Fuel Economy Calculator

Level: Easy
Concepts: Precedence, Integer vs Floating‑Point Division, Type Conversion

Scenario

You log trip distance in meters and fuel consumed in milliliters on an embedded logger. You must compute fuel economy in km/L as a double, being careful with integer vs floating‑point division and operator precedence.

Problem Statement

Implement a function that converts the raw sensor values (distance in meters, fuel in milliliters) to fuel economy expressed in kilometres per litre (km/L). The calculation must be performed using floating‑point arithmetic; any accidental integer division must be avoided.

Requirements

• Use double for the result; inputs are int.

• Compute

  km    = distance_meters / 1000.0;          // metres → kilometres
  liters= fuel_milliliters / 1000.0;         // millilitres → litres
  kmpl  = km / liters;
  

• Ensure at least one operand of each division is a double (e.g., 1000.0 or an explicit cast) and parenthesise sub‑expressions to make the evaluation order explicit:

  *out_kmpl = (distance_meters / 1000.0) / (fuel_milliliters / 1000.0);
  

• Validate inputs:

  • distance_meters >= 0
  • fuel_milliliters > 0
  • out_kmpl is non‑NULL

• Return an error for invalid inputs; do not modify *out_kmpl on error.

Function Details

Name: compute_kmpl

Arguments:

Return Value: int — 0 on success; -1 on invalid input or null pointer.

Description:
The function validates the parameters, performs the floating‑point conversion and division as described above, stores the result in *out_kmpl, and returns 0. If any validation fails, it returns -1 and leaves *out_kmpl unchanged.

Solution Approach

1. Validate pointers – if out_kmpl == NULL → return -1.
2. Validate numeric ranges – if distance_meters < 0 or fuel_milliliters <= 0 → return -1.
3. Perform floating‑point calculation using 1000.0 (a double) to force the operands into floating‑point context and parenthesise the sub‑expressions to guarantee the intended order.
4. Store the result in *out_kmpl and return 0.

Tasks to Perform

1. Declare the function prototype:

   int compute_kmpl(int distance_meters, int fuel_milliliters, double *out_kmpl);
   

2. Implement the validation and calculation steps as outlined in the Solution Approach.

3. Write unit tests covering all required test cases (see below).

4. Verify that no integer division occurs by inspecting the generated assembly or by confirming the results with known floating‑point values.

Test Cases