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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Code to execute a parsed template.
package template
import (
"bytes"
"io"
"reflect"
"strings"
)
// Internal state for executing a Template. As we evaluate the struct,
// the data item descends into the fields associated with sections, etc.
// Parent is used to walk upwards to find variables higher in the tree.
type state struct {
parent *state // parent in hierarchy
data reflect.Value // the driver data for this section etc.
wr io.Writer // where to send output
buf [2]bytes.Buffer // alternating buffers used when chaining formatters
}
func (parent *state) clone(data reflect.Value) *state {
return &state{parent: parent, data: data, wr: parent.wr}
}
// Evaluate interfaces and pointers looking for a value that can look up the name, via a
// struct field, method, or map key, and return the result of the lookup.
func (t *Template) lookup(st *state, v reflect.Value, name string) reflect.Value {
for v.IsValid() {
typ := v.Type()
if n := v.Type().NumMethod(); n > 0 {
for i := 0; i < n; i++ {
m := typ.Method(i)
mtyp := m.Type
if m.Name == name && mtyp.NumIn() == 1 && mtyp.NumOut() == 1 {
if !isExported(name) {
t.execError(st, t.linenum, "name not exported: %s in type %s", name, st.data.Type())
}
return v.Method(i).Call(nil)[0]
}
}
}
switch av := v; av.Kind() {
case reflect.Ptr:
v = av.Elem()
case reflect.Interface:
v = av.Elem()
case reflect.Struct:
if !isExported(name) {
t.execError(st, t.linenum, "name not exported: %s in type %s", name, st.data.Type())
}
return av.FieldByName(name)
case reflect.Map:
if v := av.MapIndex(reflect.ValueOf(name)); v.IsValid() {
return v
}
return reflect.Zero(typ.Elem())
default:
return reflect.Value{}
}
}
return v
}
// indirectPtr returns the item numLevels levels of indirection below the value.
// It is forgiving: if the value is not a pointer, it returns it rather than giving
// an error. If the pointer is nil, it is returned as is.
func indirectPtr(v reflect.Value, numLevels int) reflect.Value {
for i := numLevels; v.IsValid() && i > 0; i++ {
if p := v; p.Kind() == reflect.Ptr {
if p.IsNil() {
return v
}
v = p.Elem()
} else {
break
}
}
return v
}
// Walk v through pointers and interfaces, extracting the elements within.
func indirect(v reflect.Value) reflect.Value {
loop:
for v.IsValid() {
switch av := v; av.Kind() {
case reflect.Ptr:
v = av.Elem()
case reflect.Interface:
v = av.Elem()
default:
break loop
}
}
return v
}
// If the data for this template is a struct, find the named variable.
// Names of the form a.b.c are walked down the data tree.
// The special name "@" (the "cursor") denotes the current data.
// The value coming in (st.data) might need indirecting to reach
// a struct while the return value is not indirected - that is,
// it represents the actual named field. Leading stars indicate
// levels of indirection to be applied to the value.
func (t *Template) findVar(st *state, s string) reflect.Value {
data := st.data
flattenedName := strings.TrimLeft(s, "*")
numStars := len(s) - len(flattenedName)
s = flattenedName
if s == "@" {
return indirectPtr(data, numStars)
}
for _, elem := range strings.Split(s, ".", -1) {
// Look up field; data must be a struct or map.
data = t.lookup(st, data, elem)
if !data.IsValid() {
return reflect.Value{}
}
}
return indirectPtr(data, numStars)
}
// Is there no data to look at?
func empty(v reflect.Value) bool {
v = indirect(v)
if !v.IsValid() {
return true
}
switch v.Kind() {
case reflect.Bool:
return v.Bool() == false
case reflect.String:
return v.String() == ""
case reflect.Struct:
return false
case reflect.Map:
return false
case reflect.Array:
return v.Len() == 0
case reflect.Slice:
return v.Len() == 0
}
return false
}
// Look up a variable or method, up through the parent if necessary.
func (t *Template) varValue(name string, st *state) reflect.Value {
field := t.findVar(st, name)
if !field.IsValid() {
if st.parent == nil {
t.execError(st, t.linenum, "name not found: %s in type %s", name, st.data.Type())
}
return t.varValue(name, st.parent)
}
return field
}
func (t *Template) format(wr io.Writer, fmt string, val []interface{}, v *variableElement, st *state) {
fn := t.formatter(fmt)
if fn == nil {
t.execError(st, v.linenum, "missing formatter %s for variable", fmt)
}
fn(wr, fmt, val...)
}
// Evaluate a variable, looking up through the parent if necessary.
// If it has a formatter attached ({var|formatter}) run that too.
func (t *Template) writeVariable(v *variableElement, st *state) {
// Resolve field names
val := make([]interface{}, len(v.args))
for i, arg := range v.args {
if name, ok := arg.(fieldName); ok {
val[i] = t.varValue(string(name), st).Interface()
} else {
val[i] = arg
}
}
for i, fmt := range v.fmts[:len(v.fmts)-1] {
b := &st.buf[i&1]
b.Reset()
t.format(b, fmt, val, v, st)
val = val[0:1]
val[0] = b.Bytes()
}
t.format(st.wr, v.fmts[len(v.fmts)-1], val, v, st)
}
// Execute element i. Return next index to execute.
func (t *Template) executeElement(i int, st *state) int {
switch elem := t.elems[i].(type) {
case *textElement:
st.wr.Write(elem.text)
return i + 1
case *literalElement:
st.wr.Write(elem.text)
return i + 1
case *variableElement:
t.writeVariable(elem, st)
return i + 1
case *sectionElement:
t.executeSection(elem, st)
return elem.end
case *repeatedElement:
t.executeRepeated(elem, st)
return elem.end
}
e := t.elems[i]
t.execError(st, 0, "internal error: bad directive in execute: %v %T\n", reflect.ValueOf(e).Interface(), e)
return 0
}
// Execute the template.
func (t *Template) execute(start, end int, st *state) {
for i := start; i < end; {
i = t.executeElement(i, st)
}
}
// Execute a .section
func (t *Template) executeSection(s *sectionElement, st *state) {
// Find driver data for this section. It must be in the current struct.
field := t.varValue(s.field, st)
if !field.IsValid() {
t.execError(st, s.linenum, ".section: cannot find field %s in %s", s.field, st.data.Type())
}
st = st.clone(field)
start, end := s.start, s.or
if !empty(field) {
// Execute the normal block.
if end < 0 {
end = s.end
}
} else {
// Execute the .or block. If it's missing, do nothing.
start, end = s.or, s.end
if start < 0 {
return
}
}
for i := start; i < end; {
i = t.executeElement(i, st)
}
}
// Return the result of calling the Iter method on v, or nil.
func iter(v reflect.Value) reflect.Value {
for j := 0; j < v.Type().NumMethod(); j++ {
mth := v.Type().Method(j)
fv := v.Method(j)
ft := fv.Type()
// TODO(rsc): NumIn() should return 0 here, because ft is from a curried FuncValue.
if mth.Name != "Iter" || ft.NumIn() != 1 || ft.NumOut() != 1 {
continue
}
ct := ft.Out(0)
if ct.Kind() != reflect.Chan ||
ct.ChanDir()&reflect.RecvDir == 0 {
continue
}
return fv.Call(nil)[0]
}
return reflect.Value{}
}
// Execute a .repeated section
func (t *Template) executeRepeated(r *repeatedElement, st *state) {
// Find driver data for this section. It must be in the current struct.
field := t.varValue(r.field, st)
if !field.IsValid() {
t.execError(st, r.linenum, ".repeated: cannot find field %s in %s", r.field, st.data.Type())
}
field = indirect(field)
start, end := r.start, r.or
if end < 0 {
end = r.end
}
if r.altstart >= 0 {
end = r.altstart
}
first := true
// Code common to all the loops.
loopBody := func(newst *state) {
// .alternates between elements
if !first && r.altstart >= 0 {
for i := r.altstart; i < r.altend; {
i = t.executeElement(i, newst)
}
}
first = false
for i := start; i < end; {
i = t.executeElement(i, newst)
}
}
if array := field; array.Kind() == reflect.Array || array.Kind() == reflect.Slice {
for j := 0; j < array.Len(); j++ {
loopBody(st.clone(array.Index(j)))
}
} else if m := field; m.Kind() == reflect.Map {
for _, key := range m.MapKeys() {
loopBody(st.clone(m.MapIndex(key)))
}
} else if ch := iter(field); ch.IsValid() {
for {
e, ok := ch.Recv()
if !ok {
break
}
loopBody(st.clone(e))
}
} else {
t.execError(st, r.linenum, ".repeated: cannot repeat %s (type %s)",
r.field, field.Type())
}
if first {
// Empty. Execute the .or block, once. If it's missing, do nothing.
start, end := r.or, r.end
if start >= 0 {
newst := st.clone(field)
for i := start; i < end; {
i = t.executeElement(i, newst)
}
}
return
}
}
// A valid delimiter must contain no space and be non-empty.
func validDelim(d []byte) bool {
if len(d) == 0 {
return false
}
for _, c := range d {
if isSpace(c) {
return false
}
}
return true
}