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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/

// See page 180.

// Package tempconv performs Celsius and Fahrenheit temperature computations.
package tempconv

import (
    "flag"
    "fmt"
)

type Celsius float64
type Fahrenheit float64

func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9.0/5.0 + 32.0) }
func FToC(f Fahrenheit) Celsius { return Celsius((f - 32.0) * 5.0 / 9.0) }

func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }

/*
//!+flagvalue
package flag

// Value is the interface to the value stored in a flag.
type Value interface {
    String() string
    Set(string) error
}
//!-flagvalue
*/

//!+celsiusFlag
// *celsiusFlag satisfies the flag.Value interface.
type celsiusFlag struct{ Celsius }

func (f *celsiusFlag) Set(s string) error {
    var unit string
    var value float64
    fmt.Sscanf(s, "%f%s", &value, &unit) // no error check needed
    switch unit {
    case "C", "°C":
        f.Celsius = Celsius(value)
        return nil
    case "F", "°F":
        f.Celsius = FToC(Fahrenheit(value))
        return nil
    }
    return fmt.Errorf("invalid temperature %q", s)
}

//!-celsiusFlag

//!+CelsiusFlag

// CelsiusFlag defines a Celsius flag with the specified name,
// default value, and usage, and returns the address of the flag variable.
// The flag argument must have a quantity and a unit, e.g., "100C".
func CelsiusFlag(name string, value Celsius, usage string) *Celsius {
    f := celsiusFlag{value}
    flag.CommandLine.Var(&f, name, usage)
    return &f.Celsius
}

//!-CelsiusFlag