"""Tails the lidar/camera/recording log files for the browser log panel.

The original PyQt5 scan GUIs had a single unified QPlainTextEdit showing all
subprocess stdout with a [tag] prefix — this is the web equivalent. Each
ManagedProcess re-opens its log file in "w" mode on every start (see
process_manager.py), so a tailer must detect truncation (file shrank since
last read) and reset to the top rather than seeking past EOF forever.
"""
from __future__ import annotations

from pathlib import Path
from typing import Iterable


class LogTailer:
    def __init__(self, sources: dict[str, Path]):
        self._sources = sources
        self._offsets: dict[str, int] = {tag: 0 for tag in sources}

    def read_tail(self, tag: str, max_lines: int = 200) -> list[str]:
        """Backlog for a fresh websocket connection — last N lines, and
        advances this tag's offset to end-of-file so read_new() only
        reports genuinely new lines afterwards."""
        path = self._sources.get(tag)
        if path is None or not path.exists():
            return []
        try:
            with open(path, "r", errors="replace") as f:
                lines = f.readlines()
                self._offsets[tag] = f.tell()
        except OSError:
            return []
        return [ln.rstrip("\n") for ln in lines[-max_lines:]]

    def read_new(self) -> Iterable[tuple[str, str]]:
        """Yields (tag, line) for every line appended since the last call."""
        for tag, path in self._sources.items():
            if not path.exists():
                continue
            try:
                size = path.stat().st_size
                if size < self._offsets.get(tag, 0):
                    self._offsets[tag] = 0  # file was truncated (process restarted)
                with open(path, "r", errors="replace") as f:
                    f.seek(self._offsets[tag])
                    new_text = f.read()
                    self._offsets[tag] = f.tell()
            except OSError:
                continue
            if not new_text:
                continue
            for line in new_text.splitlines():
                if line:
                    yield tag, line