// scanner.go
package text

import (
	"fmt"
	"strings"
)

type ScannerFlag uint16

const (
	ValueRequired = ScannerFlag(1 << iota)
	DirectValue
	EncryptValue
)

// Flags symbols in the abova iota order
const flagSymbols = "!=*"

type VariableHandler interface {
	Handle(varSpec string, flags ScannerFlag) (value string, err error)
}

func Scan(handler VariableHandler, source string) (result string, err error) {
	var spec, value string
	var flags ScannerFlag
	var backSlash bool
	var sb strings.Builder
	offset := 0
	for offset < len(source) {
		ch := source[offset]
		offset++
		if backSlash {
			switch ch {
			case '\\':
				sb.WriteByte('\\')
			case varIntro:
				sb.WriteByte(varIntro)
			default:
				sb.WriteByte(ch)
			}
			backSlash = false
		} else if ch == '\\' {
			backSlash = true
		} else if ch == varIntro {
			if spec, flags, offset, err = getVarSpec(source, offset); err != nil {
				//err = fmt.Errorf("Source:\n%s", source)
				return
			}
			if handler != nil {
				if value, err = handler.Handle(spec, flags); err != nil {
					return
				}
			}
			sb.WriteString(value)
		} else {
			sb.WriteByte(ch)
		}
	}
	result = sb.String()
	return
}

func getVarSpec(source string, startOffset int) (spec string, flags ScannerFlag, offset int, err error) {
	var ch byte
	var lastOpenBrace int = -1

	braceCount := 0
	if startOffset < len(source) && source[startOffset] == '{' {
		lastOpenBrace = startOffset
		braceCount++
		startOffset++
	}
	count := 0
	beginning := true
	for offset = startOffset; offset < len(source); offset++ {
		ch = source[offset]
		if ch == '{' && braceCount > 0 {
			lastOpenBrace = offset
			count++
			braceCount++
		} else if ch == '}' {
			braceCount--
			if braceCount == 0 {
				break
			}
			count++
		} else if braceCount == 0 && strings.IndexByte("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_", ch) < 0 {
			break
		} else if braceCount == 1 && beginning {
			if flagIndex := strings.IndexByte(flagSymbols, ch); flagIndex >= 0 {
				startOffset++
				flags |= ScannerFlag(1 << flagIndex)
			} else {
				count++
				beginning = false
			}
		} else {
			count++
			beginning = false
		}
	}
	if err == nil {
		if braceCount > 0 {
			err = fmt.Errorf("unbalanced open braces at offset %d", lastOpenBrace)
		} else {
			if offset < len(source) && ch == '}' {
				offset++
			}
			//fmt.Println("count:", count)
			spec = source[startOffset : startOffset+count]
		}
	}
	return
}