Add ratio calculation to take the ratio of two calculations.

perf/go/parser/parser.go

 package parser
 
 import (
         "fmt"
 
         "skia.googlesource.com/buildbot.git/perf/go/types"
 )
 
 type NodeType int
 
@@ skipping 52 matching lines @@
 
 // NewContext create a new parsing context that includes the basic functions.
 func NewContext(tile *types.Tile) *Context {
         return &Context{
                 Tile: tile,
                 Funcs: map[string]Func{
                         "filter": filterFunc,
                         "norm":   normFunc,
                         "fill":   fillFunc,
                         "ave":    aveFunc,
+                        "ratio":  ratioFunc,
                 },
         }
 }
 
 // Eval parses and evaluates the given string expression and returns the Traces, or
 // an error.
 func (ctx *Context) Eval(exp string) ([]*types.PerfTrace, error) {
         ctx.formula = exp
         n, err := parse(exp)
         if err != nil {
@@ skipping 70 matching lines @@
                         l.nextItem()
                         continue
                 case itemRParen:
                         break Loop
                 default:
                         return fmt.Errorf("Invalid token in args: %d", it.typ)
                 }
         }
         return nil
 }