// Copyright 2021 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package lexer provides a basic lexer for the Tint intrinsic definition
// language
package lexer

import (
	"fmt"
	"unicode"

	"dawn.googlesource.com/tint/tools/src/cmd/intrinsic-gen/tok"
)

// Lex produces a list of tokens for the given source code
func Lex(src []rune, filepath string) ([]tok.Token, error) {
	l := lexer{
		tok.Location{Line: 1, Column: 1, Rune: 0, Filepath: filepath},
		src,
		[]tok.Token{},
	}
	if err := l.lex(); err != nil {
		return nil, err
	}
	return l.tokens, nil
}

type lexer struct {
	loc    tok.Location
	runes  []rune
	tokens []tok.Token
}

// lex() lexes the source, populating l.tokens
func (l *lexer) lex() error {
	for {
		switch l.peek(0) {
		case 0:
			return nil
		case ' ', '\t':
			l.next()
		case '\n':
			l.next()
		case '<':
			l.tok(1, tok.Lt)
		case '>':
			l.tok(1, tok.Gt)
		case '(':
			l.tok(1, tok.Lparen)
		case ')':
			l.tok(1, tok.Rparen)
		case '{':
			l.tok(1, tok.Lbrace)
		case '}':
			l.tok(1, tok.Rbrace)
		case ':':
			l.tok(1, tok.Colon)
		case ',':
			l.tok(1, tok.Comma)
		case '|':
			l.tok(1, tok.Or)
		case '"':
			start := l.loc
			l.next() // Skip opening quote
			n := l.count(toFirst('\n', '"'))
			if l.peek(n) != '"' {
				return fmt.Errorf("%v unterminated string", start)
			}
			l.tok(n, tok.String)
			l.next() // Skip closing quote
		default:
			switch {
			case l.peek(1) == '/':
				l.skip(l.count(toFirst('\n')))
				l.next() // Consume newline
			case l.match("[[", tok.Ldeco):
			case l.match("]]", tok.Rdeco):
			case l.match("->", tok.Arrow):
			case l.match("fn", tok.Function):
			case l.match("enum", tok.Enum):
			case l.match("type", tok.Type):
			case l.match("match", tok.Match):
			case unicode.IsLetter(l.peek(0)):
				l.tok(l.count(alphaNumericOrUnderscore), tok.Identifier)
			case unicode.IsNumber(l.peek(0)):
				l.tok(l.count(unicode.IsNumber), tok.Integer)
			default:
				return fmt.Errorf("%v: unexpected '%v'", l.loc, string(l.runes[0]))
			}
		}
	}
}

// next() consumes and returns the next rune in the source, or 0 if reached EOF
func (l *lexer) next() rune {
	if len(l.runes) > 0 {
		r := l.runes[0]
		l.runes = l.runes[1:]
		l.loc.Rune++
		if r == '\n' {
			l.loc.Line++
			l.loc.Column = 1
		} else {
			l.loc.Column++
		}
		return r
	}
	return 0
}

// skip() consumes the next `n` runes in the source
func (l *lexer) skip(n int) {
	for i := 0; i < n; i++ {
		l.next()
	}
}

// peek() returns the rune `i` runes ahead of the current position
func (l *lexer) peek(i int) rune {
	if i >= len(l.runes) {
		return 0
	}
	return l.runes[i]
}

// predicate is a function that can be passed to count()
type predicate func(r rune) bool

// count() returns the number of sequential runes from the current position that
// match the predicate `p`
func (l *lexer) count(p predicate) int {
	for i := 0; i < len(l.runes); i++ {
		if !p(l.peek(i)) {
			return i
		}
	}
	return len(l.runes)
}

// tok() appends a new token of kind `k` using the next `n` runes.
// The next `n` runes are consumed by tok().
func (l *lexer) tok(n int, k tok.Kind) {
	start := l.loc
	runes := l.runes[:n]
	l.skip(n)
	end := l.loc

	src := tok.Source{S: start, E: end}
	l.tokens = append(l.tokens, tok.Token{Kind: k, Source: src, Runes: runes})
}

// match() checks whether the next runes are equal to `s`. If they are, then
// these runes are used to append a new token of kind `k`, and match() returns
// true. If the next runes are not equal to `s` then false is returned, and no
// runes are consumed.
func (l *lexer) match(s string, kind tok.Kind) bool {
	runes := []rune(s)
	if len(l.runes) < len(runes) {
		return false
	}
	for i, r := range runes {
		if l.runes[i] != r {
			return false
		}
	}
	l.tok(len(runes), kind)
	return true
}

// toFirst() returns a predicate that returns true if the rune is not in `runes`
// toFirst() is intended to be used with count(), so `count(toFirst('x'))` will
// count up to, but not including the number of consecutive runes that are not
// 'x'.
func toFirst(runes ...rune) predicate {
	return func(r rune) bool {
		for _, t := range runes {
			if t == r {
				return false
			}
		}
		return true
	}
}

// alphaNumericOrUnderscore() returns true if the rune `r` is a number, letter
// or underscore.
func alphaNumericOrUnderscore(r rune) bool {
	return r == '_' || unicode.IsLetter(r) || unicode.IsNumber(r)
}