Candle Replay

Practice trading one candle at a time—without seeing the future.

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Inspiration

When traders review historical charts, future price movements are often already visible. This creates hindsight bias and makes it difficult to practice realistic decision-making.

I built Candle Replay to provide a training environment where users can move through historical market data one candle at a time without seeing what happens next.

What it does

Candle Replay is a browser-based trading practice simulator that hides future candles and reveals market data step by step.

Users can:

  • Start immediately with a one-click demo
  • Replay market data using play, pause, step, and speed controls
  • View 1-minute, 3-minute, 5-minute, and 15-minute charts
  • Switch between one-chart and two-chart layouts
  • Practice simulated Buy and Sell orders
  • Close positions and review profit and loss
  • Review trade history and session statistics
  • Add trade notes, moving averages, and horizontal price levels
  • Save sessions and review them later
  • Export trading results as CSV
  • Use the interface in Japanese and English

Candle Replay is designed for education and trading practice. It does not provide investment advice or guarantee trading results.

How we built it

Candle Replay was built as a web application that processes CSV market data and controls how much information is visible at each point in the replay.

The replay system establishes a strict time boundary so that future candles are not displayed. Source data is aggregated into multiple timeframes while keeping each chart synchronized with the current replay time.

During Build Week, I extended the project by adding:

  • A one-click demo that works without uploading a CSV file
  • A bilingual Japanese and English interface
  • A Quick Start guide
  • Synthetic demo prices instead of the original price series
  • Additional automated tests and production checks

I used Codex throughout the development cycle to translate requirements into implementation tasks, review future-data safeguards, investigate browser behavior, improve tests, and verify production readiness.

Challenges we ran into

The main challenge was preventing future information from appearing across multiple synchronized timeframes.

Another challenge was keeping the replay controls, simulated positions, profit and loss, trade history, and saved sessions synchronized while processing more than 100,000 rows of demo data.

I also needed to make the project immediately testable without requiring judges to prepare their own CSV file.

Accomplishments that we're proud of

  • Deployed a working production application on Vercel
  • Added a one-click demo with 105,993 rows of sample data
  • Implemented synchronized multi-timeframe candle replay
  • Completed the workflow from simulated entry to trade review
  • Added session saving and CSV result export
  • Passed 362 automated tests
  • Passed linting and the production build
  • Verified the main workflow in a real browser

What we learned

This project reinforced the importance of defining domain rules before implementation, especially when time boundaries and financial calculations affect every part of an application.

I also learned that Codex is most effective when requirements, acceptance criteria, and verification steps are clearly documented and continuously reviewed.

What's next for Candle Replay

After Build Week, I plan to conduct user testing, simplify the production interface, and continue improving the trade-review workflow.

I also plan to refine accessibility and make the tool easier for individual traders to use repeatedly as part of deliberate practice.

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