RCR 294: Array#interpolate

This rcr was discussed here [].

[1, 2, 3].inject(nil){|a,b| a ? (a+[0, b]) : [b]}  #=> [1, 0, 2, 0, 3]

[1,2,3].map{|x|[x,0]}.flatten[0..-2]   #=> [1, 0, 2, 0, 3]

However I like the following because of its readability.

res = []
[1, 2, 3].each_with_index do |a,i|

  res << 0 if i!=0
  res << a

end
res    #=> [1, 0, 2, 0, 3]

None of these are really nice solutions.

1 argument, without block:
[1, 2, 3, 4].interpolate("a") #=> [1, "a", 2, "a", 3, "a", 4]

no arguments, with a block:
[8,6,4,2,0].interpolate{|a,b|(a+b)/2}  #-> [8,7,6,5,4,3,2,1,0]

More refined versions could be:

no arguments, with block:
[0, 10, 100, 1000].interpolate{|a,b,index|"#{(a+b)/2} #{index}"}
#=> [0, "5 0", 10, "55 1", 100, "550 2", 1000]

with arguments and block:
[0, 10, 100, 1000,
10000].interpolate(+1,-1){|a,b,index,sign|"#{(a+b)/2*sign} #{index}"}
#=> [0, "5 0", 10, "-55 1", 100, "550 2", 1000, "-5500 3", 10000]

2+ arguments, without block:
[1, 2, 3, 4, 5].interpolate("a","b","c") 
#=> [1, "a", 2, "b", 3, "c", 4,
"a", 5]

class Array

  def interpolate(*args,&block)
    out = Array.new
    first = true
    last = nil
    if not block and args.empty?
      raise ArgumentError,"wrong number of arguments (0 for 1)"
    elsif not args.empty? and block
      apos = 0
      self.each_index do |i|
        if not first
            out << block.call(last,self[i],i-1,args[apos])
          apos+=1
          apos%=args.size
        end
        out << self[i]
        last = self[i]
        first=false
      end
      out
    elsif not args.empty?
      apos = 0
      self.each do |i|
        if not first
          out << args[apos]
          apos+=1
          apos%=args.size
        end
        out << i
        last = i
        first=false
      end
      out
    else
      self.each_index do |i|
        if not first
            out << block.call(last,self[i],i-1)
        end
        out << self[i]
        last = self[i]
        first=false
      end
      out
    end
  end
  def interpolate!(*args,&block)
    self.replace(self.interpolate(*args,&block))
  end

end