// Copyright 2023 The CUE 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 cueimports provides support for reading the import
// section of a CUE file without needing to read the rest of it.
package cueimports

import (
	"bufio"
	"io"
	"unicode/utf8"

	"cuelang.org/go/cue/errors"
	"cuelang.org/go/cue/token"
)

type importReader struct {
	b    *bufio.Reader
	buf  []byte
	peek byte
	err  errors.Error
	eof  bool
	nerr int
}

func isIdent(c byte) bool {
	return 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || '0' <= c && c <= '9' || c == '_' || c >= utf8.RuneSelf
}

var (
	errSyntax = errors.Newf(token.NoPos, "syntax error") // TODO: remove
	errNUL    = errors.Newf(token.NoPos, "unexpected NUL in input")
)

// syntaxError records a syntax error, but only if an I/O error has not already been recorded.
func (r *importReader) syntaxError() {
	if r.err == nil {
		r.err = errSyntax
	}
}

// readByte reads the next byte from the input, saves it in buf, and returns it.
// If an error occurs, readByte records the error in r.err and returns 0.
func (r *importReader) readByte() byte {
	c, err := r.b.ReadByte()
	if err == nil {
		r.buf = append(r.buf, c)
		if c == 0 {
			err = errNUL
		}
	}
	if err != nil {
		if err == io.EOF {
			r.eof = true
		} else if r.err == nil {
			r.err = errors.Wrapf(err, token.NoPos, "readByte")
		}
		c = 0
	}
	return c
}

// peekByte returns the next byte from the input reader but does not advance beyond it.
// If skipSpace is set, peekByte skips leading spaces and comments.
func (r *importReader) peekByte(skipSpace bool) byte {
	if r.err != nil {
		if r.nerr++; r.nerr > 10000 {
			panic("import reader looping")
		}
		return 0
	}

	// Use r.peek as first input byte.
	// Don't just return r.peek here: it might have been left by peekByte(false)
	// and this might be peekByte(true).
	c := r.peek
	if c == 0 {
		c = r.readByte()
	}
	for r.err == nil && !r.eof {
		if skipSpace {
			// For the purposes of this reader, commas are never necessary to
			// understand the input and are treated as spaces.
			switch c {
			case ' ', '\f', '\t', '\r', '\n', ',':
				c = r.readByte()
				continue

			case '/':
				c = r.readByte()
				if c == '/' {
					for c != '\n' && r.err == nil && !r.eof {
						c = r.readByte()
					}
				} else {
					r.syntaxError()
				}
				c = r.readByte()
				continue
			}
		}
		break
	}
	r.peek = c
	return r.peek
}

// nextByte is like peekByte but advances beyond the returned byte.
func (r *importReader) nextByte(skipSpace bool) byte {
	c := r.peekByte(skipSpace)
	r.peek = 0
	return c
}

// readKeyword reads the given keyword from the input.
// If the keyword is not present, readKeyword records a syntax error.
func (r *importReader) readKeyword(kw string) {
	r.peekByte(true)
	for i := 0; i < len(kw); i++ {
		if r.nextByte(false) != kw[i] {
			r.syntaxError()
			return
		}
	}
	if isIdent(r.peekByte(false)) {
		r.syntaxError()
	}
}

// readIdent reads an identifier from the input.
// If an identifier is not present, readIdent records a syntax error.
func (r *importReader) readIdent() {
	c := r.peekByte(true)
	if !isIdent(c) {
		r.syntaxError()
		return
	}
	for isIdent(r.peekByte(false)) {
		r.peek = 0
	}
}

// readString reads a quoted string literal from the input.
// If an identifier is not present, readString records a syntax error.
func (r *importReader) readString() {
	switch r.nextByte(true) {
	case '"':
		// Note: although the syntax in the specification only allows
		// a simple string literal here, the cue/parser package also
		// allows #"..."# and """ literals, so there's some impedance-mismatch here.
		for r.err == nil {
			c := r.nextByte(false)
			if c == '"' {
				break
			}
			if r.eof || c == '\n' {
				r.syntaxError()
			}
			if c == '\\' {
				r.nextByte(false)
			}
		}
	default:
		r.syntaxError()
	}
}

// readImport reads an import clause - optional identifier followed by quoted string -
// from the input.
func (r *importReader) readImport() {
	c := r.peekByte(true)
	if isIdent(c) {
		r.readIdent()
	}
	r.readString()
}

// Read is like io.ReadAll, except that it expects a CUE file as
// input and stops reading the input once the imports have completed.
func Read(f io.Reader) ([]byte, errors.Error) {
	r := &importReader{b: bufio.NewReader(f)}

	r.readKeyword("package")
	r.readIdent()
	for r.peekByte(true) == 'i' {
		r.readKeyword("import")
		if r.peekByte(true) == '(' {
			r.nextByte(false)
			for r.peekByte(true) != ')' && r.err == nil {
				r.readImport()
			}
			r.nextByte(false)
		} else {
			r.readImport()
		}
	}

	// If we stopped successfully before EOF, we read a byte that told us we were done.
	// Return all but that last byte, which would cause a syntax error if we let it through.
	if r.err == nil && !r.eof {
		return r.buf[:len(r.buf)-1], nil
	}

	// If we stopped for a syntax error, consume the whole file so that
	// we are sure we don't change the errors that the cue/parser returns.
	if r.err == errSyntax {
		r.err = nil
		for r.err == nil && !r.eof {
			r.readByte()
		}
	}

	return r.buf, r.err
}