Add ratio calculation to take the ratio of two calculations.

perf/go/parser/funcs.go

 package parser
 
 import (
         "fmt"
+        "math"
         "net/url"
         "strconv"
 
         "skia.googlesource.com/buildbot.git/perf/go/config"
         "skia.googlesource.com/buildbot.git/perf/go/types"
         "skia.googlesource.com/buildbot.git/perf/go/vec"
 )
 
 type FilterFunc struct{}
 
@@ skipping 153 matching lines @@
                 }
         }
         return []*types.PerfTrace{ret}, nil
 }
 
 func (AveFunc) Describe() string {
         return `ave() averages the values of all argument traces into a single trace.`
 }
 
 var aveFunc = AveFunc{}
+
+type RatioFunc struct{}
+
+func (RatioFunc) Eval(ctx *Context, node *Node) ([]*types.PerfTrace, error) {
+        if len(node.Args) != 2 {
+                return nil, fmt.Errorf("ratio() takes two arguments")
+        }
+
+        tracesA, err := node.Args[0].Eval(ctx)
+        if err != nil {
+                return nil, fmt.Errorf("ratio() argument failed to evaluate: %s", err)
+        }
+
+        tracesB, err := node.Args[1].Eval(ctx)
+        if err != nil {
+                return nil, fmt.Errorf("ratio() argument failed to evaluate: %s", err)
+        }
+
+        ret := types.NewPerfTraceN(len(tracesA[0].Values))
+        for i, _ := range ret.Values {
+                ret.Values[i] = tracesA[0].Values[i] / tracesB[0].Values[i]
+                if math.IsInf(ret.Values[i], 0) {
+                        ret.Values[i] = config.MISSING_DATA_SENTINEL
+                }
+        }
+        return []*types.PerfTrace{ret}, nil
+}
+
+func (RatioFunc) Describe() string {
+        return `ratio(a, b) returns the point by point ratio of two traces.
+                That is, it returns a trace with a[i]/b[i] for every point in a and b.`
+}
+
+var ratioFunc = RatioFunc{}