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class Quantity
attr_reader :amount, :unit
def initialize(amount, unit)
@amount = amount
@unit = UnitOfMeasure.for(unit)
end
def to(target_unit)
Quantity.new(
UnitOfMeasure.for(target_unit).convert(amount, unit),
target_unit
)
end
def to_f
@amount.to_f
end
def +(other)
Quantity.new(amount + amount_from(other), unit)
end
def -(other)
Quantity.new(amount - amount_from(other), unit)
end
def /(other)
Quantity.new(amount / amount_from(other), unit)
end
def *(other)
Quantity.new(amount * amount_from(other), unit)
end
def >(other)
amount > amount_from(other)
end
def >=(other)
self.>(other) || eql?(other)
end
def <(other)
amount < amount_from(other)
end
def coerce(other)
[self, other]
end
def hash
amount.hash + unit.class.hash
end
def eql?(other, delta = 0.1)
(amount - amount_from(other)).abs <= delta
end
def ==(other)
eql?(other)
end
def to_s
to_f.to_s
end
def pretty_print
"#{to_f} #{unit}"
end
def to_h
{ amount: amount, unit: unit.to_s }
end
def to_hash
to_h
end
private
def amount_from(quantity)
quantity.respond_to?(:to) ? quantity.to(unit).amount : quantity
end
class UnitOfMeasure
def self.for(unit)
case unit
when :lbs, :lb
Pound.new
when :kg, :kgs
Kilogram.new
else
unit
end
end
end
class Pound < UnitOfMeasure
def convert(amount, unit)
case unit
when Kilogram
amount * 2.20462
else
amount
end
end
def to_s
"lbs"
end
end
class Kilogram < UnitOfMeasure
def convert(amount, unit)
case unit
when Pound
amount * 0.453592
else
amount
end
end
def to_s
"kg"
end
end
end
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