template: for range on a map, sort the keys if feasible.

Fixes #2696.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5543055

src/pkg/text/template/doc.go

@@ -50,7 +50,9 @@ data, defined in detail below.
 		The value of the pipeline must be an array, slice, or map. If
 		the value of the pipeline has length zero, nothing is output;
 		otherwise, dot is set to the successive elements of the array,
-		slice, or map and T1 is executed.
+		slice, or map and T1 is executed. If the value is a map and the
+		keys are of basic type with a defined order ("comparable"), the
+		elements will be visited in sorted key order.
 
 	{{range pipeline}} T1 {{else}} T0 {{end}}
 		The value of the pipeline must be an array, slice, or map. If

src/pkg/text/template/exec.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"reflect"
 	"runtime"
+	"sort"
 	"strings"
 	"text/template/parse"
 )
@@ -234,7 +235,7 @@ func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
 		if val.Len() == 0 {
 			break
 		}
-		for _, key := range val.MapKeys() {
+		for _, key := range sortKeys(val.MapKeys()) {
 			oneIteration(key, val.MapIndex(key))
 		}
 		return
@@ -676,3 +677,44 @@ func (s *state) printValue(n parse.Node, v reflect.Value) {
 	}
 	fmt.Fprint(s.wr, v.Interface())
 }
+
+// Types to help sort the keys in a map for reproducible output.
+
+type rvs []reflect.Value
+
+func (x rvs) Len() int      { return len(x) }
+func (x rvs) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
+
+type rvInts struct{ rvs }
+
+func (x rvInts) Less(i, j int) bool { return x.rvs[i].Int() < x.rvs[j].Int() }
+
+type rvUints struct{ rvs }
+
+func (x rvUints) Less(i, j int) bool { return x.rvs[i].Uint() < x.rvs[j].Uint() }
+
+type rvFloats struct{ rvs }
+
+func (x rvFloats) Less(i, j int) bool { return x.rvs[i].Float() < x.rvs[j].Float() }
+
+type rvStrings struct{ rvs }
+
+func (x rvStrings) Less(i, j int) bool { return x.rvs[i].String() < x.rvs[j].String() }
+
+// sortKeys sorts (if it can) the slice of reflect.Values, which is a slice of map keys.
+func sortKeys(v []reflect.Value) []reflect.Value {
+	if len(v) <= 1 {
+		return v
+	}
+	switch v[0].Kind() {
+	case reflect.Float32, reflect.Float64:
+		sort.Sort(rvFloats{v})
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		sort.Sort(rvInts{v})
+	case reflect.String:
+		sort.Sort(rvStrings{v})
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		sort.Sort(rvUints{v})
+	}
+	return v
+}

src/pkg/text/template/exec_test.go

@@ -11,7 +11,6 @@ import (
 	"fmt"
 	"os"
 	"reflect"
-	"sort"
 	"strings"
 	"testing"
 )
@@ -169,18 +168,6 @@ func (t *T) MAdd(a int, b []int) []int {
 	return v
 }
 
-// MSort is used to sort map keys for stable output. (Nice trick!)
-func (t *T) MSort(m map[string]int) []string {
-	keys := make([]string, len(m))
-	i := 0
-	for k := range m {
-		keys[i] = k
-		i++
-	}
-	sort.Strings(keys)
-	return keys
-}
-
 // EPERM returns a value and an error according to its argument.
 func (t *T) EPERM(error bool) (bool, error) {
 	if error {
@@ -410,9 +397,9 @@ var execTests = []execTest{
 	{"range empty else", "{{range .SIEmpty}}-{{.}}-{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
 	{"range []bool", "{{range .SB}}-{{.}}-{{end}}", "-true--false-", tVal, true},
 	{"range []int method", "{{range .SI | .MAdd .I}}-{{.}}-{{end}}", "-20--21--22-", tVal, true},
-	{"range map", "{{range .MSI | .MSort}}-{{.}}-{{end}}", "-one--three--two-", tVal, true},
+	{"range map", "{{range .MSI}}-{{.}}-{{end}}", "-1--3--2-", tVal, true},
 	{"range empty map no else", "{{range .MSIEmpty}}-{{.}}-{{end}}", "", tVal, true},
-	{"range map else", "{{range .MSI | .MSort}}-{{.}}-{{else}}EMPTY{{end}}", "-one--three--two-", tVal, true},
+	{"range map else", "{{range .MSI}}-{{.}}-{{else}}EMPTY{{end}}", "-1--3--2-", tVal, true},
 	{"range empty map else", "{{range .MSIEmpty}}-{{.}}-{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
 	{"range empty interface", "{{range .Empty3}}-{{.}}-{{else}}EMPTY{{end}}", "-7--8-", tVal, true},
 	{"range empty nil", "{{range .Empty0}}-{{.}}-{{end}}", "", tVal, true},