Tagged Union Telemetry Frame (Variant Payload with Summary)

Title: Process Variant Telemetry Using a Tagged Union (Sum/Length/Clamp)
Level: Difficult
Concepts: struct with tag + union (tagged union), safe variant handling via switch on enum, fixed‑size arrays in union members, validation, deterministic outputs

Scenario

A device sends telemetry frames where the payload type varies per frame. To avoid dynamic allocation and keep a fixed footprint, each frame is a tagged union:

• An array of integers,
• A scalar double,
• A small text message (C‑string) in a fixed‑length buffer.

You must write a function that summarizes any frame based on its tag:

• For INT_ARRAY: compute sum and count.
• For SCALAR: clamp into [min_allowed, max_allowed] and report whether clamping happened.
• For TEXT: compute string length (up to buffer size, stopping at '\0'), and report presence of printable‑only (ASCII 32..126).

Problem Statement

Define a struct that contains a tag (enum) and a union with three payload shapes. Implement a function that, given a const pointer to a frame and some by‑value controls, fills a summary struct with results appropriate to the tag. The function must validate inputs and set outputs deterministically.

Requirements

• Frame definition (suggested):
  • enum FrameTag { FT_INT_ARRAY=0, FT_SCALAR=1, FT_TEXT=2 };
  • struct Frame { enum FrameTag tag; union { struct { int data[8]; int count; } ints; double scalar; char text[32]; } payload; };
    • For FT_INT_ARRAY, use payload.ints.count in [0..8].
    • For FT_TEXT, treat payload.text as a C‑string up to 31 chars plus '\0'.
• Summary output (suggested):
  • struct Summary { enum FrameTag tag; long sum; int count; double clamped_scalar; bool scalar_was_clamped; int text_len; bool text_all_printable; };
• Function behavior per tag:
  • INT_ARRAY: sum first count elements into a long; set count; ignore scalar/text fields.
  • SCALAR: if scalar < min_allowed set to min_allowed; if > max_allowed set to max_allowed; report scalar_was_clamped.
  • TEXT: compute length up to first '\0' or 31; test all characters are printable ASCII (32..126).
• Validation:
  • Pointers frame, out_summary must be non‑NULL.
  • frame->tag must be a valid enum.
  • For FT_INT_ARRAY, 0 ≤ count ≤ 8.
  • min_allowed ≤ max_allowed.
• No dynamic memory; fixed sizes only.
• Time: O(n) per case (n ≤ 8 for ints, n ≤ 31 for text).
• Space: O(1).

Function Details

• Name: summarize_frame
• Arguments:
  • const struct Frame *frame
  • double min_allowed
  • double max_allowed
  • struct Summary *out_summary
• Return Value:
  int — 0 on success; -1 on invalid input (no output written).
• Description:
  Use a switch (frame->tag) to select the active union member and compute a tag‑appropriate summary. Do not read inactive union members. Validate ranges and inputs; on failure return -1 without modifying *out_summary.

Solution Approach

• Validate frame != NULL, out_summary != NULL, min_allowed ≤ max_allowed.
• Validate frame->tag ∈ {FT_INT_ARRAY, FT_SCALAR, FT_TEXT}.
• Initialize all summary fields to safe defaults; set summary.tag = frame->tag.
• INT_ARRAY:
  • Validate 0 ≤ count ≤ 8.
  • Accumulate long sum over data[0..count-1]; set summary.sum=sum, summary.count=count.
• SCALAR:
  • Let x = frame->payload.scalar; clamp into [min_allowed, max_allowed]; set summary.clamped_scalar, summary.scalar_was_clamped.
• TEXT:
  • Walk up to 31 characters or until '\0'; compute len.
  • Check 32 ≤ ch ≤ 126 for each character to set text_all_printable.
  • Set summary.text_len=len and summary.text_all_printable.
• Write *out_summary and return 0.

Tasks to Perform

1. Define enum FrameTag and the struct Frame with the union containing:
   • struct { int data[8]; int count; } ints;
   • double scalar;
   • char text[32];
2. Define struct Summary with fields listed above.
3. Implement summarize_frame:
   • Validate pointers and ranges (min_allowed ≤ max_allowed).
   • Switch on frame->tag and process the matching union member.
   • Fill only relevant summary fields; initialize others deterministically (e.g., zeros/false).
4. Handle all error cases by returning -1 without writing outputs.

Test Cases