new exercise to encode/decode variable length quantity

config.json

@@ -75,7 +75,8 @@
     "word-search",
     "connect",
     "ledger",
-    "poker"
+    "poker",
+    "variable-length-quantity"
   ],
   "deprecated": [
     "accumulate",

exercises/variable-length-quantity/example.go

@@ -0,0 +1,56 @@
+package variablelengthquantity
+
+// EncodeVarint returns the varint encoding of x.
+func EncodeVarint(x uint32) []byte {
+	if x>>7 == 0 {
+		return []byte{
+			byte(x),
+		}
+	}
+
+	if x>>14 == 0 {
+		return []byte{
+			byte(0x80 | x>>7),
+			byte(127 & x),
+		}
+	}
+
+	if x>>21 == 0 {
+		return []byte{
+			byte(0x80 | x>>14),
+			byte(0x80 | x>>7),
+			byte(127 & x),
+		}
+	}
+
+	return []byte{
+		byte(0x80 | x>>21),
+		byte(0x80 | x>>14),
+		byte(0x80 | x>>7),
+		byte(127 & x),
+	}
+}
+
+// DecodeVarint reads a varint-encoded integer from the slice.
+// It returns the integer and the number of bytes consumed, or
+// zero if there is not enough.
+func DecodeVarint(buf []byte) (x uint32, n int) {
+	if len(buf) < 1 {
+		return 0, 0
+	}
+
+	if buf[0] <= 0x80 {
+		return uint32(buf[0]), 1
+	}
+
+	var b byte
+	for n, b = range buf {
+		x = x << 7
+		x |= uint32(b) & 0x7F
+		if (b & 0x80) == 0 {
+			return x, n
+		}
+	}
+
+	return x, n
+}

exercises/variable-length-quantity/variable_length_quantity_test.go

@@ -0,0 +1,36 @@
+package variablelengthquantity
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestEncodeDecodeVarint(t *testing.T) {
+	testCases := []struct {
+		input  []byte
+		output uint32
+	}{
+		0: {[]byte{0x7F}, 127},
+		1: {[]byte{0x81, 0x00}, 128},
+		2: {[]byte{0xC0, 0x00}, 8192},
+		3: {[]byte{0xFF, 0x7F}, 16383},
+		4: {[]byte{0x81, 0x80, 0x00}, 16384},
+		5: {[]byte{0xFF, 0xFF, 0x7F}, 2097151},
+		6: {[]byte{0x81, 0x80, 0x80, 0x00}, 2097152},
+		7: {[]byte{0xC0, 0x80, 0x80, 0x00}, 134217728},
+		8: {[]byte{0xFF, 0xFF, 0xFF, 0x7F}, 268435455},
+
+		9:  {[]byte{0x82, 0x00}, 256},
+		10: {[]byte{0x81, 0x10}, 144},
+	}
+
+	for i, tc := range testCases {
+		t.Logf("test case %d - %#v\n", i, tc.input)
+		if o, _ := DecodeVarint(tc.input); o != tc.output {
+			t.Fatalf("expected %d\ngot\n%d\n", tc.output, o)
+		}
+		if encoded := EncodeVarint(tc.output); bytes.Compare(encoded, tc.input) != 0 {
+			t.Fatalf("%d - expected %#v\ngot\n%#v\n", tc.output, tc.input, encoded)
+		}
+	}
+}