Reconcile Writable Config with Read-Only Defaults (Tolerances, Dry-Run)

Title: Array Reconciliation Using const int* Defaults and By-Value Controls
Level: Difficult
Concepts: const pointer for read-only input, writable pointer for in-place updates, by-value parameters (tolerance, flags), output via pointers, aliasing safety, input validation

Scenario

Your device stores default configuration values in a read-only table (flash/ROM). At runtime, a writable configuration array may drift from defaults. You must reconcile the writable array against defaults as follows:

• If abs(current[i] - defaults[i]) > tolerance, the element is out-of-spec.
• If dry_run == false, overwrite current[i] with defaults[i].
• If dry_run == true, do not modify current, but still report how many elements would be updated and the first index that would change.

The defaults table must never be modified; it is provided as a const int*. Control inputs (tolerance, dry_run) are by value.

Problem Statement

Implement a function that compares two same-length arrays, counts out-of-spec entries, optionally updates the writable array to match defaults (depending on dry_run), and reports how many entries were actually updated (or would be updated), plus the first mismatch index (or -1 if none). Guard against invalid inputs and aliasing (if defaults and current point to the same buffer, treat as an error).

Requirements

• Inputs:
  • const int *defaults — read-only table; must not be modified.
  • int *current — writable config array.
  • int n — array length (n >= 0).
  • int tolerance — inclusive tolerance (≥ 0).
  • bool dry_run — by-value control flag.
• Outputs:
  • int *out_mismatches — count of indices with abs(diff) > tolerance.
  • int *out_updates — number of elements updated (or would update if dry_run==true).
  • int *out_first_index — smallest index that is out-of-spec, or -1 if none.
• Rules:
  • If any pointer is NULL, n < 0, or tolerance < 0, return error without modifying outputs.
  • If n == 0, succeed with all outputs as 0 and -1 (no work).
  • If defaults == current (same address), return error: cannot read-only compare and write in-place safely.
  • Time: O(n), Space: O(1).

Function Details

• Name: reconcile_config_with_defaults
• Arguments:
  • const int *defaults
  • int *current
  • int n
  • int tolerance
  • bool dry_run
  • int *out_mismatches
  • int *out_updates
  • int *out_first_index
• Return Value:
  • int — 0 on success; -1 on invalid input; -2 on aliasing (defaults == current).
• Description:
  Scan arrays and compute diff = current[i] - defaults[i]. If diff > tolerance or diff < -tolerance, mark as mismatch. Count mismatches and record the first mismatch index. If dry_run==false, set current[i] = defaults[i] and increment updates; if dry_run==true, leave current untouched but still increment updates for each out-of-spec slot. Use const int* for defaults to prevent accidental writes. tolerance and dry_run are by value; any changes inside the function must not affect caller variables.

Solution Approach

• Validate inputs: pointers not NULL, n >= 0, tolerance >= 0.
• If n == 0: write *out_mismatches = 0; *out_updates = 0; *out_first_index = -1; return 0;.
• If defaults == current: return -2 (cannot safely reconcile).
• Initialize locals: mismatches = 0; updates = 0; first = -1;.
• Loop i = 0..n-1:
  • Compute diff = current[i] - defaults[i].
  • If diff > tolerance || diff < -tolerance:
    • mismatches++; if first < 0, set first = i.
    • If !dry_run, do current[i] = defaults[i]; and updates++; else updates++ only.
• Set outputs and return 0.

Tasks to Perform

1. Validate: defaults, current, out_mismatches, out_updates, out_first_index are not NULL; n >= 0; tolerance >= 0.
2. Check aliasing: if defaults == current, return -2.
3. Handle n == 0: set outputs to 0, 0, -1 and return 0.
4. Iterate through arrays; detect out-of-spec entries with inclusive tolerance.
5. If dry_run==false, update current[i] to default; otherwise leave it unchanged.
6. Track mismatches, updates, and the earliest index requiring change.
7. Write outputs and return 0.

Test Cases