// 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.

// Package token defines constants representing the lexical tokens of the Go
// programming language and basic operations on tokens (printing, predicates).
//
package token

import "strconv"

// Token is the set of lexical tokens of the Go programming language.
type Token int

// The list of tokens.
const (
	// Special tokens
	ILLEGAL Token = iota
	EOL
	EOF
	COMMENT

	literal_beg
	// Identifiers and basic type literals
	// (these tokens stand for classes of literals)
	IDENT  // main
	STRING // "abc"
	literal_end

	operator_beg
	// Operators and delimiters
	ASSIGN // =
	LBRACK // [
	RBRACK // ]
	operator_end
)

var tokens = [...]string{
	ILLEGAL: "ILLEGAL",

	EOL:     "EOL",
	EOF:     "EOF",
	COMMENT: "COMMENT",

	IDENT:  "IDENT",
	STRING: "STRING",

	ASSIGN: "=",
	LBRACK: "[",
	RBRACK: "]",
}

// String returns the string corresponding to the token tok.
// For operators, delimiters, and keywords the string is the actual
// token character sequence (e.g., for the token ADD, the string is
// "+"). For all other tokens the string corresponds to the token
// constant name (e.g. for the token IDENT, the string is "IDENT").
//
func (tok Token) String() string {
	s := ""
	if 0 <= tok && tok < Token(len(tokens)) {
		s = tokens[tok]
	}
	if s == "" {
		s = "token(" + strconv.Itoa(int(tok)) + ")"
	}
	return s
}

// A set of constants for precedence-based expression parsing.
// Non-operators have lowest precedence, followed by operators
// starting with precedence 1 up to unary operators. The highest
// precedence corresponds serves as "catch-all" precedence for
// selector, indexing, and other operator and delimiter tokens.
//
const (
	LowestPrec  = 0 // non-operators
	UnaryPrec   = 6
	HighestPrec = 7
)

// Precedence returns the operator precedence of the binary
// operator op. If op is not a binary operator, the result
// is LowestPrecedence.
//
func (op Token) Precedence() int {
	switch op {
	case LOR:
		return 1
	case LAND:
		return 2
	case EQL, NEQ, LSS, LEQ, GTR, GEQ:
		return 3
	case ADD, SUB, OR, XOR:
		return 4
	case MUL, QUO, REM, SHL, SHR, AND, AND_NOT:
		return 5
	}
	return LowestPrec
}

var keywords map[string]Token

func init() {
	keywords = make(map[string]Token)
	for i := keyword_beg + 1; i < keyword_end; i++ {
		keywords[tokens[i]] = i
	}
}

// Lookup maps an identifier to its keyword token or IDENT (if not a keyword).
//
func Lookup(ident string) Token {
	if tok, is_keyword := keywords[ident]; is_keyword {
		return tok
	}
	return IDENT
}

// Predicates

// IsLiteral returns true for tokens corresponding to identifiers
// and basic type literals; it returns false otherwise.
//
func (tok Token) IsLiteral() bool { return literal_beg < tok && tok < literal_end }

// IsOperator returns true for tokens corresponding to operators and
// delimiters; it returns false otherwise.
//
func (tok Token) IsOperator() bool { return operator_beg < tok && tok < operator_end }

// IsKeyword returns true for tokens corresponding to keywords;
// it returns false otherwise.
//
func (tok Token) IsKeyword() bool { return keyword_beg < tok && tok < keyword_end }