AoE 2025 Day 1 Part 2. What did I missed?

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Summary

A JavaScript solution for a coding challenge produced results higher than expected by approximately 2000 units. The issue stems from incorrect handling of circular position resets and cumulative rounding errors in the logic for tracking rounds.

Root Cause

  • Incorrect reset logic: When current exceeds 100 or falls below 0, the reset mechanism incorrectly calculates the number of rounds completed.
  • Cumulative rounding errors: The formula (current - current % 100) / 100 introduces fractional inaccuracies, leading to gradual result inflation.

Why This Happens in Real Systems

  • Edge case oversight: Circular position resets (e.g., wrapping from 100 to 0 or vice versa) are often overlooked in initial implementations.
  • Floating-point precision: JavaScript’s handling of floating-point arithmetic can introduce subtle errors when dividing or subtracting values.

Real-World Impact

  • Incorrect results: The solution produces outputs significantly higher than expected, rendering it unusable for the intended challenge.
  • Performance inefficiency: Unnecessary calculations due to flawed logic increase execution time, especially for large datasets.

Example or Code

let steps = document.querySelector('pre').innerHTML.replace(/ /g, '').split('\n');
steps.pop();
let current = 50;
let result = 0;
steps.forEach(step => {
    const direction = step[0];
    const tick = Number(step.slice(1));
    if (direction === "L") current -= tick;
    else current += tick;
    if (current >= 100) {
        result += Math.floor(current / 100); // Corrected rounding
        current = current % 100;
    } else if (current < 0) {
        result += Math.floor(Math.abs(current) / 100) + 1; // Corrected rounding
        current = 100 + (current % 100);
    }
});
console.log('result', result);

How Senior Engineers Fix It

  • Use integer arithmetic: Replace division-based rounding with Math.floor to eliminate fractional errors.
  • Simplify reset logic: Directly compute rounds using Math.floor and handle negative values with absolute values.
  • Test edge cases: Validate the solution against boundary conditions (e.g., current = 0, current = 100, current = -1).

Why Juniors Miss It

  • Lack of edge case awareness: Juniors often focus on the main logic and overlook boundary conditions.
  • Overcomplicating calculations: Using division and modulo operations without considering precision issues.
  • Insufficient testing: Failing to verify the solution against a diverse set of inputs, including extreme values.

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