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pretty.go
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pretty.go
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package sexpr
// This file implements the algorithm described in Derek C. Oppen's
// 1979 Stanford technical report, "Pretty Printing".
import (
"bytes"
"fmt"
"reflect"
)
func MarshalIndent(v interface{}) ([]byte, error) {
p := printer{width: margin}
if err := pretty(&p, reflect.ValueOf(v)); err != nil {
return nil, err
}
return p.Bytes(), nil
}
const margin = 80
type token struct {
kind rune // one of "s ()" (string, blank, start, end)
str string
size int
}
type printer struct {
tokens []*token // FIFO buffer
stack []*token // stack of open ' ' and '(' tokens
rtotal int // total number of spaces needed to print stream
bytes.Buffer
indents []int
width int // remaining space
}
func (p *printer) string(str string) {
tok := &token{kind: 's', str: str, size: len(str)}
if len(p.stack) == 0 {
p.print(tok)
} else {
p.tokens = append(p.tokens, tok)
p.rtotal += len(str)
}
}
func (p *printer) pop() (top *token) {
last := len(p.stack) - 1
top, p.stack = p.stack[last], p.stack[:last]
return
}
func (p *printer) begin() {
if len(p.stack) == 0 {
p.rtotal = 1
}
t := &token{kind: '(', size: -p.rtotal}
p.tokens = append(p.tokens, t)
p.stack = append(p.stack, t) // push
p.string("(")
}
func (p *printer) end() {
p.string(")")
p.tokens = append(p.tokens, &token{kind: ')'})
x := p.pop()
x.size += p.rtotal
if x.kind == ' ' {
p.pop().size += p.rtotal
}
if len(p.stack) == 0 {
for _, tok := range p.tokens {
p.print(tok)
}
p.tokens = nil
}
}
func (p *printer) space() {
last := len(p.stack) - 1
x := p.stack[last]
if x.kind == ' ' {
x.size += p.rtotal
p.stack = p.stack[:last] // pop
}
t := &token{kind: ' ', size: -p.rtotal}
p.tokens = append(p.tokens, t)
p.stack = append(p.stack, t)
p.rtotal++
}
func (p *printer) print(t *token) {
switch t.kind {
case 's':
p.WriteString(t.str)
p.width -= len(t.str)
case '(':
p.indents = append(p.indents, p.width)
case ')':
p.indents = p.indents[:len(p.indents)-1] // pop
case ' ':
if t.size > p.width {
p.width = p.indents[len(p.indents)-1] - 1
fmt.Fprintf(&p.Buffer, "\n%*s", margin-p.width, "")
} else {
p.WriteByte(' ')
p.width--
}
}
}
func (p *printer) stringf(format string, args ...interface{}) {
p.string(fmt.Sprintf(format, args...))
}
func pretty(p *printer, v reflect.Value) error {
switch v.Kind() {
case reflect.Invalid:
p.string("nil")
case reflect.Int, reflect.Int8, reflect.Int16,
reflect.Int32, reflect.Int64:
p.stringf("%d", v.Int())
case reflect.Uint, reflect.Uint8, reflect.Uint16,
reflect.Uint32, reflect.Uint64, reflect.Uintptr:
p.stringf("%d", v.Uint())
case reflect.String:
p.stringf("%q", v.String())
case reflect.Array, reflect.Slice: // (value ...)
p.begin()
for i := 0; i < v.Len(); i++ {
if i > 0 {
p.space()
}
if err := pretty(p, v.Index(i)); err != nil {
return err
}
}
p.end()
case reflect.Struct: // ((name value ...)
p.begin()
for i := 0; i < v.NumField(); i++ {
if i > 0 {
p.space()
}
p.begin()
p.string(v.Type().Field(i).Name)
p.space()
if err := pretty(p, v.Field(i)); err != nil {
return err
}
p.end()
}
p.end()
case reflect.Map: // ((key value ...)
p.begin()
for i, key := range v.MapKeys() {
if i > 0 {
p.space()
}
p.begin()
if err := pretty(p, key); err != nil {
return err
}
p.space()
if err := pretty(p, v.MapIndex(key)); err != nil {
return err
}
p.end()
}
p.end()
case reflect.Ptr:
return pretty(p, v.Elem())
default: // float, complex, bool, chan, func, interface
return fmt.Errorf("unsupported type: %s", v.Type())
}
return nil
}