import React, { useCallback, useEffect, useMemo, useState } from "react";

import {
  Alert, Box, Button, Checkbox, Chip, IconButton, LinearProgress, Paper, Skeleton, Stack,
  TextField, Tooltip, Typography,
} from "@mui/material";
import DeleteOutlineIcon from "@mui/icons-material/DeleteOutline";
import ArrowUpwardIcon from "@mui/icons-material/ArrowUpward";
import ArrowDownwardIcon from "@mui/icons-material/ArrowDownward";

import { getApiErrorMessage } from "../api";
import {
  CHECKLIST_CATEGORIES, Checklist, ChecklistItem, applicationChecklistApi,
} from "../applicationWorkspace";

// Phase 5 Milestone 2 — the application checklist.
//
// One workflow surface, not a new tracker: the system items arrive already ticked wherever the
// existing readiness signals say the work is done (CV attached, cover letter written, follow-up
// scheduled...). Everything here is the user's to tick, add to, reorder or dismiss.
// docs/architecture/application-workspace.md.
export default function ApplicationChecklist({ jobId, onChanged }: { jobId: number; onChanged?: () => void }) {
  const [checklist, setChecklist] = useState<Checklist | null>(null);
  const [error, setError] = useState<string | null>(null);
  const [busy, setBusy] = useState(false);
  const [draft, setDraft] = useState("");

  const load = useCallback(async () => {
    try {
      setChecklist(await applicationChecklistApi.get(jobId));
      setError(null);
    } catch (err) {
      setError(getApiErrorMessage(err, "Could not load the checklist."));
    }
  }, [jobId]);

  useEffect(() => {
    load();
  }, [load]);

  // Every mutation re-reads: the backend re-syncs the auto-completed items on each read, so the
  // response is the only thing that knows the real state.
  const mutate = useCallback(async (run: () => Promise<unknown>) => {
    setBusy(true);
    try {
      await run();
      await load();
      onChanged?.();
    } catch (err) {
      setError(getApiErrorMessage(err, "Could not update the checklist."));
    } finally {
      setBusy(false);
    }
  }, [load, onChanged]);

  const toggle = (item: ChecklistItem) =>
    mutate(() => applicationChecklistApi.update(jobId, item.id, {
      status: item.status === "done" ? "pending" : "done",
    }));

  const remove = (item: ChecklistItem) => mutate(() => applicationChecklistApi.remove(jobId, item.id));

  const move = (item: ChecklistItem, delta: -1 | 1) => {
    if (!checklist) return;
    const ids = checklist.items.map((i) => i.id);
    const from = ids.indexOf(item.id);
    const to = from + delta;
    if (to < 0 || to >= ids.length) return;
    [ids[from], ids[to]] = [ids[to], ids[from]];
    return mutate(() => applicationChecklistApi.reorder(jobId, ids));
  };

  const add = (e: React.FormEvent) => {
    e.preventDefault();
    const title = draft.trim();
    if (!title) return;
    setDraft("");
    return mutate(() => applicationChecklistApi.add(jobId, title));
  };

  const grouped = useMemo(() => {
    const live = checklist?.items.filter((i) => i.status !== "dismissed") ?? [];
    return CHECKLIST_CATEGORIES
      .map((c) => ({ ...c, items: live.filter((i) => i.category === c.key) }))
      .filter((c) => c.items.length > 0);
  }, [checklist]);

  if (!checklist && !error) {
    return <Stack spacing={1}>{[0, 1, 2].map((i) => <Skeleton key={i} variant="rounded" height={52} />)}</Stack>;
  }

  const progress = checklist?.progress;

  return (
    <Stack spacing={2}>
      {error && <Alert severity="error" onClose={() => setError(null)}>{error}</Alert>}

      {progress && (
        <Paper sx={{ p: 2, borderRadius: 3 }}>
          <Stack direction="row" alignItems="baseline" justifyContent="space-between" sx={{ mb: 1 }}>
            <Typography variant="subtitle2" sx={{ fontWeight: 800 }}>Application checklist</Typography>
            <Typography variant="body2" color="text.secondary">
              {progress.completed} of {progress.total} done
            </Typography>
          </Stack>
          <LinearProgress
            variant="determinate"
            value={progress.percent}
            aria-label="Checklist completion"
            sx={{ height: 8, borderRadius: 4 }}
          />
        </Paper>
      )}

      {grouped.map((group) => (
        <Paper key={group.key} sx={{ p: 2, borderRadius: 3 }}>
          <Typography variant="overline" color="text.secondary">{group.label}</Typography>
          <Stack sx={{ mt: 0.5 }}>
            {group.items.map((item) => (
              <Stack
                key={item.id}
                direction="row"
                alignItems="flex-start"
                spacing={1}
                sx={{ py: 0.75, borderRadius: 2, "&:hover .checklist-actions": { opacity: 1 } }}
              >
                <Checkbox
                  size="small"
                  checked={item.status === "done"}
                  disabled={busy}
                  onChange={() => toggle(item)}
                  inputProps={{ "aria-label": item.title }}
                  sx={{ mt: -0.25 }}
                />
                <Box sx={{ flexGrow: 1, minWidth: 0 }}>
                  <Stack direction="row" spacing={0.75} alignItems="center" flexWrap="wrap">
                    <Typography
                      variant="body2"
                      sx={{
                        fontWeight: 600,
                        textDecoration: item.status === "done" ? "line-through" : "none",
                        color: item.status === "done" ? "text.disabled" : "text.primary",
                      }}
                    >
                      {item.title}
                    </Typography>
                    {!item.isSystemGenerated && <Chip size="small" label="Yours" variant="outlined" />}
                    {item.isAutoCompleted && <Chip size="small" label="Detected" color="success" variant="outlined" />}
                  </Stack>
                  {item.description && (
                    <Typography variant="caption" color="text.secondary">{item.description}</Typography>
                  )}
                </Box>
                <Stack direction="row" className="checklist-actions" sx={{ opacity: { xs: 1, md: 0 }, transition: "opacity .15s" }}>
                  <Tooltip title="Move up">
                    <span>
                      <IconButton size="small" disabled={busy} aria-label={`Move up: ${item.title}`} onClick={() => move(item, -1)}>
                        <ArrowUpwardIcon fontSize="inherit" />
                      </IconButton>
                    </span>
                  </Tooltip>
                  <Tooltip title="Move down">
                    <span>
                      <IconButton size="small" disabled={busy} aria-label={`Move down: ${item.title}`} onClick={() => move(item, 1)}>
                        <ArrowDownwardIcon fontSize="inherit" />
                      </IconButton>
                    </span>
                  </Tooltip>
                  <Tooltip title={item.isSystemGenerated ? "Not relevant for this role" : "Delete"}>
                    <span>
                      <IconButton size="small" disabled={busy} aria-label={`Remove: ${item.title}`} onClick={() => remove(item)}>
                        <DeleteOutlineIcon fontSize="inherit" />
                      </IconButton>
                    </span>
                  </Tooltip>
                </Stack>
              </Stack>
            ))}
          </Stack>
        </Paper>
      ))}

      <Paper component="form" onSubmit={add} sx={{ p: 2, borderRadius: 3 }}>
        <Stack direction="row" spacing={1}>
          <TextField
            fullWidth
            size="small"
            label="Add your own task"
            value={draft}
            disabled={busy}
            onChange={(e) => setDraft(e.target.value)}
          />
          <Button type="submit" variant="contained" disabled={busy || !draft.trim()}>Add</Button>
        </Stack>
      </Paper>
    </Stack>
  );
}