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
LittleGray Calculator-Infinity
LittleGray Calculator-Infinity is a high-precision large number scientific calculator that supports real-time calculations of custom functions and expressions
$4.99 · In-app purchases
PRICE
$4.99
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
★ 2.0
1 ratings · Below Average
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 included17 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 LittleGray Calculator-Infinity: map the core Productivity 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 advanced features like graphing or scientific calculations
Assumption to check: Limited functionality compared to other calculators
Validate with people who use this workflow before building.
Hypothesis 2
Implement a more modern design with smoother transitions
Assumption to check: Poor user interface and experience
Validate with people who use this workflow before building.
Hypothesis 3
Include a feature to save and view past calculations
Assumption to check: No history tracking feature
Validate with people who use this workflow before building.
Hypothesis 4
Optimize performance for a wider range of devices
Assumption to check: Inconsistent performance on older devices
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
LittleGray Calculator-Infinity is a high-precision large number scientific calculator that supports real-time calculations of custom functions and expressions. The columnar display window allows you to intuitively view the calculation process and calculation results of expressions. A variety of button layouts allow You quickly swipe to switch between the simple keyboard and the all-purpose keyboard. LittleGray Calculator-Infinity allows you to copy expressions from anywhere to the expression bar and modify the expressions by editing. It can also automatically generate historical records through the regular "=" sign of the calculator, and you can copy the parts you need in the historical records to The expression bar is recalculated. LittleGray Calculator-Infinity features: 1. Columnar layout 2. The column display window is displayed in multiple modes, and the size of the column window can be switched by clicking the column display window three times 3. Support large number calculation function, up to 9999 factorial and 9999^9999 power. 4. Unlimited display mode of large integer digits. 5. Support the four arithmetic operations of arbitrary precision of addition, subtraction, multiplication and division, and the maximum division is reserved to 1000 digits after the decimal point 6. Custom function formula function, functions can be classified. 7 multiple key modes: simple keyboard, function keyboard, hexadecimal\logic calculation keyboard, the keyboard can be switched by sliding left and right 8. History record function 9. iCloud cloud synchronization function 10. Real-time voice function 11. Multiple button color matching 12. More functions are waiting for you to discover
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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
Zustand
React Navigation
react-native-async-storage
BUILD KIT DIFFICULTY
Intermediate
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.
