refactor qbittorrent client helpers

internal/service/qbittorrent_bencode.go

@@ -0,0 +1,91 @@
+package service
+
+import (
+	"crypto/sha1" // #nosec G505 -- BitTorrent v1 info-hash is SHA-1 by protocol.
+	"encoding/hex"
+)
+
+func torrentInfoHash(data []byte) string {
+	start, end, ok := torrentInfoBounds(data)
+	if !ok {
+		return ""
+	}
+	sum := sha1.Sum(data[start:end]) // #nosec G401 -- BitTorrent v1 info-hash is SHA-1 by protocol, not a security hash.
+	return hex.EncodeToString(sum[:])
+}
+
+func torrentInfoBounds(data []byte) (int, int, bool) {
+	if len(data) == 0 || data[0] != 'd' {
+		return 0, 0, false
+	}
+	pos := 1
+	for pos < len(data) && data[pos] != 'e' {
+		keyStart, keyEnd, next, ok := parseBencodeString(data, pos)
+		if !ok {
+			return 0, 0, false
+		}
+		valueStart := next
+		valueEnd, ok := bencodeValueEnd(data, valueStart)
+		if !ok {
+			return 0, 0, false
+		}
+		if string(data[keyStart:keyEnd]) == "info" {
+			return valueStart, valueEnd, true
+		}
+		pos = valueEnd
+	}
+	return 0, 0, false
+}
+
+func parseBencodeString(data []byte, pos int) (int, int, int, bool) {
+	if pos >= len(data) || data[pos] < '0' || data[pos] > '9' {
+		return 0, 0, 0, false
+	}
+	length := 0
+	for pos < len(data) && data[pos] >= '0' && data[pos] <= '9' {
+		length = length*10 + int(data[pos]-'0')
+		pos++
+	}
+	if pos >= len(data) || data[pos] != ':' {
+		return 0, 0, 0, false
+	}
+	start := pos + 1
+	end := start + length
+	if end > len(data) {
+		return 0, 0, 0, false
+	}
+	return start, end, end, true
+}
+
+func bencodeValueEnd(data []byte, pos int) (int, bool) {
+	if pos >= len(data) {
+		return 0, false
+	}
+	switch data[pos] {
+	case 'i':
+		end := pos + 1
+		for end < len(data) && data[end] != 'e' {
+			end++
+		}
+		if end >= len(data) {
+			return 0, false
+		}
+		return end + 1, true
+	case 'l', 'd':
+		end := pos + 1
+		for end < len(data) && data[end] != 'e' {
+			next, ok := bencodeValueEnd(data, end)
+			if !ok {
+				return 0, false
+			}
+			end = next
+		}
+		if end >= len(data) {
+			return 0, false
+		}
+		return end + 1, true
+	default:
+		_, _, next, ok := parseBencodeString(data, pos)
+		return next, ok
+	}
+}