import { loadConfig } from "../config";
import { isGitRepo, getStagedDiff, getUnstagedFiles, getRepoRoot, stageFiles } from "../git";
import { selectFiles } from "../selector";
import { BACK } from "../menu";
import { collectProjectContext } from "../context";
import { EXPLAIN_SYSTEM_PROMPT, buildExplainPrompt } from "../prompt";
import { callAI } from "../ai";
import { BOLD, GREEN, RED, DIM, RESET, CYAN } from "../terminal";
import { isStdinTTY } from "../tty";
import type { StreamCallbacks } from "../types";
import type { ParsedArgs } from "../cli";

export async function handleExplain(args: ParsedArgs): Promise<number> {
  const config = await loadConfig();

  if (!config.apiKey) {
    console.error(`\n  ${RED()}Error: API key not set. Run ${BOLD()}gai config${RESET()}${RED()} to configure.${RESET()}\n`);
    return 1;
  }

  const unstaged = args.flags["unstaged"] as boolean;
  const staged = args.flags["staged"] as boolean;
  const verbose = args.flags["verbose"] as boolean;

  // Determine which diff to explain
  let diff: string;
  let sourceLabel: string;

  if (unstaged) {
    if (!(await isGitRepo())) {
      console.error(`\n  ${RED()}Error: Not a git repository.${RESET()}\n`);
      return 1;
    }
    try {
      diff = (await Bun.$`git diff`.quiet().text()).trim();
    } catch {
      diff = "";
    }
    sourceLabel = "unstaged changes";
  } else {
    // Default: staged changes (or piped)
    if (isStdinTTY()) {
      if (!(await isGitRepo())) {
        console.error(`\n  ${RED()}Error: Not a git repository.${RESET()}\n`);
        return 1;
      }
      diff = await getStagedDiff();
      sourceLabel = "staged changes";

      // If no staged changes, offer to stage unstaged files
      if (!diff) {
        const unstagedFiles = await getUnstagedFiles();
        if (unstagedFiles.length > 0) {
          const selected = await selectFiles([], unstagedFiles);
          if (selected === BACK) return 0;
          if (selected.length > 0) {
            await stageFiles(selected);
            console.log(`  ${GREEN()}Staged ${selected.length} file(s).${RESET()}`);
            diff = await getStagedDiff();
          }
        }
      }
    } else {
      // Read from pipe
      const chunks: Buffer[] = [];
      for await (const chunk of process.stdin) {
        chunks.push(typeof chunk === "string" ? Buffer.from(chunk) : chunk);
      }
      diff = Buffer.concat(chunks).toString("utf-8").trim();
      sourceLabel = "piped input";
    }
  }

  if (!diff) {
    console.log(`  ${DIM()}No ${sourceLabel} to explain.${RESET()}`);
    return 0;
  }

  if (args.flags["verbose"]) {
    console.log(`  ${DIM()}Explaining ${sourceLabel} (${diff.length} bytes)${RESET()}`);
  }

  const MAX_DIFF_SIZE = 15000;
  const truncatedDiff = diff.length > MAX_DIFF_SIZE
    ? diff.substring(0, MAX_DIFF_SIZE) + "\n... (truncated)"
    : diff;

  // Collect project context for better explanations
  let contextPrefix = "";
  try {
    if (await isGitRepo()) {
      const repoRoot = await getRepoRoot();
      const ctx = await collectProjectContext(repoRoot);
      if (ctx.packageDescription) {
        contextPrefix = `Project: ${ctx.packageDescription}\n\n`;
      }
    }
  } catch {}

  const userPrompt = contextPrefix + buildExplainPrompt(truncatedDiff);

  if (verbose) {
    console.log(`  ${DIM()}Model: ${config.model} | API: ${config.apiBase}${RESET()}`);
  }

  const tty = isStdinTTY();
  if (tty) {
    console.log(`\n  ${BOLD()}${CYAN()}Analyzing ${sourceLabel}...${RESET()}\n`);
  }

  try {
    const callbacks: StreamCallbacks = tty ? {
      onToken: (token) => process.stdout.write(token),
    } : undefined;

    const explanation = await callAI(config, EXPLAIN_SYSTEM_PROMPT, userPrompt, callbacks);
    if (callbacks) {
      process.stdout.write("\n");
    } else {
      // Non-TTY: print the result directly
      process.stdout.write(explanation + "\n");
    }
  } catch (err) {
    console.error(`\n  ${RED()}AI request failed: ${err instanceof Error ? err.message : err}${RESET()}\n`);
    return 1;
  }

  return 0;
}