dhcpv4: simplify option parsing.

option's codes and lengths were being parsed twice: once in ParseOption
and once in each option type's Parse implementation. Consolidate such
that it only happens once.

Additionally, only pass data to options that they should parse -- we
know the length before the Parse function is called, so the option only
gets to see the data it needs to see.

Also, use uio.Lexer to simplify parsing code in general. Easier to read
and reason about.

1 files changed, 5 insertions, 14 deletions

diff --git a/dhcpv4/bsdp/bsdp_option_version.go b/dhcpv4/bsdp/bsdp_option_version.go
index 8431a94..38158d7 100644
--- a/dhcpv4/bsdp/bsdp_option_version.go
+++ b/dhcpv4/bsdp/bsdp_option_version.go
@@ -4,10 +4,9 @@ import (
 	"fmt"
 
 	"github.com/insomniacslk/dhcp/dhcpv4"
+	"github.com/u-root/u-root/pkg/uio"
 )
 
-// Implements the BSDP option version. Can be one of 1.0 or 1.1
-
 // Specific versions.
 var (
 	Version1_0 = []byte{1, 0}
@@ -15,6 +14,8 @@ var (
 )
 
 // OptVersion represents a BSDP protocol version.
+//
+// Implements the BSDP option version. Can be one of 1.0 or 1.1
 type OptVersion struct {
 	Version []byte
 }
@@ -22,18 +23,8 @@ type OptVersion struct {
 // ParseOptVersion constructs an OptVersion struct from a sequence of
 // bytes and returns it, or an error.
 func ParseOptVersion(data []byte) (*OptVersion, error) {
-	if len(data) < 4 {
-		return nil, dhcpv4.ErrShortByteStream
-	}
-	code := dhcpv4.OptionCode(data[0])
-	if code != OptionVersion {
-		return nil, fmt.Errorf("expected option %v, got %v instead", OptionVersion, code)
-	}
-	length := int(data[1])
-	if length != 2 {
-		return nil, fmt.Errorf("expected length 2, got %d instead", length)
-	}
-	return &OptVersion{data[2:4]}, nil
+	buf := uio.NewBigEndianBuffer(data)
+	return &OptVersion{buf.CopyN(2)}, buf.FinError()
 }
 
 // Code returns the option code.