class Spec::Matchers::Change

def actual_delta
  @after - @before
end

def by(amount)
  @amount = amount
  self
end

def by_at_least(minimum)
  @minimum = minimum
  self
end

def by_at_most(maximum)
  @maximum = maximum
  self
end      

def description
  "change ##{@message}"
end

def evaluate_value_proc
  @value_proc.call
end

def failure_message_for_should
  if @to
    "#{@message} should have been changed to #{@to.inspect}, but is now #{@after.inspect}"
  elsif @from
    "#{@message} should have initially been #{@from.inspect}, but was #{@before.inspect}"
  elsif @amount
    "#{@message} should have been changed by #{@amount.inspect}, but was changed by #{actual_delta.inspect}"
  elsif @minimum
    "#{@message} should have been changed by at least #{@minimum.inspect}, but was changed by #{actual_delta.inspect}"
  elsif @maximum
    "#{@message} should have been changed by at most #{@maximum.inspect}, but was changed by #{actual_delta.inspect}"
  else
    "#{@message} should have changed, but is still #{@before.inspect}"
  end
end

def failure_message_for_should_not
  "#{@message} should not have changed, but did change from #{@before.inspect} to #{@after.inspect}"
end

def from (from)
  @from = from
  self
end

def initialize(receiver=nil, message=nil, &block)
  @message = message || "result"
  @value_proc = block || lambda {receiver.__send__(message)}
  @to = @from = @minimum = @maximum = @amount = nil
end

def matches?(event_proc)
  raise_block_syntax_error if block_given?
  
  @before = evaluate_value_proc
  event_proc.call
  @after = evaluate_value_proc
  
  return (@to = false) if @from unless @from == @before
  return false if @to unless @to == @after
  return (@before + @amount == @after) if @amount
  return ((@after - @before) >= @minimum) if @minimum
  return ((@after - @before) <= @maximum) if @maximum        
  return @before != @after
end

def raise_block_syntax_error
  raise MatcherError.new(<<-MESSAGE
passed to should or should_not change must use {} instead of do/end
E
  )
end

def to(to)
  @to = to
  self
end