added range support to continue interrupted download

internal/http/range.go

@@ -0,0 +1,70 @@
+package http
+
+import (
+	"strconv"
+	"strings"
+)
+
+type RangeResult int
+
+const (
+	// no header, invalid syntax or multiple ranges: serve the full body with 200
+	RangeNone RangeResult = iota
+	// serve bytes [start, end] (inclusive) with 206
+	RangeOK
+	// respond with 416
+	RangeUnsatisfiable
+)
+
+// ParseRange parses a "Range" header value for a resource of the given size.
+// Only a single byte range is supported: "bytes=a-b", "bytes=a-", "bytes=-n".
+// Anything else is ignored (RangeNone), which RFC 9110 §14.2 allows.
+func ParseRange(header string, size int64) (start, end int64, r RangeResult) {
+	header = strings.TrimSpace(header)
+	if header == "" {
+		return 0, 0, RangeNone
+	}
+	spec, ok := strings.CutPrefix(header, "bytes=")
+	if !ok || strings.ContainsRune(spec, ',') {
+		return 0, 0, RangeNone
+	}
+	firstStr, lastStr, ok := strings.Cut(spec, "-")
+	if !ok {
+		return 0, 0, RangeNone
+	}
+	firstStr = strings.TrimSpace(firstStr)
+	lastStr = strings.TrimSpace(lastStr)
+
+	if firstStr == "" {
+		// suffix range: last n bytes
+		n, ok := parseNonNegative(lastStr)
+		if !ok {
+			return 0, 0, RangeNone
+		}
+		if n == 0 || size == 0 {
+			return 0, 0, RangeUnsatisfiable
+		}
+		return max(size-n, 0), size - 1, RangeOK
+	}
+
+	first, ok := parseNonNegative(firstStr)
+	if !ok {
+		return 0, 0, RangeNone
+	}
+	last := size - 1
+	if lastStr != "" {
+		last, ok = parseNonNegative(lastStr)
+		if !ok || last < first {
+			return 0, 0, RangeNone
+		}
+	}
+	if first >= size {
+		return 0, 0, RangeUnsatisfiable
+	}
+	return first, min(last, size-1), RangeOK
+}
+
+func parseNonNegative(s string) (int64, bool) {
+	n, err := strconv.ParseUint(s, 10, 63)
+	return int64(n), err == nil
+}