go-spew/spew/common.go

/*
 * Copyright (c) 2013 Dave Collins <dave@davec.name>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

package spew

import (
	"fmt"
	"io"
	"reflect"
	"strconv"
	"unsafe"
)

// reflectValue mirrors the struct layout of the reflect package Value type.
var reflectValue struct {
	typ  unsafe.Pointer
	val  unsafe.Pointer
	flag uintptr
}

// flagIndir indicates whether the value field of a reflect.Value is the actual
// data or a pointer to the data.
const flagIndir = 1 << 1

// unsafeReflectValue converts the passed reflect.Value into a one that bypasses
// the typical safety restrictions preventing access to unaddressable and
// unexported data.  It works by digging the raw pointer to the underlying
// value out of the protected value and generating a new unprotected (unsafe)
// reflect.Value to it.
//
// This allows us to check for implementations of the Stringer and error
// interfaces to be used for pretty printing ordinarily unaddressable and
// inaccessible values such as unexported struct fields.
func unsafeReflectValue(v reflect.Value) (rv reflect.Value) {
	indirects := 1
	vt := v.Type()
	upv := unsafe.Pointer(uintptr(unsafe.Pointer(&v)) + unsafe.Offsetof(reflectValue.val))
	rvf := *(*uintptr)(unsafe.Pointer(uintptr(unsafe.Pointer(&v)) + unsafe.Offsetof(reflectValue.flag)))
	if rvf&flagIndir != 0 {
		vt = reflect.PtrTo(v.Type())
		indirects++
	}

	pv := reflect.NewAt(vt, upv)
	rv = pv
	for i := 0; i < indirects; i++ {
		rv = rv.Elem()
	}
	return rv
}

// Some constants in the form of bytes to avoid string overhead.  This mirrors
// the technique used in the fmt package.
var (
	panicBytes            = []byte("(PANIC=")
	plusBytes             = []byte("+")
	iBytes                = []byte("i")
	trueBytes             = []byte("true")
	falseBytes            = []byte("false")
	interfaceBytes        = []byte("(interface {})")
	commaNewlineBytes     = []byte(",\n")
	newlineBytes          = []byte("\n")
	openBraceBytes        = []byte("{")
	openBraceNewlineBytes = []byte("{\n")
	closeBraceBytes       = []byte("}")
	asteriskBytes         = []byte("*")
	colonBytes            = []byte(":")
	colonSpaceBytes       = []byte(": ")
	openParenBytes        = []byte("(")
	closeParenBytes       = []byte(")")
	spaceBytes            = []byte(" ")
	pointerChainBytes     = []byte("->")
	nilAngleBytes         = []byte("<nil>")
	maxNewlineBytes       = []byte("<max depth reached>\n")
	maxShortBytes         = []byte("<max>")
	circularBytes         = []byte("<already shown>")
	circularShortBytes    = []byte("<shown>")
	invalidAngleBytes     = []byte("<invalid>")
	openBracketBytes      = []byte("[")
	closeBracketBytes     = []byte("]")
	percentBytes          = []byte("%")
	precisionBytes        = []byte(".")
	openAngleBytes        = []byte("<")
	closeAngleBytes       = []byte(">")
	openMapBytes          = []byte("map[")
	closeMapBytes         = []byte("]")
)

// hexDigits is used to map a decimal value to a hex digit.
var hexDigits = "0123456789abcdef"

// catchPanic handles any panics that might occur during the handleMethods
// calls.
func catchPanic(w io.Writer, v reflect.Value) {
	if err := recover(); err != nil {
		w.Write(panicBytes)
		fmt.Fprintf(w, "%v", err)
		w.Write(closeParenBytes)
	}
}

// handleMethods attempts to call the Error and String methods on the underlying
// type the passed reflect.Value represents and outputes the result to Writer w.
//
// It handles panics in any called methods by catching and displaying the error
// as the formatted value.
func handleMethods(cs *ConfigState, w io.Writer, v reflect.Value) (handled bool) {
	// We need an interface to check if the type implements the error or
	// Stringer interface.  However, the reflect package won't give us an
	// an interface on certain things like unexported struct fields in order
	// to enforce visibility rules.  We use unsafe to bypass these restrictions
	// since this package does not mutate the values.
	if !v.CanInterface() {
		v = unsafeReflectValue(v)
	}

	// Choose whether or not to do error and Stringer interface lookups against
	// the base type or a pointer to the base type depending on settings.
	// Technically calling one of these methods with a pointer receiver can
	// mutate the value, however, types which choose to satisify an error or
	// Stringer interface with a pointer receiver should not be mutating their
	// state inside these interface methods.
	var viface interface{}
	if !cs.DisablePointerMethods {
		if !v.CanAddr() {
			v = unsafeReflectValue(v)
		}
		viface = v.Addr().Interface()
	} else {
		viface = v.Interface()
	}

	// Is it an error or Stringer?
	switch iface := viface.(type) {
	case error:
		defer catchPanic(w, v)
		w.Write([]byte(iface.Error()))
		return true

	case fmt.Stringer:
		defer catchPanic(w, v)
		w.Write([]byte(iface.String()))
		return true
	}
	return false
}

// printBool outputs a boolean value as true or false to Writer w.
func printBool(w io.Writer, val bool) {
	if val {
		w.Write(trueBytes)
	} else {
		w.Write(falseBytes)
	}
}

// printInt outputs a signed integer value to Writer w.
func printInt(w io.Writer, val int64) {
	w.Write([]byte(strconv.FormatInt(val, 10)))
}

// printUint outputs an unsigned integer value to Writer w.
func printUint(w io.Writer, val uint64) {
	w.Write([]byte(strconv.FormatUint(val, 10)))
}

// printFloat outputs a floating point value using the specified precision,
// which is expected to be 32 or 64bit, to Writer w.
func printFloat(w io.Writer, val float64, precision int) {
	w.Write([]byte(strconv.FormatFloat(val, 'g', -1, precision)))
}

// printComplex outputs a complex value using the specified float precision
// for the real and imaginary parts to Writer w.
func printComplex(w io.Writer, c complex128, floatPrecision int) {
	r := real(c)
	w.Write(openParenBytes)
	w.Write([]byte(strconv.FormatFloat(r, 'g', -1, floatPrecision)))
	i := imag(c)
	if i >= 0 {
		w.Write(plusBytes)
	}
	w.Write([]byte(strconv.FormatFloat(i, 'g', -1, floatPrecision)))
	w.Write(iBytes)
	w.Write(closeParenBytes)
}

// printHexPtr outputs a uintptr formatted as hexidecimal with a leading '0x'
// prefix to Writer w.
func printHexPtr(w io.Writer, p uintptr) {
	// Null pointer.
	num := uint64(p)
	if num == 0 {
		w.Write(nilAngleBytes)
		return
	}

	// Max uint64 is 16 bytes in hex + 2 bytes for '0x' prefix
	buf := make([]byte, 18)

	// It's simpler to construct the hex string right to left.
	base := uint64(16)
	i := len(buf) - 1
	for num >= base {
		buf[i] = hexDigits[num%base]
		num /= base
		i--
	}
	buf[i] = hexDigits[num]

	// Add '0x' prefix.
	i--
	buf[i] = 'x'
	i--
	buf[i] = '0'

	// Strip unused leading bytes.
	buf = buf[i:]
	w.Write(buf)
}