsilon float64) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if !AlmostEqualFloat64(a[i], b[i], epsilon) {
+			return false
+		}
+	}
+	return true
+}
+
+// AlmostEqualFloat32 returns true if a and b are equal within a relative error
+// of epsilon. See http://floating-point-gui.de/errors/comparison/ for the
+// details of the applied method.
+//
+// This function is copy/paste to avoid a dependency.
+// https://github.com/beorn7/floats
+func AlmostEqualFloat32(a, b, epsilon float32) bool {
+	if a == b {
+		return true
+	}
+	absA := AbsFloat32(a)
+	absB := AbsFloat32(b)
+	diff := AbsFloat32(a - b)
+	if a == 0 || b == 0 || absA+absB < minNormalFloat32 {
+		return diff < epsilon*minNormalFloat32
+	}
+	return diff/MinFloat32(absA+absB, math.MaxFloat32) < epsilon
+}
+
+// AlmostEqualFloat32s is the slice form of AlmostEqualFloat32.
+func AlmostEqualFloat32s(a, b []float32, epsilon float32) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if !AlmostEqualFloat32(a[i], b[i], epsilon) {
+			return false
+		}
+	}
+	return true
+}
+
+// AbsFloat32 works like math.Abs, but for float32.
+func AbsFloat32(x float32) float32 {
+	switch {
+	case x < 0:
+		return -x
+	case x == 0:
+		return 0 // return correctly abs(-0)
+	}
+	return x
+}
+
+// MinFloat32 works like math.Min, but for float32.
+func MinFloat32(x, y float32) float32 {
+	if x < y {
+		return x
+	}
+	return y
+}
+
 type Match struct {
 	A    int
 	B    int