deploy: 06a1bdf735

vendor/gopl.io/ch12/sexpr/decode.go

@@ -0,0 +1,162 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 344.
+
+// Package sexpr provides a means for converting Go objects to and
+// from S-expressions.
+package sexpr
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+	"strconv"
+	"text/scanner"
+)
+
+//!+Unmarshal
+// Unmarshal parses S-expression data and populates the variable
+// whose address is in the non-nil pointer out.
+func Unmarshal(data []byte, out interface{}) (err error) {
+	lex := &lexer{scan: scanner.Scanner{Mode: scanner.GoTokens}}
+	lex.scan.Init(bytes.NewReader(data))
+	lex.next() // get the first token
+	defer func() {
+		// NOTE: this is not an example of ideal error handling.
+		if x := recover(); x != nil {
+			err = fmt.Errorf("error at %s: %v", lex.scan.Position, x)
+		}
+	}()
+	read(lex, reflect.ValueOf(out).Elem())
+	return nil
+}
+
+//!-Unmarshal
+
+//!+lexer
+type lexer struct {
+	scan  scanner.Scanner
+	token rune // the current token
+}
+
+func (lex *lexer) next()        { lex.token = lex.scan.Scan() }
+func (lex *lexer) text() string { return lex.scan.TokenText() }
+
+func (lex *lexer) consume(want rune) {
+	if lex.token != want { // NOTE: Not an example of good error handling.
+		panic(fmt.Sprintf("got %q, want %q", lex.text(), want))
+	}
+	lex.next()
+}
+
+//!-lexer
+
+// The read function is a decoder for a small subset of well-formed
+// S-expressions.  For brevity of our example, it takes many dubious
+// shortcuts.
+//
+// The parser assumes
+// - that the S-expression input is well-formed; it does no error checking.
+// - that the S-expression input corresponds to the type of the variable.
+// - that all numbers in the input are non-negative decimal integers.
+// - that all keys in ((key value) ...) struct syntax are unquoted symbols.
+// - that the input does not contain dotted lists such as (1 2 . 3).
+// - that the input does not contain Lisp reader macros such 'x and #'x.
+//
+// The reflection logic assumes
+// - that v is always a variable of the appropriate type for the
+//   S-expression value.  For example, v must not be a boolean,
+//   interface, channel, or function, and if v is an array, the input
+//   must have the correct number of elements.
+// - that v in the top-level call to read has the zero value of its
+//   type and doesn't need clearing.
+// - that if v is a numeric variable, it is a signed integer.
+
+//!+read
+func read(lex *lexer, v reflect.Value) {
+	switch lex.token {
+	case scanner.Ident:
+		// The only valid identifiers are
+		// "nil" and struct field names.
+		if lex.text() == "nil" {
+			v.Set(reflect.Zero(v.Type()))
+			lex.next()
+			return
+		}
+	case scanner.String:
+		s, _ := strconv.Unquote(lex.text()) // NOTE: ignoring errors
+		v.SetString(s)
+		lex.next()
+		return
+	case scanner.Int:
+		i, _ := strconv.Atoi(lex.text()) // NOTE: ignoring errors
+		v.SetInt(int64(i))
+		lex.next()
+		return
+	case '(':
+		lex.next()
+		readList(lex, v)
+		lex.next() // consume ')'
+		return
+	}
+	panic(fmt.Sprintf("unexpected token %q", lex.text()))
+}
+
+//!-read
+
+//!+readlist
+func readList(lex *lexer, v reflect.Value) {
+	switch v.Kind() {
+	case reflect.Array: // (item ...)
+		for i := 0; !endList(lex); i++ {
+			read(lex, v.Index(i))
+		}
+
+	case reflect.Slice: // (item ...)
+		for !endList(lex) {
+			item := reflect.New(v.Type().Elem()).Elem()
+			read(lex, item)
+			v.Set(reflect.Append(v, item))
+		}
+
+	case reflect.Struct: // ((name value) ...)
+		for !endList(lex) {
+			lex.consume('(')
+			if lex.token != scanner.Ident {
+				panic(fmt.Sprintf("got token %q, want field name", lex.text()))
+			}
+			name := lex.text()
+			lex.next()
+			read(lex, v.FieldByName(name))
+			lex.consume(')')
+		}
+
+	case reflect.Map: // ((key value) ...)
+		v.Set(reflect.MakeMap(v.Type()))
+		for !endList(lex) {
+			lex.consume('(')
+			key := reflect.New(v.Type().Key()).Elem()
+			read(lex, key)
+			value := reflect.New(v.Type().Elem()).Elem()
+			read(lex, value)
+			v.SetMapIndex(key, value)
+			lex.consume(')')
+		}
+
+	default:
+		panic(fmt.Sprintf("cannot decode list into %v", v.Type()))
+	}
+}
+
+func endList(lex *lexer) bool {
+	switch lex.token {
+	case scanner.EOF:
+		panic("end of file")
+	case ')':
+		return true
+	}
+	return false
+}
+
+//!-readlist