feat: add PercentageFloat method for decimal percentage support

[walle250ai]

Problem

Percentage(int64) only supports whole-number rates. Real-world tax and discount calculations frequently require decimal percentages (12.5% VAT, 6.25% sales tax, etc.) that cannot be represented.

Solution

Adds PercentageFloat(percentage float64) (*Money, error) that accepts a float64 rate in [0.0, 100.0] while keeping all arithmetic in integer space.

Implementation approach

Instead of multiplying amount by a float, the method scales the percentage to a large integer numerator/denominator pair (precision = 1e9) and delegates to the existing calculator.allocate(amount, numerator, denominator) path:

const precision = 1e9
numerator  := int64(percentage * precision)   // e.g. 12.5 → 12_500_000_000
denominator := int64(100 * precision)          // always 100_000_000_000

This avoids floating-point amounts entirely, consistent with the library's design.

Truncation note: results are truncated towards zero (e.g. 33.3% of $3.00 → $0.99, not $1.00). This matches the behaviour of Allocate and Split.

Tests

Case	Input	Expected
12.5% of $200	PercentageFloat(12.5) on 200	25
6.25% of $800	PercentageFloat(6.25) on 800	50
33.3% of $300 (truncation)	PercentageFloat(33.3) on 300	99
Boundary 0.0%	PercentageFloat(0.0)	0
Boundary 100.0%	PercentageFloat(100.0)	100
Invalid -0.1	error	—
Invalid 100.1	error	—

All existing tests continue to pass.

🤖 Generated with Claude Code

Summary by Sourcery

Add integer- and float-based percentage calculation helpers to Money with validation and coverage tests.

New Features:

• Introduce Money.Percentage to compute whole-number percentage values of a Money amount with range validation.
• Introduce Money.PercentageFloat to compute decimal percentage values of a Money amount using high-precision integer arithmetic and truncation toward zero.

Tests:

• Add unit tests for Money.Percentage, including normal, boundary, and invalid percentage cases.
• Add unit tests for Money.PercentageFloat, including fractional, negative-amount, boundary, and invalid percentage cases.

[sourcery-ai]

Reviewer's Guide

Adds percentage-based calculation helpers to Money, including a new float-based API that keeps arithmetic in integer space, with comprehensive validation and tests for both integer and float percentages.

Sequence diagram for PercentageFloat delegation to calculator

sequenceDiagram
    actor Caller
    participant Money
    participant mutate
    participant calc

    Caller ->> Money: PercentageFloat(percentage float64)
    Money ->> Money: validate percentage in [0.0, 100.0]
    Money ->> Money: compute precision, numerator, denominator
    Money ->> mutate: use calc to allocate amount
    mutate ->> calc: allocate(m.amount, numerator, denominator)
    calc -->> mutate: allocatedAmount int64
    mutate -->> Money: allocatedAmount int64
    Money ->> Money: construct new Money(allocatedAmount, m.currency)
    Money -->> Caller: *Money, nil

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Class diagram for Money with new percentage helpers

classDiagram
    class Money {
        int64 amount
        Currency currency
        Allocate(rs ...int) []*Money, error
        Percentage(percentage int64) *Money, error
        PercentageFloat(percentage float64) *Money, error
        Display() string
    }

    class Mutate {
        Calculator calc
    }

    class Calculator {
        allocate(amount int64, numerator int64, denominator int64) int64
    }

    Money --> Mutate : uses mutate
    Mutate --> Calculator : uses calc

    %% New methods introduced in this PR
    Money : Percentage(percentage int64) *Money, error
    Money : PercentageFloat(percentage float64) *Money, error

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File-Level Changes

Change	Details	Files
Introduce integer-based Percentage helper on Money with input validation	Add Money.Percentage(int64) method that validates the rate is between 0 and 100 inclusive Implement percentage calculation by delegating to mutate.calc.allocate with numerator=percentage and denominator=100 Return a typed error message on invalid input and construct a new Money with the computed amount and original currency	money.go
Add PercentageFloat helper that accepts decimal percentages while keeping integer arithmetic	Add Money.PercentageFloat(float64) with validation for range [0.0, 100.0] and shared error message format Convert float percentage into a scaled integer fraction using precision 1e9 and call mutate.calc.allocate(amount, numerator, denominator) Document truncation-towards-zero behaviour for non-integer results and preserve original currency in the returned Money	money.go
Add unit tests covering normal, boundary, invalid, and truncation scenarios for Percentage and PercentageFloat	Introduce TestMoney_Percentage, TestMoney_Percentage_BoundaryValues, and TestMoney_Percentage_InvalidInput to cover valid, boundary, and out-of-range integer percentages Introduce TestMoney_PercentageFloat and TestMoney_PercentageFloat_InvalidInput to validate decimal percentages, truncation behaviour, negative amounts, and error cases Verify returned error messages match the expected string and that invalid inputs yield nil Money results	money_test.go

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[sourcery-ai]

Hey - I've left some high level feedback:

• In PercentageFloat, consider rounding the scaled percentage (e.g., int64(math.Round(percentage * precision))) instead of truncating the float64 multiplication result to reduce the impact of floating-point representation error on boundary cases.
• The range checks and error messages for Percentage and PercentageFloat are duplicated; extracting a shared validation/helper or a shared error value would keep behavior consistent and simplify future changes.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `PercentageFloat`, consider rounding the scaled percentage (e.g., `int64(math.Round(percentage * precision))`) instead of truncating the float64 multiplication result to reduce the impact of floating-point representation error on boundary cases.
- The range checks and error messages for `Percentage` and `PercentageFloat` are duplicated; extracting a shared validation/helper or a shared error value would keep behavior consistent and simplify future changes.

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