Feature spec
Goal: Send a clear invoice after finishing a job.
- Add a client, line items, a due date, and payment instructions.
- Preview the total before exporting a PDF.
- Keep an editable draft if export fails.
App Overview
Ningen Calc X
For those who can no longer be satisfied with ordinary calculator apps.
$1.99 · In-app purchases
PRICE
$1.99
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
★ 5.0
1 ratings · Top 5%
Evidence scope: No current overall US chart position is available. Category-chart positions shown in discovery are a separate scope. Revenue is a directional model estimate, not verified earnings.
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Build Kit
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A product spec, screen plans, and build steps for your coding agent.
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What's included15 files
A product spec, phased build plan, differentiation notes, and screen references.
- START_HERE
- AGENTS
- EVIDENCE
- SPEC
- BUILD_PLAN
- DIFFERENTIATION
- skills/
- screens/
Preview a sample Build Kit No email needed
Illustrative example: an invoice app for independent contractors. Your kit is tailored to the app you choose; this is not its generated content.
Screen plan
Invoice editor: Client at the top, editable line items in the middle, total and preview action at the bottom.
States: Empty draft, validation errors beside each field, exporting, and a retry action that preserves the draft.
Differentiate and validate
Hypothesis: Contractors need faster repeat invoices more than more templates.
First experiment: Test duplicating a previous job with five contractors. Watch where they hesitate before expanding the feature set.
The full kit adds a phased build plan, evidence notes, tool prompts, and implementation guidance. It is a plan for your coding agent, not a finished app.
Usage: Mechanics are fair game; never reuse the original name, branding, assets, or verbatim copy.
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Free quick prompt For a smaller starting point
Quick prompt
Good fit for mobile-first iOS and Android prototypes with managed app scaffolding.
// RORK SYSTEM PROMPT
Rork plan preview for Ningen Calc X: map the core Entertainment workflow, choose the smallest differentiated feature set, define the data and monetization boundaries, then prototype the riskiest user journey first.
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Opportunities
AI-GENERATED OPPORTUNITY HYPOTHESES
These are generated suggestions, not verified review quotations. Check current App Store reviews and speak with users before treating a possible gap as a requirement.
Hypothesis 1
Add features like graphing or scientific functions.
Assumption to check: Limited functionality compared to advanced calculators
Validate with people who use this workflow before building.
Hypothesis 2
Implement dark mode for better user experience.
Assumption to check: No dark mode option
Validate with people who use this workflow before building.
Hypothesis 3
Allow users to customize button themes and layouts.
Assumption to check: Lack of customization options
Validate with people who use this workflow before building.
Hypothesis 4
Enhance the history screen with search and filter options.
Assumption to check: Limited history visibility
Validate with people who use this workflow before building.
App evidence
Original App Store material. Ratings and screenshots describe the existing app, not proof of demand for your version.
App Store description
For those who can no longer be satisfied with ordinary calculator apps. The age of numbers made from people has arrived! Ningen Calc X — the calculator app where numbers literally walk onto the screen. Have you ever thought this while using a normal calculator? - The results appear way too fast - You’re tired of cold, lifeless digital numbers - Store-bought calculators just aren’t cute Ningen Calc X solves all of these problems instantly! Press a button and a mysterious crowd of people comes marching in, forming human shapes to display the numbers you entered. It’s a wonderfully ridiculous calculator experience. Since it takes time for the human numerals to assemble, the result appears absurdly slowly for a calculator. Honestly, doing the math in your head would probably be faster! Perfect when you want to: - Kill some time - Pretend you're calculating while buying time - Smash the time-efficiency obsession of modern life Ningen Calc X — highly recommended for anyone who wants a calculator experience like no other. ---------------- [Extra Features in the Full Version] This app is the full version of Ningen Calc X. Compared to Ningen Calc X Lite, it offers the following: - No ads — safe and distraction-free, even for children - Choose from over a dozen unique stages (backgrounds and colour themes) - A convenient backspace key to easily correct input mistakes - Two button style options, with the ability to hide buttons entirely - Supports landscape mode, iPad, and Mac — including keyboard input
Written user reviews are not shown here. Check current App Store reviews >
Screenshots
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Build scope
Plan a focused first version with your coding agent. These are planning assumptions, not a delivery guarantee.
Define the first version
Choose one audience and one core workflow. Use the kit to agree on its screens, data, and acceptance criteria before building.
Set boundaries
Decide which secondary features, integrations, and platform support can wait. Your version does not need to reproduce everything in the original.
Check the hard parts
Validate external services, specialist technology, data access, and ongoing costs for your chosen scope.
SUGGESTED BUILD STACK
React Native
Expo
Redux
react-navigation
@react-native-async-storage/async-storage
expo-sound
BUILD KIT DIFFICULTY
Starter
From the Build Kit assessment.
A reliable timeline needs an agreed scope and a technical check. Ask your agent to estimate the phases in BUILD_PLAN.md after that review.
