FloorDiv bug fix, Example (and regression test)

base/math.go

@@ -44,23 +44,25 @@ func Horner(x float64, c ...float64) float64 {
 // It uses integer math only, so is more efficient than using floating point
 // intermediate values.  This function can be used in many places where INT()
 // appears in AA.  As with built in integer division, it panics with y == 0.
-func FloorDiv(x, y int) int {
-	if (x < 0) == (y < 0) {
-		return x / y
+func FloorDiv(x, y int) (q int) {
+	q = x / y
+	if (x < 0) != (y < 0) && x%y != 0 {
+		q--
 	}
-	return x/y - 1
+	return
 }
 
 // FloorDiv64 returns the integer floor of the fractional value (x / y).
 //
 // It uses integer math only, so is more efficient than using floating point
 // intermediate values.  This function can be used in many places where INT()
 // appears in AA.  As with built in integer division, it panics with y == 0.
-func FloorDiv64(x, y int64) int64 {
-	if (x < 0) == (y < 0) {
-		return x / y
+func FloorDiv64(x, y int64) (q int64) {
+	q = x / y
+	if (x < 0) != (y < 0) && x%y != 0 {
+		q--
 	}
-	return x/y - 1
+	return
 }
 
 // Cmp compares two float64s and returns -1, 0, or 1 if a is <, ==, or > b,

base/math_test.go

@@ -11,6 +11,56 @@ import (
 	"github.com/soniakeys/meeus/base"
 )
 
+func ExampleFloorDiv() {
+	// compare to / operator examples in Go spec at
+	// https://golang.org/ref/spec#Arithmetic_operators
+	fmt.Println(base.FloorDiv(+5, +3))
+	fmt.Println(base.FloorDiv(-5, +3))
+	fmt.Println(base.FloorDiv(+5, -3))
+	fmt.Println(base.FloorDiv(-5, -3))
+	fmt.Println()
+	// exact divisors, no remainders
+	fmt.Println(base.FloorDiv(+6, +3))
+	fmt.Println(base.FloorDiv(-6, +3))
+	fmt.Println(base.FloorDiv(+6, -3))
+	fmt.Println(base.FloorDiv(-6, -3))
+	// Output:
+	// 1
+	// -2
+	// -2
+	// 1
+	//
+	// 2
+	// -2
+	// -2
+	// 2
+}
+
+func ExampleFloorDiv64() {
+	// compare to / operator examples in Go spec at
+	// https://golang.org/ref/spec#Arithmetic_operators
+	fmt.Println(base.FloorDiv64(+5, +3))
+	fmt.Println(base.FloorDiv64(-5, +3))
+	fmt.Println(base.FloorDiv64(+5, -3))
+	fmt.Println(base.FloorDiv64(-5, -3))
+	fmt.Println()
+	// exact divisors, no remainders
+	fmt.Println(base.FloorDiv64(+6, +3))
+	fmt.Println(base.FloorDiv64(-6, +3))
+	fmt.Println(base.FloorDiv64(+6, -3))
+	fmt.Println(base.FloorDiv64(-6, -3))
+	// Output:
+	// 1
+	// -2
+	// -2
+	// 1
+	//
+	// 2
+	// -2
+	// -2
+	// 2
+}
+
 // Section "Trigonometric functions of large angles":
 //
 // Meeus makes his point, but an example with integer values is a bit unfair