from __future__ import annotations

import re
from dataclasses import dataclass

_FIELD_PATTERN = re.compile(r"(?<!\\)\$\{(.*?)(?<!\\)\}")


@dataclass
class MailTemplate:
    template_string: str = ""

    def set_template_string(self, template: str) -> "MailTemplate":
        self.template_string = template
        return self

    def get_used_fields(self) -> set[str]:
        return set(_FIELD_PATTERN.findall(self.template_string))

    def apply_values(self, values: dict[str, str], *, keep_missing: bool = True) -> str:
        def replace(match: re.Match[str]) -> str:
            key = match.group(1)
            if key in values:
                return values[key]
            return match.group(0) if keep_missing else ""

        rendered = _FIELD_PATTERN.sub(replace, self.template_string)
        return rendered.replace(r"\${", "${").replace(r"\}", "}")