From 4605d3907ce31b503743d1e7514ede90850a4da5 Mon Sep 17 00:00:00 2001
From: ChillerDragon <ChillerDragon@gmail.com>
Date: Mon, 17 Jun 2024 12:23:54 +0800
Subject: [PATCH] Add int packer and unpacker

---
 packer/packer.go      |  67 +++++++++++++++++
 packer/packer_test.go | 163 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 230 insertions(+)
 create mode 100644 packer/packer.go
 create mode 100644 packer/packer_test.go

diff --git a/packer/packer.go b/packer/packer.go
new file mode 100644
index 0000000..3f442bf
--- /dev/null
+++ b/packer/packer.go
@@ -0,0 +1,67 @@
+package packer
+
+import "errors"
+
+func PackInt(num int) ([]byte, error) {
+	dstLen := 4
+	res := []byte{0x00}
+	idx := 0
+	if num < 0 {
+		res[0] |= 0x40 // set sign bit
+		num = ^num
+	}
+
+	res[0] |= byte(num & 0x3F) // pack 6 bit into dst
+	num >>= 6                  // discard 6 bits
+	for num != 0 {
+		if idx > dstLen {
+			return nil, errors.New("Int too big")
+		}
+
+		res = append(res, 0x00)
+
+		res[idx] |= 0x80 // set extend bit
+		idx++
+		res[idx] = byte(num & 0x7F) // pack 7 bit
+		num >>= 7                   // discard 7 bits
+	}
+
+	return res, nil
+}
+
+func UnpackInt(data []byte) int {
+	sign := int(data[0]>>6) & 1
+	res := int(data[0] & 0x3F)
+	i := 0
+	// fake loop should only loop once
+	// its the poor mans goto hack
+	for {
+		if (data[i] & 0x80) == 0 {
+			break
+		}
+		i += 1
+		res |= int(data[i]&0x7F) << 6
+
+		if (data[i] & 0x80) == 0 {
+			break
+		}
+		i += 1
+		res |= int(data[i]&0x7F) << (6 + 7)
+
+		if (data[i] & 0x80) == 0 {
+			break
+		}
+		i += 1
+		res |= int(data[i]&0x7F) << (6 + 7 + 7)
+
+		if (data[i] & 0x80) == 0 {
+			break
+		}
+		i += 1
+		res |= int(data[i]&0x7F) << (6 + 7 + 7 + 7)
+		break
+	}
+
+	res ^= -sign
+	return res
+}
diff --git a/packer/packer_test.go b/packer/packer_test.go
new file mode 100644
index 0000000..4057a13
--- /dev/null
+++ b/packer/packer_test.go
@@ -0,0 +1,163 @@
+package packer
+
+import (
+	"reflect"
+	"testing"
+)
+
+// pack
+
+func TestPackSmallPositiveInts(t *testing.T) {
+	got, err := PackInt(1)
+	want := []byte{0x01}
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+}
+
+func TestPackMultiBytePositiveInts(t *testing.T) {
+	got, err := PackInt(63)
+	want := []byte{0x3F}
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+
+	got, err = PackInt(64)
+	want = []byte{0x80, 0x01}
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+
+	got, err = PackInt(65)
+	want = []byte{0x81, 0x01}
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+}
+
+func TestPackSmallNegativeInts(t *testing.T) {
+	got, err := PackInt(-1)
+	want := []byte{0x40}
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+
+	got, err = PackInt(-2)
+	want = []byte{0x41}
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+}
+
+func TestPackMultiByteNegativeInts(t *testing.T) {
+	got, err := PackInt(-63)
+	want := []byte{0x7E}
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+
+	got, err = PackInt(-64)
+	want = []byte{0x7F}
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+
+	got, err = PackInt(-65)
+	want = []byte{0xC0, 0x01}
+
+	if err != nil {
+		t.Errorf("Unexpected error: %v", err)
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+}
+
+// unpack
+
+func TestUnpackSmallPositiveInts(t *testing.T) {
+	got := UnpackInt([]byte{0x01})
+	want := 1
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+
+	got = UnpackInt([]byte{0x02})
+	want = 2
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+
+	got = UnpackInt([]byte{0x03})
+	want = 3
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+}
+
+func TestUnpackMultiBytePositiveInts(t *testing.T) {
+	got := UnpackInt([]byte{0x3f})
+	want := 63
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+
+	got = UnpackInt([]byte{0x80, 0x01})
+	want = 64
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+
+	got = UnpackInt([]byte{0x81, 0x01})
+	want = 65
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v, wanted %v", got, want)
+	}
+}