package parsekit

import (
	"fmt"
	"runtime"
	"strings"
	"unicode/utf8"
)

// ParseHandler defines the type of function that must be implemented to handle
// a parsing state in a Parser state machine.
//
// A ParseHandler function gets a ParseAPI struct as its input. This struct holds
// all the internal state for the parsing state machine and provides the
// interface that the ParseHandler uses to interact with the parser.
type ParseHandler func(*ParseAPI)

// ParseAPI holds the internal state of a parse run and provides an API to
// ParseHandler methods to communicate with the parser.
type ParseAPI struct {
	input        string          // the input that is being scanned by the parser
	inputPos     int             // current byte cursor position in the input
	loopCheck    map[string]bool // used for parser loop detection
	cursorLine   int             // current rune cursor row number in the input
	cursorColumn int             // current rune cursor column position in the input
	len          int             // the total length of the input in bytes
	newline      bool            // keep track of when we have scanned a newline
	expecting    string          // a description of what the current state expects to find (see P.Expects())
	buffer       stringBuffer    // an efficient buffer, used to build string values (see P.Accept())
	err          *Error          // error during parsing, retrieved by Error(), further ParseAPI calls are ignored
	stopped      bool            // a boolean set to true by Stop(), further ParseAPI calls are ignored

	LastMatch string // a string representation of the last matched input data
}

// panicWhenStoppedOrInError will panic when the parser has produced an error
// or when it has been stopped. It is used from the ParseAPI methods, to
// prevent further calls to the ParseAPI on these occasions.
//
// Basically, this guard ensures proper coding of parsers, making sure
// that clean routes are followed. You can consider this check a runtime
// unit test.
func (p *ParseAPI) panicWhenStoppedOrInError() {
	if !p.isStoppedOrInError() {
		return
	}

	called, _ := p.getCaller(1)
	parts := strings.Split(called, ".")
	calledShort := parts[len(parts)-1]
	caller, filepos := p.getCaller(2)

	after := "Error()"
	if p.stopped {
		after = "Stop()"
	}

	panic(fmt.Sprintf("Illegal call to ParseAPI.%s() from %s at %s: no calls allowed after ParseAPI.%s", calledShort, caller, filepos, after))
}

func (p *ParseAPI) isStoppedOrInError() bool {
	return p.stopped || p.err != nil
}

func (p *ParseAPI) checkForLoops() {
	caller, filepos := p.getCaller(2)
	if _, ok := p.loopCheck[filepos]; ok {
		panic(fmt.Sprintf("Loop detected in parser in %s at %s", caller, filepos))
	}
	p.loopCheck[filepos] = true
}

// peek returns but does not advance the cursor to the next rune in the input.
// Returns the rune, its width in bytes and a boolean.
//
// The boolean will be false in case no upcoming rune can be peeked
// (end of data or invalid UTF8 character). In this case, the returned rune
// will be one of eofRune or invalidRune.
func (p *ParseAPI) peek(byteOffset int) (rune, int, bool) {
	r, w := utf8.DecodeRuneInString(p.input[p.inputPos+byteOffset:])
	return handleRuneError(r, w)
}

// eofRune is a special rune that is used to indicate an end of file when
// reading a character from the input.
const eofRune rune = -1

// invalidRune is a special rune that is used to indicate an invalid UTF8
// rune on the input.
const invalidRune rune = utf8.RuneError

// handleRuneError is used to create specific rune value in case of errors.
// When an error occurs, then utf8.RuneError will be in the rune.
// This can however indicate one of two situations:
// 1) w == 0: end of file is reached
// 2) w == 1: invalid UTF character on input
// This function lets these two cases return respectively the
// package's own eofRune or invalidRune, to make it easy for calling code
// to distinct between these two cases.
func handleRuneError(r rune, w int) (rune, int, bool) {
	if r == utf8.RuneError {
		if w == 0 {
			return eofRune, 0, false
		}
		return invalidRune, w, false
	}
	return r, w, true
}

func (p *ParseAPI) getCaller(depth int) (string, string) {
	// No error handling, because we call this method ourselves with safe depth values.
	pc, file, line, _ := runtime.Caller(depth + 1)
	filepos := fmt.Sprintf("%s:%d", file, line)
	caller := runtime.FuncForPC(pc)
	return caller.Name(), filepos
}