Select an Object by a Dynamic Integer Field

Problem Statement

You are asked to implement a utility function that selects a single object (represented as a dictionary) from a collection based on a specified field name and an extreme condition (MIN or MAX).

Each object is a mapping of string keys to integer values. Crucially, records are semi-structured: not every object is guaranteed to contain the requested field.

Your function should:

1. Filter for records that contain the target field.
2. Find and return the record whose value for field is either the minimum or maximum, depending on mode.
3. In the event of a tie, return the first record encountered in the original sequence.
4. If no records contain the specified field, or the input list is empty, return None.
5. Raise a ValueError if mode is neither "MIN" nor "MAX" (case-insensitive).

Function Signature

from typing import Any

def select_by_field(
    records: list[dict[str, int]], 
    field: str, 
    mode: str = "MAX"
) -> dict[str, int] | None:
    pass

Constraints

• 0 <= len(records) <= 100,000
• 0 <= len(record.keys()) <= 50 for each object
• Integer values can be negative, zero, or positive (-10^9 <= value <= 10^9).
• The algorithm should operate in O(N) time and O(1) auxiliary space, where N is the number of records.

Examples

Example 1

records = [
    {"a": 10, "b": 20},
    {"a": 5, "c": 15},
    {"a": 10, "d": 99}
]

# Call:
select_by_field(records, field="a", mode="MAX")
# Output: {"a": 10, "b": 20}
# Explanation: Both records at index 0 and 2 have value 10 for "a". Index 0 is returned to break ties by first occurrence.

Example 2

records = [
    {"id": 1, "score": -5},
    {"id": 2},
    {"id": 3, "score": -20},
    {"id": 4, "score": 0}
]

# Call:
select_by_field(records, field="score", mode="MIN")
# Output: {"id": 3, "score": -20}
# Explanation: The record with id 2 does not have "score" and is ignored. -20 is the minimum among [-5, -20, 0].

Example 3

records = [{"x": 1}, {"y": 2}]

# Call:
select_by_field(records, field="z", mode="MAX")
# Output: None
# Explanation: No record contains key "z".