feat(bitfield): bitfield pieces

pkg/bitfield/bitfield.go

@@ -0,0 +1,27 @@
+package bitfield
+
+// Bitfield represents the pieces that a peer has
+type Bitfield []byte
+
+// HasPiece teels if a bitfield has a particular index
+func (bf Bitfield) HasPiece(index int) bool {
+	byteIndex := index / 8
+	offset := index % 8
+	if byteIndex < 0 || byteIndex >= len(bf) {
+		return false
+	}
+	return bf[byteIndex]>>(7-offset)&1 != 0
+}
+
+// SetPiece sets a bit in the bitfield
+func (bf Bitfield) SetPiece(index int) {
+	byteIndex := index / 8
+	offset := index % 8
+
+	// silently ignore invalid bounded index
+	if byteIndex < 0 || byteIndex >= len(bf) {
+		return
+	}
+
+	bf[byteIndex] |= 1 << (7 - offset)
+}

pkg/bitfield/bitfield_test.go

@@ -0,0 +1,49 @@
+package bitfield
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestHasPiece(t *testing.T) {
+	bf := Bitfield{0b01010100, 0b01010100}
+	outputs := []bool{false, true, false, true, false, true, false, false, false, true, false, true, false, true, false, false, false, false, false, false}
+	for i := 0; i < len(outputs); i++ {
+		assert.Equal(t, outputs[i], bf.HasPiece(i))
+	}
+}
+
+func TestSetPiece(t *testing.T) {
+	tests := []struct {
+		input  Bitfield
+		index  int
+		output Bitfield
+	}{
+		{
+			input:  Bitfield{0b01010100, 0b01010100},
+			index:  4, //          v (set)
+			output: Bitfield{0b01011100, 0b01010100},
+		},
+		{
+			input:  Bitfield{0b01010100, 0b01010100},
+			index:  9, //                   v (noop)
+			output: Bitfield{0b01010100, 0b01010100},
+		},
+		{
+			input:  Bitfield{0b01010100, 0b01010100},
+			index:  15, //                        v (set)
+			output: Bitfield{0b01010100, 0b01010101},
+		},
+		{
+			input:  Bitfield{0b01010100, 0b01010100},
+			index:  19, //                            v (noop)
+			output: Bitfield{0b01010100, 0b01010100},
+		},
+	}
+	for _, test := range tests {
+		bf := test.input
+		bf.SetPiece(test.index)
+		assert.Equal(t, test.output, bf)
+	}
+}