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"""

Problem Bank

------------

Debugging challenges across three difficulty tiers.



Each problem is a dict with:

  - name:           short identifier

  - description:    what the function should do (natural language)

  - buggy_code:     the broken implementation

  - function_name:  entry-point the tests call

  - tests:          list of (args_tuple, expected_return_value)

"""

from __future__ import annotations

from dataclasses import dataclass, field
from typing import Any, List, Tuple


@dataclass
class Problem:
    name: str
    description: str
    buggy_code: str
    function_name: str
    tests: List[Tuple[tuple, Any]] = field(default_factory=list)


# ---------------------------------------------------------------------------
# EASY  --  Syntax errors
# ---------------------------------------------------------------------------

EASY_PROBLEMS: List[Problem] = [
    Problem(
        name="missing_colon",
        description=(
            "Write a function `is_even(n)` that returns True when n is even "
            "and False otherwise."
        ),
        buggy_code=(
            "def is_even(n)\n"
            "    if n % 2 == 0:\n"
            "        return True\n"
            "    return False\n"
        ),
        function_name="is_even",
        tests=[
            ((0,), True),
            ((1,), False),
            ((2,), True),
            ((-3,), False),
            ((100,), True),
        ],
    ),
    Problem(
        name="unmatched_paren",
        description=(
            "Write a function `farewell(name)` that returns the string "
            "'Goodbye, <name>' (e.g. farewell('Alice') -> 'Goodbye, Alice')."
        ),
        buggy_code=(
            "def farewell(name):\n"
            '    return "Goodbye, " + str(name\n'
        ),
        function_name="farewell",
        tests=[
            (("Alice",), "Goodbye, Alice"),
            (("Bob",), "Goodbye, Bob"),
            (("",), "Goodbye, "),
            (("World",), "Goodbye, World"),
        ],
    ),
    Problem(
        name="bad_indentation",
        description=(
            "Write a function `sum_list(lst)` that returns the sum of all "
            "elements in a list of numbers."
        ),
        buggy_code=(
            "def sum_list(lst):\n"
            "    total = 0\n"
            "    for item in lst:\n"
            "    total += item\n"
            "    return total\n"
        ),
        function_name="sum_list",
        tests=[
            (([],), 0),
            (([1, 2, 3],), 6),
            (([10, -5, 3],), 8),
            (([-1, -2, -3],), -6),
            (([100],), 100),
        ],
    ),
]

# ---------------------------------------------------------------------------
# MEDIUM  --  Logic errors
# ---------------------------------------------------------------------------

MEDIUM_PROBLEMS: List[Problem] = [
    Problem(
        name="off_by_one_max",
        description=(
            "Write a function `find_max(lst)` that returns the largest "
            "element in a non-empty list.  Return None for an empty list."
        ),
        buggy_code=(
            "def find_max(lst):\n"
            "    if not lst:\n"
            "        return None\n"
            "    max_val = lst[0]\n"
            "    for i in range(1, len(lst) - 1):\n"
            "        if lst[i] > max_val:\n"
            "            max_val = lst[i]\n"
            "    return max_val\n"
        ),
        function_name="find_max",
        tests=[
            (([],), None),
            (([5],), 5),
            (([1, 2, 3],), 3),
            (([3, 1, 2],), 3),
            (([1, 3, 2],), 3),
            (([-5, -1, -10],), -1),
        ],
    ),
    Problem(
        name="wrong_operator_palindrome",
        description=(
            "Write a function `is_palindrome(s)` that returns True if the "
            "string s reads the same forwards and backwards (case-insensitive)."
        ),
        buggy_code=(
            "def is_palindrome(s):\n"
            "    s = s.lower()\n"
            "    left, right = 0, len(s) - 1\n"
            "    while left > right:\n"
            "        if s[left] != s[right]:\n"
            "            return False\n"
            "        left += 1\n"
            "        right -= 1\n"
            "    return True\n"
        ),
        function_name="is_palindrome",
        tests=[
            (("",), True),
            (("a",), True),
            (("Racecar",), True),
            (("hello",), False),
            (("abba",), True),
            (("abcd",), False),
        ],
    ),
    Problem(
        name="missing_edge_case_flatten",
        description=(
            "Write a function `flatten(nested)` that takes a list which may "
            "contain integers or sublists of integers (one level deep) and "
            "returns a flat list of all the integers."
        ),
        buggy_code=(
            "def flatten(nested):\n"
            "    result = []\n"
            "    for item in nested:\n"
            "        if isinstance(item, list):\n"
            "            for sub in item:\n"
            "                result.append(sub)\n"
            "    return result\n"
        ),
        function_name="flatten",
        tests=[
            (([],), []),
            (([1, 2, 3],), [1, 2, 3]),
            (([[1, 2], 3],), [1, 2, 3]),
            (([[1], [2, 3]],), [1, 2, 3]),
            (([1, [2], 3, [4, 5]],), [1, 2, 3, 4, 5]),
        ],
    ),
]

# ---------------------------------------------------------------------------
# HARD  --  Algorithmic / subtle bugs
# ---------------------------------------------------------------------------

HARD_PROBLEMS: List[Problem] = [
    Problem(
        name="binary_search_bounds",
        description=(
            "Write a function `binary_search(arr, target)` that returns the "
            "index of target in a sorted list, or -1 if not found."
        ),
        buggy_code=(
            "def binary_search(arr, target):\n"
            "    left, right = 0, len(arr)\n"
            "    while left <= right:\n"
            "        mid = (left + right) // 2\n"
            "        if arr[mid] == target:\n"
            "            return mid\n"
            "        elif arr[mid] < target:\n"
            "            left = mid + 1\n"
            "        else:\n"
            "            right = mid\n"
            "    return -1\n"
        ),
        function_name="binary_search",
        tests=[
            (([], 5), -1),
            (([1], 1), 0),
            (([1], 2), -1),
            (([1, 3, 5, 7, 9], 5), 2),
            (([1, 3, 5, 7, 9], 1), 0),
            (([1, 3, 5, 7, 9], 9), 4),
            (([1, 3, 5, 7, 9], 4), -1),
            (([2, 4, 6, 8, 10, 12], 12), 5),
        ],
    ),
    Problem(
        name="merge_sorted_wrong",
        description=(
            "Write a function `merge_sorted(a, b)` that merges two sorted "
            "lists into a single sorted list."
        ),
        buggy_code=(
            "def merge_sorted(a, b):\n"
            "    result = []\n"
            "    i = j = 0\n"
            "    while i < len(a) and j < len(b):\n"
            "        if a[i] <= b[j]:\n"
            "            result.append(a[i])\n"
            "            i += 1\n"
            "        else:\n"
            "            result.append(b[j])\n"
            "            j += 1\n"
            "    return result\n"
        ),
        function_name="merge_sorted",
        tests=[
            (([], []), []),
            (([1], []), [1]),
            (([], [2]), [2]),
            (([1, 3, 5], [2, 4, 6]), [1, 2, 3, 4, 5, 6]),
            (([1, 2], [3, 4, 5]), [1, 2, 3, 4, 5]),
            (([1, 1, 1], [1, 1]), [1, 1, 1, 1, 1]),
        ],
    ),
    Problem(
        name="matrix_spiral_bug",
        description=(
            "Write a function `spiral_order(matrix)` that returns the elements "
            "of an MxN matrix in spiral order (clockwise from top-left). "
            "Return an empty list for an empty matrix."
        ),
        buggy_code=(
            "def spiral_order(matrix):\n"
            "    if not matrix or not matrix[0]:\n"
            "        return []\n"
            "    result = []\n"
            "    top, bottom = 0, len(matrix) - 1\n"
            "    left, right = 0, len(matrix[0]) - 1\n"
            "    while top <= bottom and left <= right:\n"
            "        for col in range(left, right + 1):\n"
            "            result.append(matrix[top][col])\n"
            "        top += 1\n"
            "        for row in range(top, bottom + 1):\n"
            "            result.append(matrix[row][right])\n"
            "        right -= 1\n"
            "        for col in range(right, left - 1, -1):\n"
            "            result.append(matrix[bottom][col])\n"
            "        bottom -= 1\n"
            "        for row in range(bottom, top - 1, -1):\n"
            "            result.append(matrix[row][left])\n"
            "        left += 1\n"
            "    return result\n"
        ),
        function_name="spiral_order",
        tests=[
            (([],), []),
            (([[1]],), [1]),
            (([[1, 2], [3, 4]],), [1, 2, 4, 3]),
            (([[1, 2, 3], [4, 5, 6], [7, 8, 9]],), [1, 2, 3, 6, 9, 8, 7, 4, 5]),
            (([[1, 2, 3, 4]],), [1, 2, 3, 4]),
            (([[1], [2], [3]],), [1, 2, 3]),
        ],
    ),
]

# ---------------------------------------------------------------------------
# Lookup
# ---------------------------------------------------------------------------

TASK_PROBLEMS = {
    "fix_syntax": EASY_PROBLEMS,
    "fix_logic": MEDIUM_PROBLEMS,
    "fix_algorithm": HARD_PROBLEMS,
}

ALL_TASK_NAMES = list(TASK_PROBLEMS.keys())