Efficient Nested JSON Parsing

HardPython

Problem

Wissen Technology data pipelines receive JSON payloads with nested objects and arrays. Implement flatten_json to convert any JSON-compatible value into a dictionary mapping each leaf path to its value, without using recursive function calls.

Formal Specification

Input is one JSON-compatible Python value: a dictionary, list, string, number, boolean, or None. Object keys contain only letters, digits, and underscores. Return a dictionary whose keys are paths and whose values are scalar JSON values. Use $ for the root, .key for object fields, and [index] for array elements. None, booleans, strings, and numbers are leaves. Empty dictionaries and lists are also emitted as values at their own paths.

The implementation must use an explicit stack so it remains safe for nesting depths that could exceed Python's recursion limit. The input must not be modified.

Constraints

At most 10^5 total JSON values
Maximum nesting depth is 10^5
Object keys match [A-Za-z0-9_]+
The input is valid JSON-compatible data
The input must not be modified

Function Signature

def flatten_json(data):

Formal Specification

The implementation must use an explicit stack so it remains safe for nesting depths that could exceed Python's recursion limit. The input must not be modified.