package usecase

import (
	"encoding/json"
	"strings"
	"unicode/utf8"
)

// boundedDiagnosticJSON returns a JSON string prefix and whether it represents
// the complete normalized input. It retains at most the useful prefix.
func boundedDiagnosticJSON(value string, maxBytes int) ([]byte, bool) {
	if maxBytes < 2 {
		return nil, false
	}
	limit := maxBytes - 2
	var prefix strings.Builder
	used := 0
	more := false
	for i := 0; i < len(value); {
		r, size := utf8.DecodeRuneInString(value[i:])
		piece := value[i : i+size]
		if r == utf8.RuneError && size == 1 {
			piece = "\uFFFD"
			i++
			for i < len(value) {
				r, size = utf8.DecodeRuneInString(value[i:])
				if r != utf8.RuneError || size != 1 {
					break
				}
				i++
			}
		} else {
			i += size
		}
		if used+len(piece) > limit {
			more = true
			break
		}
		prefix.WriteString(piece)
		used += len(piece)
	}
	normalized := prefix.String()
	encoded, _ := json.Marshal(normalized)
	if len(encoded) > maxBytes {
		return truncateDiagnosticJSONValue(normalized, maxBytes), false
	}
	return encoded, !more
}