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// SPDX-License-Identifier: MIT
// Copyright (c) 2026 Paul Buetow
package perc
import (
"fmt"
"regexp"
"strconv"
"strings"
)
// CalculationType represents the type of calculation performed.
type CalculationType int
const (
// PercentOfY: "X% of Y" → "X.00% of Y.00 = Z.00"
PercentOfY CalculationType = iota
// IsWhatPercentOfY: "X is what % of Y" → "X.00 is P.00% of Y.00"
IsWhatPercentOfY
// IsYPercentOfWhat: "X is Y% of what" → "X.00 is Y.00% of W.00"
IsYPercentOfWhat
)
// Calculation represents the result of a percentage calculation.
type Calculation struct {
Type CalculationType
Percent float64
Base float64
Result float64
Steps string
}
// Format returns the formatted calculation result.
func (c *Calculation) Format() string {
var baseStr string
switch c.Type {
case PercentOfY:
baseStr = fmt.Sprintf("%.2f%% of %.2f = %.2f", c.Percent, c.Base, c.Result)
case IsWhatPercentOfY:
// percent is the result, base is the "whole"
baseStr = fmt.Sprintf("%.2f is %.2f%% of %.2f", c.Result, c.Percent, c.Base)
case IsYPercentOfWhat:
// percent is the known value, base is the "what"
baseStr = fmt.Sprintf("%.2f is %.2f%% of %.2f", c.Result, c.Percent, c.Base)
}
if c.Steps != "" {
return baseStr + "\n Steps: " + c.Steps
}
return baseStr
}
// ParsingStrategy represents a parsing function that attempts to parse input.
// Returns a Calculation if handled, or error if not.
type ParsingStrategy func(input string) (*Calculation, bool, error)
// strategyRegistry maintains a registry of parsing strategies.
type strategyRegistry struct {
strategies []ParsingStrategy
}
// newStrategyRegistry creates a new strategy registry.
func newStrategyRegistry() *strategyRegistry {
return &strategyRegistry{
strategies: make([]ParsingStrategy, 0),
}
}
// register adds a parsing strategy to the registry.
func (r *strategyRegistry) register(strategy ParsingStrategy) {
r.strategies = append(r.strategies, strategy)
}
// parse attempts to parse input using registered strategies in order.
func (r *strategyRegistry) parse(input string) (*Calculation, bool, error) {
for _, strategy := range r.strategies {
if result, handled, err := strategy(input); handled {
return result, true, err
}
}
return nil, false, nil
}
// Parse parses a percentage calculation input string and returns the result as a formatted string.
// It handles formats like "20% of 150", "30 is what % of 150", and "30 is 20% of what".
// Note: This function only handles percentage calculations, not RPN expressions.
func Parse(input string) (string, error) {
input = strings.ToLower(strings.TrimSpace(input))
input = strings.ReplaceAll(input, "what is ", "")
input = strings.TrimSpace(input)
// Create registry and register percentage parsing strategies
registry := newStrategyRegistry()
registry.register(parseXPercentOfY)
registry.register(parseXIsWhatPercentOfY)
registry.register(parseXIsYPercentOfWhat)
calc, ok, err := registry.parse(input)
if ok {
return calc.Format(), nil
}
if err != nil {
return "", fmt.Errorf("perc: unable to parse input %q: %w", input, err)
}
return "", fmt.Errorf("perc: unable to parse input %q: unknown error", input)
}
// ParseCalculation parses a percentage calculation input string and returns the Calculation object.
// It handles formats like "20% of 150", "30 is what % of 150", and "30 is 20% of what".
// This provides callers with more flexibility to access raw values and formatting options.
func ParseCalculation(input string) (*Calculation, error) {
input = strings.ToLower(strings.TrimSpace(input))
input = strings.ReplaceAll(input, "what is ", "")
input = strings.TrimSpace(input)
// Create registry and register percentage parsing strategies
registry := newStrategyRegistry()
registry.register(parseXPercentOfY)
registry.register(parseXIsWhatPercentOfY)
registry.register(parseXIsYPercentOfWhat)
calc, ok, err := registry.parse(input)
if ok {
return calc, nil
}
if err != nil {
return nil, err
}
return nil, fmt.Errorf("perc: unable to parse input %q. See usage for examples", input)
}
// parseXPercentOfY calculates "X% of Y" and returns a Calculation.
func parseXPercentOfY(input string) (*Calculation, bool, error) {
re := regexp.MustCompile(`^(\d+(?:\.\d+)?)\s*%\s*(?:of\s+)?(\d+(?:\.\d+)?)$`)
matches := re.FindStringSubmatch(input)
if matches == nil {
return nil, false, nil
}
percent, err := strconv.ParseFloat(matches[1], 64)
if err != nil {
return nil, false, err
}
base, err := strconv.ParseFloat(matches[2], 64)
if err != nil {
return nil, false, err
}
result := (percent / 100.0) * base
calc := &Calculation{
Type: PercentOfY,
Percent: percent,
Base: base,
Result: result,
Steps: fmt.Sprintf("(%.2f / 100) * %.2f = %.2f * %.2f = %.2f", percent, base, percent/100.0, base, result),
}
return calc, true, nil
}
// parseXIsWhatPercentOfY calculates "X is what % of Y" and returns a Calculation.
func parseXIsWhatPercentOfY(input string) (*Calculation, bool, error) {
re := regexp.MustCompile(`^(\d+(?:\.\d+)?)\s+is\s+what\s*%\s*(?:of\s+)?(\d+(?:\.\d+)?)$`)
matches := re.FindStringSubmatch(input)
if matches == nil {
return nil, false, nil
}
part, err := strconv.ParseFloat(matches[1], 64)
if err != nil {
return nil, false, err
}
whole, err := strconv.ParseFloat(matches[2], 64)
if err != nil {
return nil, false, err
}
if whole == 0 {
return nil, false, fmt.Errorf("division by zero")
}
percent := (part / whole) * 100.0
calc := &Calculation{
Type: IsWhatPercentOfY,
Percent: percent,
Base: whole,
Result: part,
Steps: fmt.Sprintf("(%.2f / %.2f) * 100 = %.2f * 100 = %.2f%%", part, whole, part/whole, percent),
}
return calc, true, nil
}
// parseXIsYPercentOfWhat calculates "X is Y% of what" and returns a Calculation.
func parseXIsYPercentOfWhat(input string) (*Calculation, bool, error) {
re := regexp.MustCompile(`^(\d+(?:\.\d+)?)\s+is\s+(\d+(?:\.\d+)?)\s*%\s*(?:of\s+)?what$`)
matches := re.FindStringSubmatch(input)
if matches == nil {
return nil, false, nil
}
part, err := strconv.ParseFloat(matches[1], 64)
if err != nil {
return nil, false, err
}
percent, err := strconv.ParseFloat(matches[2], 64)
if err != nil {
return nil, false, err
}
if percent == 0 {
return nil, false, fmt.Errorf("division by zero")
}
whole := (part / percent) * 100.0
calc := &Calculation{
Type: IsYPercentOfWhat,
Percent: percent,
Base: whole,
Result: part,
Steps: fmt.Sprintf("(%.2f / %.2f) * 100 = %.2f * 100 = %.2f", part, percent, part/percent, whole),
}
return calc, true, nil
}
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