import fs from "node:fs/promises";
import { getResolvedLoggerSettings } from "../../logging.js";
import {
  ErrorCodes,
  errorShape,
  formatValidationErrors,
  validateLogsTailParams,
} from "../protocol/index.js";
import type { GatewayRequestHandlers } from "./types.js";

const DEFAULT_LIMIT = 500;
const DEFAULT_MAX_BYTES = 250_000;
const MAX_LIMIT = 5000;
const MAX_BYTES = 1_000_000;

function clamp(value: number, min: number, max: number) {
  return Math.max(min, Math.min(max, value));
}

async function readLogSlice(params: {
  file: string;
  cursor?: number;
  limit: number;
  maxBytes: number;
}) {
  const stat = await fs.stat(params.file).catch(() => null);
  if (!stat) {
    return {
      cursor: 0,
      size: 0,
      lines: [] as string[],
      truncated: false,
      reset: false,
    };
  }

  const size = stat.size;
  const maxBytes = clamp(params.maxBytes, 1, MAX_BYTES);
  const limit = clamp(params.limit, 1, MAX_LIMIT);
  let cursor =
    typeof params.cursor === "number" && Number.isFinite(params.cursor)
      ? Math.max(0, Math.floor(params.cursor))
      : undefined;
  let reset = false;
  let truncated = false;
  let start = 0;

  if (cursor != null) {
    if (cursor > size) {
      reset = true;
      start = Math.max(0, size - maxBytes);
      truncated = start > 0;
    } else {
      start = cursor;
      if (size - start > maxBytes) {
        reset = true;
        truncated = true;
        start = Math.max(0, size - maxBytes);
      }
    }
  } else {
    start = Math.max(0, size - maxBytes);
    truncated = start > 0;
  }

  if (size === 0 || size <= start) {
    return {
      cursor: size,
      size,
      lines: [] as string[],
      truncated,
      reset,
    };
  }

  const handle = await fs.open(params.file, "r");
  try {
    let prefix = "";
    if (start > 0) {
      const prefixBuf = Buffer.alloc(1);
      const prefixRead = await handle.read(prefixBuf, 0, 1, start - 1);
      prefix = prefixBuf.toString("utf8", 0, prefixRead.bytesRead);
    }

    const length = Math.max(0, size - start);
    const buffer = Buffer.alloc(length);
    const readResult = await handle.read(buffer, 0, length, start);
    const text = buffer.toString("utf8", 0, readResult.bytesRead);
    let lines = text.split("\n");
    if (start > 0 && prefix !== "\n") {
      lines = lines.slice(1);
    }
    if (lines.length > 0 && lines[lines.length - 1] === "") {
      lines = lines.slice(0, -1);
    }
    if (lines.length > limit) {
      lines = lines.slice(lines.length - limit);
    }

    cursor = size;

    return {
      cursor,
      size,
      lines,
      truncated,
      reset,
    };
  } finally {
    await handle.close();
  }
}

export const logsHandlers: GatewayRequestHandlers = {
  "logs.tail": async ({ params, respond }) => {
    if (!validateLogsTailParams(params)) {
      respond(
        false,
        undefined,
        errorShape(
          ErrorCodes.INVALID_REQUEST,
          `invalid logs.tail params: ${formatValidationErrors(validateLogsTailParams.errors)}`,
        ),
      );
      return;
    }

    const p = params as { cursor?: number; limit?: number; maxBytes?: number };
    const file = getResolvedLoggerSettings().file;
    try {
      const result = await readLogSlice({
        file,
        cursor: p.cursor,
        limit: p.limit ?? DEFAULT_LIMIT,
        maxBytes: p.maxBytes ?? DEFAULT_MAX_BYTES,
      });
      respond(true, { file, ...result }, undefined);
    } catch (err) {
      respond(
        false,
        undefined,
        errorShape(ErrorCodes.UNAVAILABLE, `log read failed: ${String(err)}`),
      );
    }
  },
};