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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.
+
+/* The datafmt package implements syntax-directed, type-driven formatting
+ of arbitrary data structures. Formatting a data structure consists of
+ two phases: first, a parser reads a format specification and builds a
+ "compiled" format. Then, the format can be applied repeatedly to
+ arbitrary values. Applying a format to a value evaluates to a []byte
+ containing the formatted value bytes, or nil.
+
+ A format specification is a set of package declarations and format rules:
+
+ Format = [ Entry { ";" Entry } [ ";" ] ] .
+ Entry = PackageDecl | FormatRule .
+
+ (The syntax of a format specification is presented in the same EBNF
+ notation as used in the Go language specification. The syntax of white
+ space, comments, identifiers, and string literals is the same as in Go.)
+
+ A package declaration binds a package name (such as 'ast') to a
+ package import path (such as '"go/ast"'). Each package used (in
+ a type name, see below) must be declared once before use.
+
+ PackageDecl = PackageName ImportPath .
+ PackageName = identifier .
+ ImportPath = string .
+
+ A format rule binds a rule name to a format expression. A rule name
+ may be a type name or one of the special names 'default' or '/'.
+ A type name may be the name of a predeclared type (for example, 'int',
+ 'float32', etc.), the package-qualified name of a user-defined type
+ (for example, 'ast.MapType'), or an identifier indicating the structure
+ of unnamed composite types ('array', 'chan', 'func', 'interface', 'map',
+ or 'ptr'). Each rule must have a unique name; rules can be declared in
+ any order.
+
+ FormatRule = RuleName "=" Expression .
+ RuleName = TypeName | "default" | "/" .
+ TypeName = [ PackageName "." ] identifier .
+
+ To format a value, the value's type name is used to select the format rule
+ (there is an override mechanism, see below). The format expression of the
+ selected rule specifies how the value is formatted. Each format expression,
+ when applied to a value, evaluates to a byte sequence or nil.
+
+ In its most general form, a format expression is a list of alternatives,
+ each of which is a sequence of operands:
+
+ Expression = [ Sequence ] { "|" [ Sequence ] } .
+ Sequence = Operand { Operand } .
+
+ The formatted result produced by an expression is the result of the first
+ alternative sequence that evaluates to a non-nil result; if there is no
+ such alternative, the expression evaluates to nil. The result produced by
+ an operand sequence is the concatenation of the results of its operands.
+ If any operand in the sequence evaluates to nil, the entire sequence
+ evaluates to nil.
+
+ There are five kinds of operands:
+
+ Operand = Literal | Field | Group | Option | Repetition .
+
+ Literals evaluate to themselves, with two substitutions. First,
+ %-formats expand in the manner of fmt.Printf, with the current value
+ passed as the parameter. Second, the current indentation (see below)
+ is inserted after every newline or form feed character.
+
+ Literal = string .
+
+ This table shows string literals applied to the value 42 and the
+ corresponding formatted result:
+
+ "foo" foo
+ "%x" 2a
+ "x = %d" x = 42
+ "%#x = %d" 0x2a = 42
+
+ A field operand is a field name optionally followed by an alternate
+ rule name. The field name may be an identifier or one of the special
+ names @ or *.
+
+ Field = FieldName [ ":" RuleName ] .
+ FieldName = identifier | "@" | "*" .
+
+ If the field name is an identifier, the current value must be a struct,
+ and there must be a field with that name in the struct. The same lookup
+ rules apply as in the Go language (for instance, the name of an anonymous
+ field is the unqualified type name). The field name denotes the field
+ value in the struct. If the field is not found, formatting is aborted
+ and an error message is returned. (TODO consider changing the semantics
+ such that if a field is not found, it evaluates to nil).
+
+ The special name '@' denotes the current value.
+
+ The meaning of the special name '*' depends on the type of the current
+ value:
+
+ array, slice types array, slice element (inside {} only, see below)
+ interfaces value stored in interface
+ pointers value pointed to by pointer
+
+ (Implementation restriction: channel, function and map types are not
+ supported due to missing reflection support).
+
+ Fields are evaluated as follows: If the field value is nil, or an array
+ or slice element does not exist, the result is nil (see below for details
+ on array/slice elements). If the value is not nil the field value is
+ formatted (recursively) using the rule corresponding to its type name,
+ or the alternate rule name, if given.
+
+ The following example shows a complete format specification for a
+ struct 'myPackage.Point'. Assume the package
+
+ package myPackage // in directory myDir/myPackage
+ type Point struct {
+ name string;
+ x, y int;
+ }
+
+ Applying the format specification
+
+ myPackage "myDir/myPackage";
+ int = "%d";
+ hexInt = "0x%x";
+ string = "---%s---";
+ myPackage.Point = name "{" x ", " y:hexInt "}";
+
+ to the value myPackage.Point{"foo", 3, 15} results in
+
+ ---foo---{3, 0xf}
+
+ Finally, an operand may be a grouped, optional, or repeated expression.
+ A grouped expression ("group") groups a more complex expression (body)
+ so that it can be used in place of a single operand:
+
+ Group = "(" [ Indentation ">>" ] Body ")" .
+ Indentation = Expression .
+ Body = Expression .
+
+ A group body may be prefixed by an indentation expression followed by '>>'.
+ The indentation expression is applied to the current value like any other
+ expression and the result, if not nil, is appended to the current indentation
+ during the evaluation of the body (see also formatting state, below).
+
+ An optional expression ("option") is enclosed in '[]' brackets.
+
+ Option = "[" Body "]" .
+
+ An option evaluates to its body, except that if the body evaluates to nil,
+ the option expression evaluates to an empty []byte. Thus an option's purpose
+ is to protect the expression containing the option from a nil operand.
+
+ A repeated expression ("repetition") is enclosed in '{}' braces.
+
+ Repetition = "{" Body [ "/" Separator ] "}" .
+ Separator = Expression .
+
+ A repeated expression is evaluated as follows: The body is evaluated
+ repeatedly and its results are concatenated until the body evaluates
+ to nil. The result of the repetition is the (possibly empty) concatenation,
+ but it is never nil. An implicit index is supplied for the evaluation of
+ the body: that index is used to address elements of arrays or slices. If
+ the corresponding elements do not exist, the field denoting the element
+ evaluates to nil (which in turn may terminate the repetition).
+
+ The body of a repetition may be followed by a '/' and a "separator"
+ expression. If the separator is present, it is invoked between repetitions
+ of the body.
+
+ The following example shows a complete format specification for formatting
+ a slice of unnamed type. Applying the specification
+
+ int = "%b";
+ array = { * / ", " }; // array is the type name for an unnamed slice
+
+ to the value '[]int{2, 3, 5, 7}' results in
+
+ 10, 11, 101, 111
+
+ Default rule: If a format rule named 'default' is present, it is used for
+ formatting a value if no other rule was found. A common default rule is
+
+ default = "%v"
+
+ to provide default formatting for basic types without having to specify
+ a specific rule for each basic type.
+
+ Global separator rule: If a format rule named '/' is present, it is
+ invoked with the current value between literals. If the separator
+ expression evaluates to nil, it is ignored.
+
+ For instance, a global separator rule may be used to punctuate a sequence
+ of values with commas. The rules:
+
+ default = "%v";
+ / = ", ";
+
+ will format an argument list by printing each one in its default format,
+ separated by a comma and a space.
+*/
+package datafmt
+
+import (
+ "bytes"
+ "fmt"
+ "go/token"
+ "io"
+ "os"
+ "reflect"
+ "runtime"
+)
+
+
+// ----------------------------------------------------------------------------
+// Format representation
+
+// Custom formatters implement the Formatter function type.
+// A formatter is invoked with the current formatting state, the
+// value to format, and the rule name under which the formatter
+// was installed (the same formatter function may be installed
+// under different names). The formatter may access the current state
+// to guide formatting and use State.Write to append to the state's
+// output.
+//
+// A formatter must return a boolean value indicating if it evaluated
+// to a non-nil value (true), or a nil value (false).
+//
+type Formatter func(state *State, value interface{}, ruleName string) bool
+
+
+// A FormatterMap is a set of custom formatters.
+// It maps a rule name to a formatter function.
+//
+type FormatterMap map[string]Formatter
+
+
+// A parsed format expression is built from the following nodes.
+//
+type (
+ expr interface{}
+
+ alternatives []expr // x | y | z
+
+ sequence []expr // x y z
+
+ literal [][]byte // a list of string segments, possibly starting with '%'
+
+ field struct {
+ fieldName string // including "@", "*"
+ ruleName string // "" if no rule name specified
+ }
+
+ group struct {
+ indent, body expr // (indent >> body)
+ }
+
+ option struct {
+ body expr // [body]
+ }
+
+ repetition struct {
+ body, separator expr // {body / separator}
+ }
+
+ custom struct {
+ ruleName string
+ fun Formatter
+ }
+)
+
+
+// A Format is the result of parsing a format specification.
+// The format may be applied repeatedly to format values.
+//
+type Format map[string]expr
+
+
+// ----------------------------------------------------------------------------
+// Formatting
+
+// An application-specific environment may be provided to Format.Apply;
+// the environment is available inside custom formatters via State.Env().
+// Environments must implement copying; the Copy method must return an
+// complete copy of the receiver. This is necessary so that the formatter
+// can save and restore an environment (in case of an absent expression).
+//
+// If the Environment doesn't change during formatting (this is under
+// control of the custom formatters), the Copy function can simply return
+// the receiver, and thus can be very light-weight.
+//
+type Environment interface {
+ Copy() Environment
+}
+
+
+// State represents the current formatting state.
+// It is provided as argument to custom formatters.
+//
+type State struct {
+ fmt Format // format in use
+ env Environment // user-supplied environment
+ errors chan os.Error // not chan *Error (errors <- nil would be wrong!)
+ hasOutput bool // true after the first literal has been written
+ indent bytes.Buffer // current indentation
+ output bytes.Buffer // format output
+ linePos token.Position // position of line beginning (Column == 0)
+ default_ expr // possibly nil
+ separator expr // possibly nil
+}
+
+
+func newState(fmt Format, env Environment, errors chan os.Error) *State {
+ s := new(State)
+ s.fmt = fmt
+ s.env = env
+ s.errors = errors
+ s.linePos = token.Position{Line: 1}
+
+ // if we have a default rule, cache it's expression for fast access
+ if x, found := fmt["default"]; found {
+ s.default_ = x
+ }
+
+ // if we have a global separator rule, cache it's expression for fast access
+ if x, found := fmt["/"]; found {
+ s.separator = x
+ }
+
+ return s
+}
+
+
+// Env returns the environment passed to Format.Apply.
+func (s *State) Env() interface{} { return s.env }
+
+
+// LinePos returns the position of the current line beginning
+// in the state's output buffer. Line numbers start at 1.
+//
+func (s *State) LinePos() token.Position { return s.linePos }
+
+
+// Pos returns the position of the next byte to be written to the
+// output buffer. Line numbers start at 1.
+//
+func (s *State) Pos() token.Position {
+ offs := s.output.Len()
+ return token.Position{Line: s.linePos.Line, Column: offs - s.linePos.Offset, Offset: offs}
+}
+
+
+// Write writes data to the output buffer, inserting the indentation
+// string after each newline or form feed character. It cannot return an error.
+//
+func (s *State) Write(data []byte) (int, os.Error) {
+ n := 0
+ i0 := 0
+ for i, ch := range data {
+ if ch == '\n' || ch == '\f' {
+ // write text segment and indentation
+ n1, _ := s.output.Write(data[i0 : i+1])
+ n2, _ := s.output.Write(s.indent.Bytes())
+ n += n1 + n2
+ i0 = i + 1
+ s.linePos.Offset = s.output.Len()
+ s.linePos.Line++
+ }
+ }
+ n3, _ := s.output.Write(data[i0:])
+ return n + n3, nil
+}
+
+
+type checkpoint struct {
+ env Environment
+ hasOutput bool
+ outputLen int
+ linePos token.Position
+}
+
+
+func (s *State) save() checkpoint {
+ saved := checkpoint{nil, s.hasOutput, s.output.Len(), s.linePos}
+ if s.env != nil {
+ saved.env = s.env.Copy()
+ }
+ return saved
+}
+
+
+func (s *State) restore(m checkpoint) {
+ s.env = m.env
+ s.output.Truncate(m.outputLen)
+}
+
+
+func (s *State) error(msg string) {
+ s.errors <- os.NewError(msg)
+ runtime.Goexit()
+}
+
+
+// TODO At the moment, unnamed types are simply mapped to the default
+// names below. For instance, all unnamed arrays are mapped to
+// 'array' which is not really sufficient. Eventually one may want
+// to be able to specify rules for say an unnamed slice of T.
+//
+
+func typename(typ reflect.Type) string {
+ switch typ.(type) {
+ case *reflect.ArrayType:
+ return "array"
+ case *reflect.SliceType:
+ return "array"
+ case *reflect.ChanType:
+ return "chan"
+ case *reflect.FuncType:
+ return "func"
+ case *reflect.InterfaceType:
+ return "interface"
+ case *reflect.MapType:
+ return "map"
+ case *reflect.PtrType:
+ return "ptr"
+ }
+ return typ.String()
+}
+
+func (s *State) getFormat(name string) expr {
+ if fexpr, found := s.fmt[name]; found {
+ return fexpr
+ }
+
+ if s.default_ != nil {
+ return s.default_
+ }
+
+ s.error(fmt.Sprintf("no format rule for type: '%s'", name))
+ return nil
+}
+
+
+// eval applies a format expression fexpr to a value. If the expression
+// evaluates internally to a non-nil []byte, that slice is appended to
+// the state's output buffer and eval returns true. Otherwise, eval
+// returns false and the state remains unchanged.
+//
+func (s *State) eval(fexpr expr, value reflect.Value, index int) bool {
+ // an empty format expression always evaluates
+ // to a non-nil (but empty) []byte
+ if fexpr == nil {
+ return true
+ }
+
+ switch t := fexpr.(type) {
+ case alternatives:
+ // append the result of the first alternative that evaluates to
+ // a non-nil []byte to the state's output
+ mark := s.save()
+ for _, x := range t {
+ if s.eval(x, value, index) {
+ return true
+ }
+ s.restore(mark)
+ }
+ return false
+
+ case sequence:
+ // append the result of all operands to the state's output
+ // unless a nil result is encountered
+ mark := s.save()
+ for _, x := range t {
+ if !s.eval(x, value, index) {
+ s.restore(mark)
+ return false
+ }
+ }
+ return true
+
+ case literal:
+ // write separator, if any
+ if s.hasOutput {
+ // not the first literal
+ if s.separator != nil {
+ sep := s.separator // save current separator
+ s.separator = nil // and disable it (avoid recursion)
+ mark := s.save()
+ if !s.eval(sep, value, index) {
+ s.restore(mark)
+ }
+ s.separator = sep // enable it again
+ }
+ }
+ s.hasOutput = true
+ // write literal segments
+ for _, lit := range t {
+ if len(lit) > 1 && lit[0] == '%' {
+ // segment contains a %-format at the beginning
+ if lit[1] == '%' {
+ // "%%" is printed as a single "%"
+ s.Write(lit[1:])
+ } else {
+ // use s instead of s.output to get indentation right
+ fmt.Fprintf(s, string(lit), value.Interface())
+ }
+ } else {
+ // segment contains no %-formats
+ s.Write(lit)
+ }
+ }
+ return true // a literal never evaluates to nil
+
+ case *field:
+ // determine field value
+ switch t.fieldName {
+ case "@":
+ // field value is current value
+
+ case "*":
+ // indirection: operation is type-specific
+ switch v := value.(type) {
+ case *reflect.ArrayValue:
+ if v.Len() <= index {
+ return false
+ }
+ value = v.Elem(index)
+
+ case *reflect.SliceValue:
+ if v.IsNil() || v.Len() <= index {
+ return false
+ }
+ value = v.Elem(index)
+
+ case *reflect.MapValue:
+ s.error("reflection support for maps incomplete")
+
+ case *reflect.PtrValue:
+ if v.IsNil() {
+ return false
+ }
+ value = v.Elem()
+
+ case *reflect.InterfaceValue:
+ if v.IsNil() {
+ return false
+ }
+ value = v.Elem()
+
+ case *reflect.ChanValue:
+ s.error("reflection support for chans incomplete")
+
+ case *reflect.FuncValue:
+ s.error("reflection support for funcs incomplete")
+
+ default:
+ s.error(fmt.Sprintf("error: * does not apply to `%s`", value.Type()))
+ }
+
+ default:
+ // value is value of named field
+ var field reflect.Value
+ if sval, ok := value.(*reflect.StructValue); ok {
+ field = sval.FieldByName(t.fieldName)
+ if field == nil {
+ // TODO consider just returning false in this case
+ s.error(fmt.Sprintf("error: no field `%s` in `%s`", t.fieldName, value.Type()))
+ }
+ }
+ value = field
+ }
+
+ // determine rule
+ ruleName := t.ruleName
+ if ruleName == "" {
+ // no alternate rule name, value type determines rule
+ ruleName = typename(value.Type())
+ }
+ fexpr = s.getFormat(ruleName)
+
+ mark := s.save()
+ if !s.eval(fexpr, value, index) {
+ s.restore(mark)
+ return false
+ }
+ return true
+
+ case *group:
+ // remember current indentation
+ indentLen := s.indent.Len()
+
+ // update current indentation
+ mark := s.save()
+ s.eval(t.indent, value, index)
+ // if the indentation evaluates to nil, the state's output buffer
+ // didn't change - either way it's ok to append the difference to
+ // the current identation
+ s.indent.Write(s.output.Bytes()[mark.outputLen:s.output.Len()])
+ s.restore(mark)
+
+ // format group body
+ mark = s.save()
+ b := true
+ if !s.eval(t.body, value, index) {
+ s.restore(mark)
+ b = false
+ }
+
+ // reset indentation
+ s.indent.Truncate(indentLen)
+ return b
+
+ case *option:
+ // evaluate the body and append the result to the state's output
+ // buffer unless the result is nil
+ mark := s.save()
+ if !s.eval(t.body, value, 0) { // TODO is 0 index correct?
+ s.restore(mark)
+ }
+ return true // an option never evaluates to nil
+
+ case *repetition:
+ // evaluate the body and append the result to the state's output
+ // buffer until a result is nil
+ for i := 0; ; i++ {
+ mark := s.save()
+ // write separator, if any
+ if i > 0 && t.separator != nil {
+ // nil result from separator is ignored
+ mark := s.save()
+ if !s.eval(t.separator, value, i) {
+ s.restore(mark)
+ }
+ }
+ if !s.eval(t.body, value, i) {
+ s.restore(mark)
+ break
+ }
+ }
+ return true // a repetition never evaluates to nil
+
+ case *custom:
+ // invoke the custom formatter to obtain the result
+ mark := s.save()
+ if !t.fun(s, value.Interface(), t.ruleName) {
+ s.restore(mark)
+ return false
+ }
+ return true
+ }
+
+ panic("unreachable")
+ return false
+}
+
+
+// Eval formats each argument according to the format
+// f and returns the resulting []byte and os.Error. If
+// an error occurred, the []byte contains the partially
+// formatted result. An environment env may be passed
+// in which is available in custom formatters through
+// the state parameter.
+//
+func (f Format) Eval(env Environment, args ...interface{}) ([]byte, os.Error) {
+ if f == nil {
+ return nil, os.NewError("format is nil")
+ }
+
+ errors := make(chan os.Error)
+ s := newState(f, env, errors)
+
+ go func() {
+ for _, v := range args {
+ fld := reflect.NewValue(v)
+ if fld == nil {
+ errors <- os.NewError("nil argument")
+ return
+ }
+ mark := s.save()
+ if !s.eval(s.getFormat(typename(fld.Type())), fld, 0) { // TODO is 0 index correct?
+ s.restore(mark)
+ }
+ }
+ errors <- nil // no errors
+ }()
+
+ err := <-errors
+ return s.output.Bytes(), err
+}
+
+
+// ----------------------------------------------------------------------------
+// Convenience functions
+
+// Fprint formats each argument according to the format f
+// and writes to w. The result is the total number of bytes
+// written and an os.Error, if any.
+//
+func (f Format) Fprint(w io.Writer, env Environment, args ...interface{}) (int, os.Error) {
+ data, err := f.Eval(env, args...)
+ if err != nil {
+ // TODO should we print partial result in case of error?
+ return 0, err
+ }
+ return w.Write(data)
+}
+
+
+// Print formats each argument according to the format f
+// and writes to standard output. The result is the total
+// number of bytes written and an os.Error, if any.
+//
+func (f Format) Print(args ...interface{}) (int, os.Error) {
+ return f.Fprint(os.Stdout, nil, args...)
+}
+
+
+// Sprint formats each argument according to the format f
+// and returns the resulting string. If an error occurs
+// during formatting, the result string contains the
+// partially formatted result followed by an error message.
+//
+func (f Format) Sprint(args ...interface{}) string {
+ var buf bytes.Buffer
+ _, err := f.Fprint(&buf, nil, args...)
+ if err != nil {
+ var i interface{} = args
+ fmt.Fprintf(&buf, "--- Sprint(%s) failed: %v", fmt.Sprint(i), err)
+ }
+ return buf.String()
+}
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