1 files changed, 98 insertions, 0 deletions
diff --git a/cmd/mybittorrent/decoder.go b/cmd/mybittorrent/decoder.go
new file mode 100644
index 0000000..10066e1
--- /dev/null
+++ b/cmd/mybittorrent/decoder.go
@@ -0,0 +1,98 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+type Decoder struct {
+	raw          string
+	decoderFuncs []DecoderFunc
+}
+
+type DecoderFunc func(string) (any, int, bool)
+
+func NewDecoder(raw string) *Decoder {
+	d := &Decoder{raw: raw}
+	return d.WithDecoderFunc(d.decodeSlice).WithDecoderFunc(d.decodeString).WithDecoderFunc(d.decodeInteger)
+}
+
+func (d *Decoder) WithDecoderFunc(f DecoderFunc) *Decoder {
+	d.decoderFuncs = append(d.decoderFuncs, f)
+	return d
+}
+
+func (d *Decoder) Decode() (any, error) {
+	for _, f := range d.decoderFuncs {
+		if r, _, ok := f(d.raw); ok {
+			return r, nil
+		}
+	}
+
+	return nil, fmt.Errorf("failed to decode value: %s", d.raw)
+}
+
+func (d *Decoder) decodeString(chunk string) (any, int, bool) {
+	if !unicode.IsDigit(rune(chunk[0])) {
+		return "", 0, false
+	}
+
+	var firstColonIndex int
+
+	for i := 0; i < len(chunk); i++ {
+		if chunk[i] == ':' {
+			firstColonIndex = i
+			break
+		}
+	}
+
+	lengthStr := chunk[:firstColonIndex]
+
+	length, err := strconv.Atoi(lengthStr)
+	if err != nil {
+		return "", 0, false
+	}
+
+	val := chunk[firstColonIndex+1 : firstColonIndex+1+length]
+
+	return val, length + 1 + len(lengthStr), true
+}
+
+func (d *Decoder) decodeInteger(chunk string) (any, int, bool) {
+	if rune(chunk[0]) != 'i' {
+		return 0, 0, false
+	}
+
+	num, err := strconv.Atoi(chunk[1:strings.IndexByte(chunk, 'e')])
+	if err != nil {
+		return 0, 0, false
+	}
+
+	return num, len(strconv.Itoa(num)) + 2, true
+}
+
+func (d *Decoder) decodeSlice(chunk string) (any, int, bool) {
+	if rune(chunk[0]) != 'l' {
+		return nil, 0, false
+	}
+
+	if rune(chunk[1]) == 'e' {
+		return []any{}, 2, true
+	}
+
+	slice := make([]any, 0, 1024)
+	offset := 1
+
+	for rune(chunk[offset]) != 'e' {
+		for _, f := range d.decoderFuncs {
+			if v, l, ok := f(chunk[offset:]); ok {
+				slice = append(slice, v)
+				offset += l
+			}
+		}
+	}
+
+	return slice, offset + 1, true
+}