Water Pump Controller with Safety & Hysteresis

Level: Easy
Concepts: if/else chains, condition ordering, guard conditions, hysteresis, clamping, boolean logic

Scenario

A domestic water tank uses a pump to maintain water level. The controller decides whether to start or stop the pump based on the tank level percentage and safety flags. To avoid frequent toggling, the system applies hysteresis (different thresholds for ON and OFF). Manual overrides must take precedence, and any safety fault must force stop the pump.

Problem Statement

Implement a function that computes the next pump command (start/stop) using if/else logic, considering safety, manual overrides, current tank level, and pump’s current state. The function should enforce hysteresis and produce a deterministic result.

Requirements

• Allowed C types only: int, long, double, char, bool, and enum (and their pointers/arrays).
• Inputs:
  • level_pct — current tank level in percent (0–100, but function should handle out-of-range gracefully via clamping).
  • pump_running — current pump state (true if currently ON).
  • manual_on — user requests pump ON (momentary).
  • manual_off — user requests pump OFF (momentary).
  • leak — leak detected (true forces pump OFF).
  • overcurrent — electrical fault (true forces pump OFF).
• Hysteresis policy:
  • Turn ON when level_pct <= 25.
  • Turn OFF when level_pct >= 80.
  • If between 26–79%, keep current state unless overridden or faulted.
• Priority (highest to lowest):
  1. Safety: if leak or overcurrent → force STOP.
  2. Manual OFF: if manual_off → STOP.
  3. Manual ON: if manual_on and no safety fault → START.
  4. Hysteresis: apply thresholds to decide START/STOP.
  5. Otherwise, hold the current state.
• Output:
  • *out_cmd = 1 to START pump, *out_cmd = 0 to STOP pump.
• Validation:
  • out_cmd must be non-null; return error and do not modify it if invalid.
  • Clamp level_pct into [0, 100] before logic.

Function Details

• Name: decide_pump_command
• Arguments:
  • int level_pct
  • bool pump_running
  • bool manual_on
  • bool manual_off
  • bool leak
  • bool overcurrent
  • int *out_cmd // 0=STOP, 1=START
• Return Value:
  • int — 0 on success; -1 if out_cmd == NULL.
• Description:
  The function enforces if/else evaluation in a clear priority order:
  1. Check safety; 2) Manual OFF; 3) Manual ON; 4) Hysteresis; 5) Hold state.
     level_pct is clamped to [0,100]. Hysteresis avoids chatter by using different ON/OFF thresholds.

Solution Approach

• Validate out_cmd != NULL; return -1 otherwise.
• Clamp level_pct to [0, 100].
• Apply strict priority using a linear if/else chain:
  • If leak || overcurrent → STOP.
  • Else if manual_off → STOP.
  • Else if manual_on → START.
  • Else if level_pct <= 25 → START.
  • Else if level_pct >= 80 → STOP.
  • Else → hold current state: START if pump_running, otherwise STOP.
• Write the final command to *out_cmd and return 0.

Tasks to Perform

1. If out_cmd == NULL, return -1 immediately (do not write output).
2. Clamp input:
   • If level_pct < 0 set to 0; if level_pct > 100 set to 100.
3. Apply priority-driven if/else logic:
   • Safety first: if leak || overcurrent → *out_cmd = 0; return 0;
   • Manual OFF next: if manual_off → *out_cmd = 0; return 0;
   • Manual ON next: if manual_on → *out_cmd = 1; return 0;
   • Hysteresis:
     • if level_pct <= 25 → *out_cmd = 1; return 0;
     • else if level_pct >= 80 → *out_cmd = 0; return 0;
   • Else hold: *out_cmd = pump_running ? 1 : 0; return 0;
4. Ensure only one path writes output and returns.

Test Cases