freezmo/components/activities/FillBlank.tsx

"use client";

import { useEffect, useRef, useState } from "react";

import type { Activity } from "@/lib/schemas";
import { cn } from "@/lib/utils";
import { Button } from "@/components/ui";

import {
  ActivityHeader,
  FeedbackPanel,
  Hint,
  OptionTile,
  PromptCard,
  type ActivityComponentProps,
} from "./shared";

type FillBlankActivity = Extract<Activity, { type: "fill_blank" }>;

const BLANK = /_{2,}/;

function normalise(value: string): string {
  return value
    .toLowerCase()
    .normalize("NFKD")
    .replace(/[\u0300-\u036f]/g, "")
    .replace(/[^a-z0-9\s]/g, " ")
    .replace(/\s+/g, " ")
    .trim();
}

function levenshtein(a: string, b: string): number {
  if (a === b) return 0;
  if (!a.length || !b.length) return Math.max(a.length, b.length);
  let previous = Array.from({ length: b.length + 1 }, (_, i) => i);
  for (let i = 1; i <= a.length; i++) {
    const current = [i];
    for (let j = 1; j <= b.length; j++) {
      current[j] = Math.min(
        previous[j] + 1,
        current[j - 1] + 1,
        previous[j - 1] + (a[i - 1] === b[j - 1] ? 0 : 1),
      );
    }
    previous = current;
  }
  return previous[b.length];
}

/** Lenient comparison: case/punctuation-insensitive, one typo allowed, "/" alternatives. */
export function isFillBlankCorrect(userValue: string, answer: string): boolean {
  const user = normalise(userValue);
  if (!user) return false;
  const candidates = answer
    .split("/")
    .map((part) => normalise(part.replace(/\(.*?\)/g, "")))
    .filter(Boolean);
  return candidates.some((candidate) => {
    if (candidate === user) return true;
    if (candidate.length >= 5 && user.length >= 4 && levenshtein(candidate, user) <= 1) return true;
    // accept the answer with/without a trailing unit or article
    const candidateWords = candidate.split(" ");
    const userWords = user.split(" ");
    if (candidateWords.length > 1 && userWords.length === 1) return candidateWords.includes(user);
    return false;
  });
}

export function FillBlank({ activity, active, onAnswered, onNext, isLast }: ActivityComponentProps<FillBlankActivity>) {
  const [value, setValue] = useState("");
  const [chosen, setChosen] = useState<string | null>(null);
  const [result, setResult] = useState<boolean | null>(null);
  const inputRef = useRef<HTMLInputElement>(null);

  const hasBlank = BLANK.test(activity.sentence);
  const [before, after = ""] = hasBlank ? activity.sentence.split(BLANK) : [activity.sentence, ""];

  useEffect(() => {
    if (active && !hasBlank && !activity.options) inputRef.current?.focus();
  }, [active, hasBlank, activity.options]);

  const submit = (answerValue: string) => {
    if (result !== null) return;
    const correct = isFillBlankCorrect(answerValue, activity.answer);
    setChosen(answerValue);
    setResult(correct);
    onAnswered(correct, { userAnswer: answerValue });
  };

  const displayAnswer = chosen ?? "";

  return (
    <>
      <ActivityHeader type="fill_blank" topic={activity.topic} />
      <PromptCard>
        <p className="text-lg font-semibold leading-loose text-frost-950 sm:text-xl dark:text-frost-50">
          {before}
          {activity.options ? (
            <span
              className={cn(
                "mx-1 inline-block min-w-24 rounded-xl border-b-4 px-3 py-0.5 text-center align-baseline",
                result === null
                  ? "border-frost-300 bg-frost-50/80 text-frost-500 dark:bg-white/10 dark:text-frost-200"
                  : result
                    ? "border-emerald-400 bg-emerald-50 text-emerald-800 dark:bg-emerald-400/15 dark:text-emerald-100"
                    : "border-rose-400 bg-rose-50 text-rose-800 dark:bg-rose-400/15 dark:text-rose-100",
              )}
            >
              {displayAnswer || "?"}
            </span>
          ) : (
            <input
              ref={inputRef}
              value={displayAnswer}
              onChange={(event) => setValue(event.target.value)}
              onKeyDown={(event) => {
                if (event.key === "Enter" && !result) {
                  event.preventDefault();
                  submit(value);
                }
                event.stopPropagation();
              }}
              disabled={result !== null}
              aria-label="Type the missing word"
              placeholder="type it"
              autoComplete="off"
              autoCapitalize="off"
              spellCheck={false}
              className={cn(
                "mx-1 inline-block rounded-xl border-b-4 bg-white/70 px-2 py-0.5 text-center text-frost-950 outline-none backdrop-blur-lg placeholder:text-frost-300 dark:bg-white/10 dark:text-frost-50 dark:placeholder:text-white/30",
                result === null
                  ? "border-frost-300 focus:border-frost-500"
                  : result
                    ? "border-emerald-400 bg-emerald-50 text-emerald-900 dark:bg-emerald-400/15 dark:text-emerald-100"
                    : "border-rose-400 bg-rose-50 text-rose-900 dark:bg-rose-400/15 dark:text-rose-100",
              )}
              style={{ width: `${Math.max(5, (displayAnswer.length || 6) + 2)}ch` }}
            />
          )}
          {after}
        </p>
      </PromptCard>

      {activity.options ? (
        <div className="grid grid-cols-2 gap-2.5" role="group" aria-label="Word bank">
          {activity.options.map((option) => {
            const state =
              result === null
                ? "idle"
                : normalise(option) === normalise(activity.answer)
                  ? "correct"
                  : chosen && normalise(chosen) === normalise(option) && result === false
                    ? "wrong"
                    : "dim";
            return (
              <OptionTile
                key={option}
                state={state}
                disabled={result !== null}
                onClick={() => submit(option)}
                className="justify-center text-center"
              >
                {option}
              </OptionTile>
            );
          })}
        </div>
      ) : result === null ? (
        <div className="flex gap-2">
          <Button size="lg" onClick={() => submit(value)} disabled={!value.trim()} className="flex-1">
            Check answer
          </Button>
        </div>
      ) : null}

      {result === null && !activity.options ? (
        <Hint>Press Enter to check. Small typos are forgiven.</Hint>
      ) : null}

      {result !== null ? (
        <FeedbackPanel
          correct={result}
          explanation={activity.explanation}
          xp={activity.xp}
          isLast={isLast}
          onNext={onNext}
          detail={
            <span>
              Expected: <span className="font-bold">{activity.answer}</span>
              {!result && chosen ? (
                <>
                  {" "}
                  — you wrote <span className="italic">{chosen}</span>
                </>
              ) : null}
            </span>
          }
        />
      ) : null}
    </>
  );
}