[BREAKING CHANGE] Add offset param (#3)

example_test.go

@@ -4,10 +4,11 @@ import "fmt"
 
 func ExampleGenerator() {
 	prime := uint64(32452867)
+	offset := uint64(0)
 	random := uint64(123456)
 	bits := 30
 
-	generator, err := NewGenerator(prime, random, bits)
+	generator, err := NewGenerator(prime, random, offset, bits)
 	if err != nil {
 		panic(err)
 	}
@@ -35,3 +36,38 @@ func ExampleGenerator() {
 	// 8 => 259549784 => 8
 	// 9 => 292117339 => 9
 }
+
+func ExampleGenerator_withOffset() {
+	prime := uint64(32452867)
+	offset := uint64(10_000_000)
+	random := uint64(123456)
+	bits := 30
+
+	generator, err := NewGenerator(prime, random, offset, bits)
+	if err != nil {
+		panic(err)
+	}
+
+	for num := uint64(0); num < 10; num++ {
+		enc := generator.Encode(num)
+		dec := generator.Decode(enc)
+
+		fmt.Printf("%d => %d => %d\n", num, enc, dec)
+
+		if num != dec {
+			fmt.Printf("oops must be equal: %d != %d\n", num, dec)
+		}
+	}
+
+	// Output:
+	// 0 => 10123456 => 0
+	// 1 => 42428867 => 1
+	// 2 => 74979014 => 2
+	// 3 => 107284425 => 3
+	// 4 => 139836620 => 4
+	// 5 => 172142031 => 5
+	// 6 => 204692178 => 6
+	// 7 => 237259733 => 7
+	// 8 => 269549784 => 8
+	// 9 => 302117339 => 9
+}

obfid.go

@@ -22,11 +22,12 @@ type Generator struct {
 	prime   uint64
 	inverse uint64
 	random  uint64
+	offset  uint64
 	mask    uint64
 }
 
-// NewGenerator creates a new generator using the provided prime number and random.
-func NewGenerator(prime, random uint64, bits int) (*Generator, error) {
+// NewGenerator creates a new generator using the provided prime number, random and offset.
+func NewGenerator(prime, random, offset uint64, bits int) (*Generator, error) {
 	if bits == 0 || bits > 64 {
 		return nil, ErrTooMuchBits
 	}
@@ -41,19 +42,20 @@ func NewGenerator(prime, random uint64, bits int) (*Generator, error) {
 		prime:   prime,
 		inverse: inverse,
 		random:  random,
+		offset:  offset,
 		mask:    mask,
 	}
 	return generator, nil
 }
 
 // Encode returns obfuscated number.
 func (g *Generator) Encode(number uint64) uint64 {
-	return ((number * g.prime) & g.mask) ^ g.random
+	return ((number * g.prime) & g.mask) ^ g.random + g.offset
 }
 
 // Decode returns the original (deobfuscated) number.
 func (g *Generator) Decode(obfuscated uint64) uint64 {
-	return ((obfuscated ^ g.random) * g.inverse) & g.mask
+	return (((obfuscated - g.offset) ^ g.random) * g.inverse) & g.mask
 }
 
 // inverse calculates the inverse of prime.

obfid_test.go

@@ -7,7 +7,7 @@ import (
 )
 
 func TestGenerator(t *testing.T) {
-	generator, err := NewGenerator(32452867, 123, 30)
+	generator, err := NewGenerator(32452867, 123, 0, 30)
 	failIfErr(t, err)
 
 	for num := uint64(0); num < 30; num++ {
@@ -21,7 +21,7 @@ func TestGenerator(t *testing.T) {
 }
 
 func TestGenerator2(t *testing.T) {
-	generator, err := NewGenerator(32452867, 123, 30)
+	generator, err := NewGenerator(32452867, 123, 0, 30)
 	failIfErr(t, err)
 
 	for num := uint64(0); num < 30; num++ {
@@ -35,7 +35,7 @@ func TestGenerator2(t *testing.T) {
 }
 
 func TestGeneratorSmall(t *testing.T) {
-	generator, err := NewGenerator(32452867, 123, 5)
+	generator, err := NewGenerator(32452867, 123, 0, 5)
 	failIfErr(t, err)
 
 	res := map[uint64]struct{}{}
@@ -64,7 +64,7 @@ func Test_inverse(t *testing.T) {
 }
 
 func BenchmarkEncode(b *testing.B) {
-	generator, err := NewGenerator(32452867, 123, 30)
+	generator, err := NewGenerator(32452867, 123, 0, 30)
 	failIfErr(b, err)
 
 	var count uint64
@@ -76,7 +76,7 @@ func BenchmarkEncode(b *testing.B) {
 }
 
 func BenchmarkDecode(b *testing.B) {
-	generator, err := NewGenerator(32452867, 123, 30)
+	generator, err := NewGenerator(32452867, 123, 0, 30)
 	failIfErr(b, err)
 
 	var count uint64