# tools/csv_parser.py
# ------------------------------------------------------------
# Reads CSV / Excel, samples for very large files, and returns a
# Markdown‑formatted “quick‑scan” report: dimensions, schema,
# missing‑value profile, numeric describe(), and memory footprint.

from __future__ import annotations

import os
from typing import Union

import pandas as pd


def _safe_read(path_or_buf: Union[str, bytes], sample_rows: int = 1_000_000) -> pd.DataFrame:
    """Read CSV or Excel.  If the file has > sample_rows, read only a sample."""
    # Determine extension (best‑effort)
    ext = ".csv"
    if isinstance(path_or_buf, str):
        ext = os.path.splitext(path_or_buf)[1].lower()

    if ext in (".xls", ".xlsx"):
        # Excel — read first sheet
        df = pd.read_excel(path_or_buf, engine="openpyxl")
    else:  # CSV family
        # First row‑count check: pandas 1.5+ uses memory map ⇒ cheap for header only
        nrows_total = sum(1 for _ in open(path_or_buf, "rb")) if isinstance(path_or_buf, str) else None
        if nrows_total and nrows_total > sample_rows:
            # sample uniformly without loading everything
            skip = sorted(
                pd.np.random.choice(range(1, nrows_total), nrows_total - sample_rows, replace=False)
            )
            df = pd.read_csv(path_or_buf, skiprows=skip)
        else:
            df = pd.read_csv(path_or_buf)

    return df


def parse_csv_tool(file: Union[str, bytes]) -> str:
    """
    Return a **Markdown** report describing the dataset.

    Sections:
    • Dimensions
    • Schema (+ dtypes)
    • Missing‑value counts + %
    • Numeric descriptive statistics
    • Memory usage
    """
    try:
        df = _safe_read(file)
    except Exception as exc:
        return f"❌ Failed to load data: {exc}"

    n_rows, n_cols = df.shape

    # ---------- schema ----------
    schema_md = "\n".join(
        f"- **{col}** – `{dtype}`"
        for col, dtype in df.dtypes.items()
    )

    # ---------- missing ----------
    miss_ct  = df.isna().sum()
    miss_pct = (miss_ct / len(df) * 100).round(1)
    missing_md = "\n".join(
        f"- **{c}**: {miss_ct[c]} ({miss_pct[c]} %)"
        for c in df.columns if miss_ct[c] > 0
    ) or "None"

    # ---------- descriptive stats (numeric only) ----------
    if df.select_dtypes("number").shape[1]:
        desc_md = df.describe().T.round(2).to_markdown()
    else:
        desc_md = "_No numeric columns_"

    # ---------- memory ----------
    mem_mb = df.memory_usage(deep=True).sum() / 1024**2

    # ---------- assemble ----------
    return f"""
# 📊 Dataset Overview

| metric | value |
| ------ | ----- |
| Rows   | {n_rows:,} |
| Columns| {n_cols} |
| Memory | {mem_mb:.2f} MB |

## 🗂 Schema
{schema_md}

## 🛠 Missing Values
{missing_md}

## 📈 Descriptive Statistics (numeric)
{desc_md}
""".strip()