import csv
import io
from dataclasses import dataclass

from app.constants import VENDOR_FIELDS


@dataclass(slots=True)
class ExportResult:
    content: str
    columns: list[str]
    output_format: str

    def preview(self, record_limit: int) -> str:
        """Build a small preview string for the result page."""
        if self.output_format == "text":
            marker = f"--- record {record_limit + 1} ---"
            if marker in self.content:
                return self.content.split(marker, 1)[0].rstrip()
            return self.content

        lines = self.content.splitlines()
        if len(lines) <= record_limit + 1:
            return self.content
        return "\n".join(lines[: record_limit + 1])


def build_export(
    records: list[dict[str, str]],
    union_keys: list[str],
    mode: str,
    output_format: str,
) -> ExportResult:
    columns = VENDOR_FIELDS if mode == "vendor" else union_keys

    if output_format == "text":
        return ExportResult(
            content=_render_text(records, columns),
            columns=columns,
            output_format=output_format,
        )

    return ExportResult(
        content=_render_csv(records, columns),
        columns=columns,
        output_format=output_format,
    )


def _render_text(records: list[dict[str, str]], columns: list[str]) -> str:
    max_key_length = max((len(column) for column in columns), default=0)
    chunks: list[str] = []

    for index, record in enumerate(records, start=1):
        chunks.append(f"--- record {index} ---")
        for column in columns:
            value = record.get(column, "")
            chunks.append(f"  {column.ljust(max_key_length)} = {value}")

    return "\n".join(chunks)


def _render_csv(records: list[dict[str, str]], columns: list[str]) -> str:
    buffer = io.StringIO()
    writer = csv.DictWriter(buffer, fieldnames=columns, extrasaction="ignore")
    writer.writeheader()
    for record in records:
        writer.writerow({column: record.get(column, "") for column in columns})
    return buffer.getvalue()