app.py

"""Week 06 — Plagiarism Checker (Flask backend, LCS via DP)"""

from flask import Flask, jsonify, request, send_from_directory

app = Flask(__name__, static_folder="static")


# ---------------------------------------------------------------------------
# LCS — Dynamic Programming
# ---------------------------------------------------------------------------

def lcs(text_a, text_b):
    """
    Longest Common Subsequence length using bottom-up DP.
    Returns: (lcs_length, dp_table)
    """
    m, n = len(text_a), len(text_b)
    dp = [[0] * (n + 1) for _ in range(m + 1)]
    for i in range(1, m + 1):
        for j in range(1, n + 1):
            if text_a[i - 1] == text_b[j - 1]:
                dp[i][j] = dp[i - 1][j - 1] + 1
            else:
                dp[i][j] = max(dp[i - 1][j], dp[i][j - 1])
    return dp[m][n], dp


def backtrack_lcs(dp, text_a, text_b):
    """Backtrack through the DP table to recover the actual LCS string."""
    i, j = len(text_a), len(text_b)
    result = []
    while i > 0 and j > 0:
        if text_a[i - 1] == text_b[j - 1]:
            result.append(text_a[i - 1])
            i -= 1
            j -= 1
        elif dp[i - 1][j] >= dp[i][j - 1]:
            i -= 1
        else:
            j -= 1
    return "".join(reversed(result))


def similarity_score(text_a, text_b, lcs_length):
    """Similarity = LCS length / max(len(A), len(B)) * 100."""
    if not text_a and not text_b:
        return 100.0
    if not text_a or not text_b:
        return 0.0
    return lcs_length / max(len(text_a), len(text_b)) * 100


def generate_diff(dp, text_a, text_b):
    """
    Walk the DP table from (m, n) to (0, 0) and classify each character as
    'match' (in LCS), 'removed' (only in A), or 'added' (only in B).
    """
    i, j = len(text_a), len(text_b)
    diff = []
    while i > 0 or j > 0:
        if i > 0 and j > 0 and text_a[i - 1] == text_b[j - 1]:
            diff.append({"char": text_a[i - 1], "status": "match"})
            i -= 1
            j -= 1
        elif j > 0 and (i == 0 or dp[i][j - 1] >= dp[i - 1][j]):
            diff.append({"char": text_b[j - 1], "status": "added"})
            j -= 1
        else:
            diff.append({"char": text_a[i - 1], "status": "removed"})
            i -= 1
    return list(reversed(diff))


# ---------------------------------------------------------------------------
# Routes
# ---------------------------------------------------------------------------

@app.route("/")
def index():
    return send_from_directory("static", "index.html")


@app.route("/check", methods=["POST"])
def check():
    data = request.get_json() or {}
    text_a = data.get("text_a", "")
    text_b = data.get("text_b", "")

    length, dp = lcs(text_a, text_b)
    lcs_str = backtrack_lcs(dp, text_a, text_b)
    score = similarity_score(text_a, text_b, length)
    diff = generate_diff(dp, text_a, text_b)

    return jsonify({
        "similarity": round(score, 2),
        "lcs_length": length,
        "lcs_string": lcs_str,
        "len_a": len(text_a),
        "len_b": len(text_b),
        "diff": diff,
    })


if __name__ == "__main__":
    app.run(debug=True, port=5000)