use ps.List for Sample storage, so it is immutable. Have to implement…

… custom marshalling

probe/host/system_test.go

@@ -24,8 +24,8 @@ func TestGetLoad(t *testing.T) {
 		t.Fatalf("Expected 3 metrics, but got: %v", have)
 	}
 	for key, metric := range have {
-		if len(metric.Samples) != 1 {
-			t.Errorf("Expected metric %v to have 1 sample, but had: %d", key, len(metric.Samples))
+		if metric.Len() != 1 {
+			t.Errorf("Expected metric %v to have 1 sample, but had: %d", key, metric.Len())
 		}
 	}
 }

render/detailed_node.go

@@ -466,7 +466,7 @@ func hostOriginTable(nmd report.Node) (Table, bool) {
 	)
 	for _, key := range []string{host.Load1, host.Load5, host.Load15} {
 		if metric, ok := nmd.Metrics[key]; ok {
-			if len(metric.Samples) == 0 {
+			if metric.Len() == 0 {
 				continue
 			}
 			if metric.Max > maxLoad {

report/latest_map.go

@@ -79,7 +79,7 @@ func (m LatestMap) toIntermediate() map[string]LatestEntry {
 	return intermediate
 }
 
-func fromIntermediate(in map[string]LatestEntry) LatestMap {
+func (m LatestMap) fromIntermediate(in map[string]LatestEntry) LatestMap {
 	out := ps.NewMap()
 	for k, v := range in {
 		out = out.Set(k, v)
@@ -105,7 +105,7 @@ func (m *LatestMap) UnmarshalJSON(input []byte) error {
 	if err := json.NewDecoder(bytes.NewBuffer(input)).Decode(&in); err != nil {
 		return err
 	}
-	*m = fromIntermediate(in)
+	*m = LatestMap{}.fromIntermediate(in)
 	return nil
 }
 
@@ -122,6 +122,6 @@ func (m *LatestMap) GobDecode(input []byte) error {
 	if err := gob.NewDecoder(bytes.NewBuffer(input)).Decode(&in); err != nil {
 		return err
 	}
-	*m = fromIntermediate(in)
+	*m = LatestMap{}.fromIntermediate(in)
 	return nil
 }

report/metrics.go

@@ -0,0 +1,268 @@
+package report
+
+import (
+	"bytes"
+	"encoding/gob"
+	"encoding/json"
+	"time"
+
+	"github.com/mndrix/ps"
+)
+
+// Metrics is a string->metric map.
+type Metrics map[string]Metric
+
+// Merge merges two sets maps into a fresh set, performing set-union merges as
+// appropriate.
+func (m Metrics) Merge(other Metrics) Metrics {
+	result := m.Copy()
+	for k, v := range other {
+		result[k] = result[k].Merge(v)
+	}
+	return result
+}
+
+// Copy returns a value copy of the sets map.
+func (m Metrics) Copy() Metrics {
+	result := Metrics{}
+	for k, v := range m {
+		result[k] = v.Copy()
+	}
+	return result
+}
+
+// Metric is a list of timeseries data with some metadata. Clients must use the
+// Add method to add values.
+type Metric struct {
+	Samples Samples   `json:"samples"`
+	Min     float64   `json:"min"`
+	Max     float64   `json:"max"`
+	First   time.Time `json:"first"`
+	Last    time.Time `json:"last"`
+}
+
+// Sample is a single datapoint of a metric.
+type Sample struct {
+	Timestamp time.Time `json:"date"`
+	Value     float64   `json:"value"`
+}
+
+// MakeMetric makes a new Metric.
+func MakeMetric() Metric {
+	return Metric{}
+}
+
+// WithFirst returns a fresh copy of m, with First set to t.
+func (m Metric) WithFirst(t time.Time) Metric {
+	m.First = t
+	return m
+}
+
+// Len returns the number of samples in the metric.
+func (m Metric) Len() int {
+	return m.Samples.Size()
+}
+
+// Add adds the sample to the Metric. Add is the only valid way to grow a
+// Metric. Add returns the Metric to enable chaining.
+func (m Metric) Add(t time.Time, v float64) Metric {
+	newSamples := m.Samples
+	popped := Samples{}
+	for !newSamples.IsNil() {
+		head := newSamples.Head()
+		if head.Timestamp.Equal(t) {
+			// The list already has the element.
+			return m
+		}
+		if head.Timestamp.Before(t) {
+			// Reached insertion point.
+			break
+		}
+		newSamples = newSamples.Tail()
+		popped = popped.Cons(head)
+	}
+	newSamples = newSamples.Cons(Sample{Timestamp: t, Value: v})
+	// Re-add any samples after this one.
+	popped.ForEach(func(s Sample) {
+		newSamples = newSamples.Cons(s)
+	})
+	m.Samples = newSamples
+	if v > m.Max {
+		m.Max = v
+	}
+	if v < m.Min {
+		m.Min = v
+	}
+	if m.First.IsZero() || t.Before(m.First) {
+		m.First = t
+	}
+	if m.Last.IsZero() || t.After(m.Last) {
+		m.Last = t
+	}
+	return m
+}
+
+// Merge combines the two Metrics and returns a new result.
+func (m Metric) Merge(other Metric) Metric {
+	other.Samples.ForEach(func(s Sample) {
+		m = m.Add(s.Timestamp, s.Value)
+	})
+	if !other.First.IsZero() && other.First.Before(m.First) {
+		m.First = other.First
+	}
+	if !other.Last.IsZero() && other.Last.After(m.Last) {
+		m.Last = other.Last
+	}
+	if other.Min < m.Min {
+		m.Min = other.Min
+	}
+	if other.Max > m.Max {
+		m.Max = other.Max
+	}
+	return m
+}
+
+// Copy returns a value copy of the Metric. Metric is immutable, so we can skip
+// this.
+func (m Metric) Copy() Metric {
+	return m
+}
+
+// Div returns a new copy of the metric, with each value divided by n.
+func (m Metric) Div(n float64) Metric {
+	oldSamples := m.Samples
+	m.Samples = Samples{}
+	oldSamples.ForEach(func(s Sample) {
+		m = m.Add(s.Timestamp, s.Value/n)
+	})
+	m.Max = m.Max / n
+	m.Min = m.Min / n
+	return m
+}
+
+// LastSample returns the last sample in the metric, or nil if there are no
+// samples.
+func (m Metric) LastSample() *Sample {
+	if m.Samples.IsNil() {
+		return nil
+	}
+	s := m.Samples.Head()
+	return &s
+}
+
+// Samples is an immutable list of timeseries data. We have this to implement
+// proper marshalling for the ps.List, as well as fixing some if ps.List's
+// behaviour.
+type Samples struct {
+	ps.List
+}
+
+// IsNil returns true if the list is empty. Unlike ps.List, this also works if
+// the list is nil.
+func (s Samples) IsNil() bool {
+	return s.List == nil || s.List.IsNil()
+}
+
+// Cons returns a new list with val as the head
+func (s Samples) Cons(val Sample) Samples {
+	if s.List == nil {
+		s.List = ps.NewList()
+	}
+	return Samples{s.List.Cons(val)}
+}
+
+// Head returns the first element of the list;
+// panics if the list is empty
+func (s Samples) Head() Sample {
+	return s.List.Head().(Sample)
+}
+
+// Tail returns a list with all elements except the head;
+// panics if the list is empty
+func (s Samples) Tail() Samples {
+	return Samples{s.List.Tail()}
+}
+
+// Size returns the list's length.  This takes O(1) time. Unlike ps.List, this
+// also works if the list is nil
+func (s Samples) Size() int {
+	if s.List == nil {
+		return 0
+	}
+	return s.List.Size()
+}
+
+// ForEach executes a callback for each value in the list.
+func (s Samples) ForEach(f func(Sample)) {
+	if s.List == nil {
+		return
+	}
+	s.List.ForEach(func(s interface{}) {
+		f(s.(Sample))
+	})
+}
+
+// Reverse returns a list whose elements are in the opposite order as
+// the original list.
+func (s Samples) Reverse() Samples {
+	if s.List == nil {
+		return s
+	}
+	return Samples{s.List.Reverse()}
+}
+
+func (s Samples) toIntermediate() []Sample {
+	samples := []Sample{}
+	s.Reverse().ForEach(func(s Sample) {
+		samples = append(samples, s)
+	})
+	return samples
+}
+
+func (s Samples) fromIntermediate(in []Sample) Samples {
+	list := ps.NewList()
+	for _, sample := range in {
+		list = list.Cons(sample)
+	}
+	return Samples{list}
+}
+
+// MarshalJSON implements json.Marshaller
+func (s Samples) MarshalJSON() ([]byte, error) {
+	buf := bytes.Buffer{}
+	var err error
+	if s.List == nil {
+		err = json.NewEncoder(&buf).Encode(nil)
+		return buf.Bytes(), err
+	}
+
+	err = json.NewEncoder(&buf).Encode(s.toIntermediate())
+	return buf.Bytes(), err
+}
+
+// UnmarshalJSON implements json.Unmarshaler
+func (s *Samples) UnmarshalJSON(input []byte) error {
+	in := []Sample{}
+	if err := json.NewDecoder(bytes.NewBuffer(input)).Decode(&in); err != nil {
+		return err
+	}
+	*s = Samples{}.fromIntermediate(in)
+	return nil
+}
+
+// GobEncode implements gob.Marshaller
+func (s Samples) GobEncode() ([]byte, error) {
+	buf := bytes.Buffer{}
+	err := gob.NewEncoder(&buf).Encode(s.toIntermediate())
+	return buf.Bytes(), err
+}
+
+// GobDecode implements gob.Unmarshaller
+func (s *Samples) GobDecode(input []byte) error {
+	in := []Sample{}
+	if err := gob.NewDecoder(bytes.NewBuffer(input)).Decode(&in); err != nil {
+		return err
+	}
+	*s = Samples{}.fromIntermediate(in)
+	return nil
+}