Use concrete value for quad.Quad

Comparison of -short benchmarks in cayley.

$ benchcmp pointer.bench concrete.bench
benchmark                                   old ns/op     new ns/op	delta
BenchmarkNamePredicate                      1673276       1655093	-1.09%
BenchmarkLargeSetsNoIntersection            318985907     261499984	-18.02%
BenchmarkNetAndSpeed                        104403743     41516981	-60.23%
BenchmarkKeanuAndNet                        17309258      16857513	-2.61%
BenchmarkKeanuAndSpeed                      20159161      19282833	-4.35%

Comparison of pathological cases are not so happy.

benchmark                                   old ns/op       new ns/op		delta
BenchmarkVeryLargeSetsSmallIntersection     55269775527     246084606672	+345.24%
BenchmarkHelplessContainsChecker            23436501319     24308906949		+3.72%

Profiling the worst case:

Pointer:
Total: 6121 samples
    1973  32.2%  32.2%     1973  32.2% runtime.findfunc
     773  12.6%  44.9%      773  12.6% readvarint
     510   8.3%  53.2%      511   8.3% step
     409   6.7%  59.9%      410   6.7% runtime.gentraceback
     390   6.4%  66.2%      391   6.4% pcvalue
     215   3.5%  69.8%      215   3.5% runtime.funcdata
     181   3.0%  72.7%      181   3.0% checkframecopy
     118   1.9%  74.6%      119   1.9% runtime.funcspdelta
      96   1.6%  76.2%       96   1.6% runtime.topofstack
      76   1.2%  77.5%       76   1.2% scanblock

Concrete:
Total: 25027 samples
    9437  37.7%  37.7%     9437  37.7% runtime.findfunc
    3853  15.4%  53.1%     3853  15.4% readvarint
    2366   9.5%  62.6%     2366   9.5% step
    2186   8.7%  71.3%     2186   8.7% runtime.gentraceback
    1816   7.3%  78.5%     1816   7.3% pcvalue
    1016   4.1%  82.6%     1016   4.1% runtime.funcdata
     859   3.4%  86.0%      859   3.4% checkframecopy
     506   2.0%  88.1%      506   2.0% runtime.funcspdelta
     410   1.6%  89.7%      410   1.6% runtime.topofstack
     303   1.2%  90.9%      303   1.2% runtime.newstack

quad/nquads/nquads.go

@@ -33,9 +33,9 @@ import (
 // Parse returns a valid quad.Quad or a non-nil error. Parse does
 // handle comments except where the comment placement does not prevent
 // a complete valid quad.Quad from being defined.
-func Parse(str string) (*quad.Quad, error) {
+func Parse(str string) (quad.Quad, error) {
 	q, err := parse([]rune(str))
-	return &q, err
+	return q, err
 }
 
 // Decoder implements N-Quad document parsing according to the RDF
@@ -52,14 +52,14 @@ func NewDecoder(r io.Reader) *Decoder {
 }
 
 // Unmarshal returns the next valid N-Quad as a quad.Quad, or an error.
-func (dec *Decoder) Unmarshal() (*quad.Quad, error) {
+func (dec *Decoder) Unmarshal() (quad.Quad, error) {
 	dec.line = dec.line[:0]
 	var line []byte
 	for {
 		for {
 			l, pre, err := dec.r.ReadLine()
 			if err != nil {
-				return nil, err
+				return quad.Quad{}, err
 			}
 			dec.line = append(dec.line, l...)
 			if !pre {
@@ -73,9 +73,9 @@ func (dec *Decoder) Unmarshal() (*quad.Quad, error) {
 	}
 	triple, err := Parse(string(line))
 	if err != nil {
-		return nil, fmt.Errorf("failed to parse %q: %v", dec.line, err)
+		return quad.Quad{}, fmt.Errorf("failed to parse %q: %v", dec.line, err)
 	}
-	if triple == nil {
+	if !triple.IsValid() {
 		return dec.Unmarshal()
 	}
 	return triple, nil