Temperature

// Temperature: two conversion tables between Celsius and Fahrenheit, built from small functions.
//
// Each function does one job and has a name that says what. The conversions are one formula
// each; `Round` turns a converted value into whole degrees; `Width` and `PrintRight` line the
// numbers up in columns; `PrintFeeling` puts a temperature into words. `Main` only calls them, so
// it reads like a description of the tables rather than a page of arithmetic.
//
// The two scales meet at exactly one temperature, and the end of the program finds it by asking
// the conversion function, not by knowing the answer.
import Io::{ Print, PrintLine };

func CelsiusToFahrenheit(celsius: float64) -> float64 {
    return celsius * 9.0 / 5.0 + 32.0;
}

func FahrenheitToCelsius(fahrenheit: float64) -> float64 {
    return (fahrenheit - 32.0) * 5.0 / 9.0;
}

// Rounds to the nearest whole degree. `as int` on its own would only drop the fraction, turning
// 37.8 into 37 rather than 38. A negative value is rounded as its positive twin, then negated, so
// -17.8 becomes -18 and not -17.
func Round(value: float64) -> int {
    if value < 0.0 {
        return -Round(-value);
    }
    return (value + 0.5) as int;
}

// How many characters a whole number takes when printed: its digits, plus one for a minus sign.
func Width(value: int) -> int {
    if value < 0 {
        return 1 + Width(-value);
    }
    if value < 10 {
        return 1;
    }
    return 1 + Width(value / 10);
}

// Prints a number right-aligned in a column, by printing spaces first to fill the gap.
func PrintRight(value: int, width: int = 6) {
    for i in Width(value)..width {
        Print(" ");
    }
    Print("{}", value);
}

func PrintFeeling(celsius: int) {
    if celsius <= 0 {
        PrintLine("   frozen");
    } else if celsius < 10 {
        PrintLine("   cold");
    } else if celsius < 20 {
        PrintLine("   cool");
    } else if celsius < 30 {
        PrintLine("   warm");
    } else if celsius < 45 {
        PrintLine("   hot");
    } else if celsius < 100 {
        PrintLine("   too hot to touch");
    } else {
        PrintLine("   boiling");
    }
}

func Main() -> int {
    // Every multiple of 5 Celsius is a whole number of Fahrenheit, so this table is exact.
    PrintLine("     C     F");
    var celsius = -40;
    while celsius <= 100 {
        PrintRight(celsius);
        PrintRight(Round(CelsiusToFahrenheit(celsius as float64)));
        PrintFeeling(celsius);
        celsius += 10;
    }
    PrintLine();

    // The other way round the results have fractions, so they are rounded to whole degrees.
    PrintLine("     F     C");
    var fahrenheit = 0;
    while fahrenheit <= 220 {
        let converted = Round(FahrenheitToCelsius(fahrenheit as float64));
        PrintRight(fahrenheit);
        PrintRight(converted);
        PrintFeeling(converted);
        fahrenheit += 20;
    }
    PrintLine();

    // Where do the scales agree? Try every whole degree and ask.
    for degrees in -100..=100 {
        if CelsiusToFahrenheit(degrees as float64) == degrees as float64 {
            PrintLine("{} C is also {} F", degrees, degrees);
        }
    }
    return 0;
}