import { readFileSync } from "node:fs";
import { type } from "arktype";
import type { ToolContext } from "./server.ts";
import { execute, tool } from "./shared.ts";

/** Hard cap on returned content to avoid flooding the model's context window. */
const MAX_CHARS = 12000;

export const ReadFileParams = type({
  path: type.string.describe("absolute path to the file to read"),
  "start_line?": type.number.describe("start line, 1-based inclusive (default: 1)"),
  "end_line?": type.number.describe("end line, 1-based inclusive (default: end of file)"),
});

export function ReadFileTool(_ctx: ToolContext) {
  return tool({
    name: "read_file",
    description:
      "Read lines from a file. Use this to read sections of the PR diff returned by checkout_pr. " +
      `Returns at most ${MAX_CHARS} characters. ` +
      "Prefer fewer, wider reads: read large contiguous ranges rather than many small ones. " +
      "If the TOC lists 10 files, read 3-4 wide ranges that cover them rather than 10 separate calls. " +
      "Example: `read_file({ path: diffPath, start_line: 5, end_line: 200 })`.",
    parameters: ReadFileParams,
    execute: execute(async ({ path, start_line, end_line }) => {
      let content: string;
      try {
        content = readFileSync(path, "utf-8");
      } catch (err) {
        const msg = err instanceof Error ? err.message : String(err);
        throw new Error(`failed to read ${path}: ${msg}`);
      }

      const lines = content.split("\n");
      const start = Math.max(0, (start_line ?? 1) - 1);
      const end = Math.min(lines.length, end_line ?? lines.length);
      const slice = lines.slice(start, end).join("\n");

      if (slice.length <= MAX_CHARS) {
        return { content: slice };
      }

      const truncated = slice.slice(0, MAX_CHARS);
      const linesShown = truncated.split("\n").length;
      const linesTotal = end - start;
      return {
        content: truncated,
        truncated: true,
        note: `Output capped at ${MAX_CHARS} chars (showed ${linesShown}/${linesTotal} lines). Use a narrower line range to read the rest.`,
      };
    }),
  });
}