System Requirements Document for solar-start

1. Introduction

solar-start is a single-page calculator. The product intent is exactly what the authoritative requirement states: build a simple, single-page design of a calculator. There is no catalogue, no account area, no marketing site, and no multi-page navigation — the calculator is the entire product surface.

The audience is any person who needs immediate, trustworthy everyday arithmetic: a visitor who opens one page, enters numbers and operators, reads a correct result, and clears the page to start again. The experience is anonymous and immediate; nothing must be installed, configured, or signed into before the first calculation can be made.

The design register is deliberately civic and instrumental rather than playful: warm paper ground, black numerals and rules, one reserved signal colour for the equals action, and a solar-yellow marker for the clear control. Arithmetic itself is the hero of the page.

2. System Overview

solar-start is delivered as one first-party, anonymously reachable page containing the complete calculator: a top rail with the SOLAR-START wordmark, a live status marker and a concise instruction; a typographic arithmetic field; a ruled 4-by-5 keypad; and a three-line readout board. Entry of numbers and operators, display of the current entry and the result, and clearing to begin a new calculation all happen on that one page.

The only active human role is the Calculator User. There are no external providers, no outbound recipients, and no human participants beyond the person operating the calculator — a calculation is performed by that person and its outcome is observed by that same person, so no participant handoff exists anywhere in this product.

Delivery is client-side and static-first: the arithmetic is evaluated in the browser, no value is transmitted or stored, and no server, database, or deployment orchestrator is required for the accepted scope.

Current horizon: one anonymous page containing the calculator, supporting number and operator entry, the current-entry and result display, clearing, and the error and recovery states that arithmetic inherently produces.

Narrow exclusions: nothing outside that single page is in scope. Memory keys, calculation history, scientific or programmer functions, unit or currency conversion, accounts, saved state, and multi-page navigation carry no accepted requirement and must not be introduced.

2a. Product Interpretation and Delivery Boundary

solar-start is a first-party, application-owned page with no access requirement: the visitor lands directly on the calculator and can use every control immediately. There is no sign-in, registration, invitation, provisioning, or session continuity, because the accepted product produces no durable relationship, obligation, entitlement, or value transfer that would need to be bound to an identified person. All state is transient to the open page; closing the page ends it.

The delivery boundary is therefore a single anonymous public entry surface that carries the whole product, plus the internal browser computation that evaluates what the visitor types. Everything the visitor initiates, observes, and recovers from happens on that one surface. No future-phase requirement was accepted in this thread, so no future page, module, or capability is planned here.

2c. Page Content and Component Coverage

Calculator

• Purpose and state. The single anonymous public entry page and the entire product: it holds the arithmetic field, the readout board, and the keypad, and it owns entry of numbers and operators, display of the current entry and the result, and clearing to begin a new calculation. Its authoritative state is the current entry (the number being typed), the active operator, the expression trail, and the current result or error state.
• Primary actions. Pressing a digit key appends to the current entry; pressing +, −, ×, or ÷ sets the active operator and records the completed operand into the expression trail; pressing = evaluates the recorded expression and shows the result in the readout.
• Supporting actions. Pressing the decimal key adds a decimal separator to the current entry; pressing the clear control (marked with the solar-yellow disc) resets current entry, active operator, expression trail, and result to the start state; the live status marker in the top rail reflects whether the calculator is idle, mid-entry, or resolved.
• Domain entities. Current entry (the number in progress), operand (a completed number in the expression), operator (+, −, ×, ÷), active operator (the operator currently armed, shown in signal red), expression trail (the thin line of the entered expression beneath the result), result (the evaluated value), and error state (the displayed message when an operation is undefined).
• Component responsibilities.
  • Top rail: SOLAR-START wordmark, live status marker, and a concise instruction line; compact uppercase utility labelling.
  • Arithmetic field: four oversized stacked + − × ÷ glyphs in Archivo Black at roughly 18vw, with one solar-yellow disc interrupting the stack at the page-grid intersection.
  • Calculator slab: near-white surface holding the readout and keypad, aligned against the right edge of the arithmetic field, separated from it by a heavy vertical black rule.
  • Readout board: three-line information board — tiny uppercase CURRENT TOTAL label, oversized tabular numeral, thin expression trail beneath.
  • Keypad grid: continuous ruled 4-by-5 grid with shared 1px black borders, square cells and no gaps; digits, decimal key, clear control, four operators, and the signal-red = cell that spans two keypad rows in the bottom-right corner.
  • Solar disc: small yellow circle at the grid intersection, which performs an instant status blink when a calculation resolves.
• Loading state. None is shown and none is required: the page is a static document with no remote dependency, so the composition renders complete on first paint. Leading zeros are suppressed in the current entry rather than displayed as placeholder content.
• Empty state. On load the readout shows 0 in the oversized display with an empty expression trail, no operator is armed in signal red, and the keypad is fully usable.
• Success state. After = the display switches cleanly to the evaluated result, the expression trail shows the expression that produced it, and the solar disc delivers one instant status blink. A negative result is shown with a leading minus sign.
• Error state. Division by zero displays an undefined-operation error in the readout instead of a numeric result; the keypad remains visible and operable but no result is produced until the visitor clears.
• Recovery. Pressing the clear control returns the page to its start state — 0 in the display, empty expression trail, no armed operator — after which a new calculation can be entered immediately. The error clears by the same action, and no state is retained after the page is closed.

3. Functional Requirements

FR-1 — Enter numbers and operators [explicit] As a Calculator User I should enter numbers and operators so that I can build the arithmetic I want to calculate.

• Actor and access: the anonymous Calculator User on the Calculator page; no identity required.
• Trigger/input: pressing digit keys 0–9 and the operator keys +, −, ×, ÷ on the keypad.
• Observable result: each digit appends to the current entry shown on the readout; each operator records the completed operand into the expression trail, arms that operator (shown in signal red) and prepares the next operand.
• Failure/recovery: if an operator is pressed before any number, or pressed twice in succession, the most recent operator replaces the armed one and no empty operand is evaluated; the entry flow continues.
• Continuation: the visitor continues entering the next operand, presses = to evaluate, or presses the clear control to start over.

FR-2 — Read the current entry [required_inference] As a Calculator User I should see the number I am typing so that I can confirm what I entered before calculating.

• Actor and access: anonymous Calculator User on the Calculator page.
• Trigger/input: any digit or decimal key press.
• Observable result: the oversized tabular readout updates immediately to the current entry, with leading zeros suppressed and the value right-aligned within the display band; the CURRENT TOTAL label sits above it.
• Failure/recovery: if the typed value grows beyond the display band, it is rendered in a compact rounded form so the layout never breaks; the visitor can clear and re-enter.
• Continuation: the visitor keeps typing, chooses an operator, or clears.

FR-3 — Clear to begin a new calculation [explicit] As a Calculator User I should clear the calculator so that I can begin a new calculation from a known state.

• Actor and access: anonymous Calculator User on the Calculator page.
• Trigger/input: pressing the clear control, marked with the solar-yellow disc.
• Observable result: the display returns to 0, the expression trail empties, any armed operator loses its signal-red state, and any error message is removed.
• Failure/recovery: clearing is always available, including while an error is displayed, so no state can trap the visitor.
• Continuation: the visitor immediately begins a new entry on the reset page.

FR-4 — Evaluate and display the result [explicit] As a Calculator User I should press equals to get the result so that I can read the answer to the arithmetic I entered.

• Actor and access: anonymous Calculator User on the Calculator page; evaluation runs in the visitor's browser.
• Trigger/input: pressing the signal-red = key after at least one operand and one operator, or after a digit followed by =.
• Observable result: the readout switches cleanly to the evaluated result in the same oversized tabular type, the expression trail beneath shows the expression that produced it, the solar disc performs one instant status blink, and the status marker reflects the resolved state.
• Failure/recovery: if the expression is incomplete or contains no operator, = leaves the current entry displayed unchanged; no partial or misleading result is shown.
• Continuation: the displayed result becomes the starting operand for the visitor's next operation until the clear control is pressed.

FR-5 — Enter decimal numbers [required_inference] As a Calculator User I should enter decimal values so that I can calculate with everyday non-integer amounts.

• Actor and access: anonymous Calculator User on the Calculator page.
• Trigger/input: pressing the decimal key while a number is being entered.
• Observable result: a single decimal separator is added to the current entry; pressing it first inserts a leading 0 before the separator, and pressing it twice in the same number leaves the existing separator unchanged.
• Failure/recovery: no invalid numeric string is ever formed, so no error state is produced by decimal entry itself.
• Continuation: the visitor continues entering fractional digits, then selects an operator or presses =.

FR-6 — Continue from a displayed result [required_inference] As a Calculator User I should keep calculating from the value already on screen so that sequential everyday arithmetic does not force me to re-enter numbers.

• Actor and access: anonymous Calculator User on the Calculator page.
• Trigger/input: pressing an operator