package perfdata // import nocternity.net/gomonop/pkg/perfdata

import (
	"fmt"
	"strings"
)

// A state of the range parser.
type rangeParserState int

const (
	rpsInit        rangeParserState = iota // Initial state
	rpsExpectStart                         // Expect the start of the range
	rpsInStart                             // Reading the start of the range
	rpsExpectColon                         // Expect the colon that separates the start and end
	rpsExpectEnd                           // Expect the end of the range
	rpsInEnd                               // Reading the end of the range
)

// An error emitted by the range parser.
type rangeParserError struct {
	input    string
	position int
	message  string
}

// parseError creates a new range parser error.
func parseError(input string, position int, message string) *rangeParserError {
	return &rangeParserError{
		input:    input,
		position: position,
		message:  message,
	}
}

// Error implements the error interface for the range parser error type.
func (rpe *rangeParserError) Error() string {
	return fmt.Sprintf("in `%s' at position %d: %s", rpe.input, rpe.position, rpe.message)
}

// The full state of the range parser.
type rangeParser struct {
	runes        []rune           // The runes being parsed
	index        int              // The read position
	state        rangeParserState // The FSM state
	startOfValue int              // The position of the start of the value being read
	strBuilder   strings.Builder  // An accumulator for values
	output       Range            // The output being generated
}

// initRangeParser initializes the range parser from the specified input.
func initRangeParser(input string) *rangeParser {
	return &rangeParser{
		runes: []rune(input),
		state: rpsInit,
	}
}

// handleInit handles the parser's rpsInit state. It can accept a '@' or skip
// to rpsExpectStart.
func (rp *rangeParser) handleInit(current rune) {
	if current == '@' {
		rp.output.inside = true
		rp.index++
	}
	rp.state = rpsExpectStart
}

// handleExpectStart handles the parser's rpsExpectStart state. It may accept
// a colon, which will move the parser to the rpsExpectEnd state, a '~' which
// will move it to rpsExpectColon, or any other rune which will cause the
// accumulation of the "start" value to begin, and the state to be moved to
// handleInStart.
func (rp *rangeParser) handleExpectStart(current rune) {
	switch current {
	case ':':
		rp.output.start = "0"
		rp.state = rpsExpectEnd
		rp.index++
	case '~':
		rp.state = rpsExpectColon
		rp.index++
	default:
		rp.startOfValue = rp.index
		rp.state = rpsInStart
	}
}

// handleInStart handles the parser's rpsInStart state, which corresponds to the
// reading of the "start" value. If a colon is found, the value will be written
// to the range structure and validated, before switching to the rpsExpectEnd
// state. Otherwise it will simply accumulate runes.
func (rp *rangeParser) handleInStart(current rune) error {
	switch current {
	case ':':
		rp.output.start = rp.strBuilder.String()
		if !valueCheck.MatchString(rp.output.start) {
			return parseError(string(rp.runes), rp.startOfValue, "invalid start value")
		}
		rp.strBuilder.Reset()
		rp.state = rpsExpectEnd
	default:
		rp.strBuilder.WriteRune(current)
	}

	rp.index++
	return nil
}

// handleExpectColon handles the parser's rpsExpectColon state. A colon MUST be
// read, otherwise an error will be returned. The parser will then switch to the
// rpsExpectEnd state.
func (rp *rangeParser) handleExpectColon(current rune) error {
	switch current {
	case ':':
		rp.state = rpsExpectEnd
		rp.index++
	default:
		return parseError(string(rp.runes), rp.index, "expected ':'")
	}

	return nil
}

// handleExpectEnd handles the parser's rpsExpectEnd state, which sets the
// position of the beginning of the "end" value, and jumps to the rpsInEnd
// state.
func (rp *rangeParser) handleExpectEnd() {
	rp.startOfValue = rp.index
	rp.state = rpsInEnd
}

// handleInEnd handles the parser's rpsInEnd state, which accumulates runes for
// the "end" value.
func (rp *rangeParser) handleInEnd(current rune) {
	rp.strBuilder.WriteRune(current)
	rp.index++
}

// consumeInput is the parser's state machine.
func (rp *rangeParser) consumeInput() error {
	for rp.index < len(rp.runes) {
		var err error
		curRune := rp.runes[rp.index]

		switch rp.state {
		case rpsInit:
			rp.handleInit(curRune)

		case rpsExpectStart:
			rp.handleExpectStart(curRune)

		case rpsInStart:
			err = rp.handleInStart(curRune)

		case rpsExpectColon:
			err = rp.handleExpectColon(curRune)

		case rpsExpectEnd:
			rp.handleExpectEnd()

		case rpsInEnd:
			rp.handleInEnd(curRune)
		}

		if err != nil {
			return err
		}
	}

	return nil
}

// Try to parse a string into a performance data range.
func ParseRange(input string) (*Range, error) {
	parser := initRangeParser(input)
	if err := parser.consumeInput(); err != nil {
		return nil, err
	}

	if parser.state == rpsInStart {
		// The range was a single value, so that's the upper bound.
		parser.output.start = "0"
		parser.state = rpsInEnd
	}
	if parser.state != rpsInEnd {
		return nil, parseError(input, parser.index, "unexpected end of input")
	}

	parser.output.end = parser.strBuilder.String()
	if !valueCheck.MatchString(parser.output.end) {
		return nil, parseError(input, parser.startOfValue, "invalid end value")
	}

	return &parser.output, nil
}