reduce

/*
 *  call-seq:
 *     enum.inject(initial, sym) => obj
 *     enum.inject(sym)          => obj
 *     enum.inject(initial) {| memo, obj | block }  => obj
 *     enum.inject          {| memo, obj | block }  => obj
 *
 *     enum.reduce(initial, sym) => obj
 *     enum.reduce(sym)          => obj
 *     enum.reduce(initial) {| memo, obj | block }  => obj
 *     enum.reduce          {| memo, obj | block }  => obj
 *
 *  Combines all elements of <i>enum</i> by applying a binary
 *  operation, specified by a block or a symbol that names a
 *  method or operator.
 *
 *  If you specify a block, then for each element in <i>enum<i>
 *  the block is passed an accumulator value (<i>memo</i>) and the element.
 *  If you specify a symbol instead, then each element in the collection
 *  will be passed to the named method of <i>memo</i>.
 *  In either case, the result becomes the new value for <i>memo</i>.
 *  At the end of the iteration, the final value of <i>memo</i> is the
 *  return value fo the method.
 *
 *  If you do not explicitly specify an <i>initial</i> value for <i>memo</i>,
 *  then uses the first element of collection is used as the initial value
 *  of <i>memo</i>.
 *
 *  Examples:
 *
 *     # Sum some numbers
 *     (5..10).reduce(:+)                            #=> 45
 *     # Same using a block and inject
 *     (5..10).inject {|sum, n| sum + n }            #=> 45
 *     # Multiply some numbers
 *     (5..10).reduce(1, :*)                         #=> 151200
 *     # Same using a block
 *     (5..10).inject(1) {|product, n| product * n } #=> 151200
 *     # find the longest word
 *     longest = %w{ cat sheep bear }.inject do |memo,word|
 *        memo.length > word.length ? memo : word
 *     end
 *     longest                                       #=> "sheep"
 *
 */
static VALUE
enum_inject(argc, argv, obj)
    int argc;
    VALUE *argv;
    VALUE obj;
{
    VALUE memo[2];
    rb_block_call_func *iter = inject_i;

    switch (rb_scan_args(argc, argv, "02", &memo[0], &memo[1])) {
      case 0:
        memo[0] = Qundef;
        break;
      case 1:
        if (rb_block_given_p()) {
            break;
        }
        memo[1] = (VALUE)rb_to_id(memo[0]);
        memo[0] = Qundef;
        iter = inject_op_i;
        break;
      case 2:
        if (rb_block_given_p()) {
            rb_warning("given block not used");
        }
        memo[1] = (VALUE)rb_to_id(memo[1]);
        iter = inject_op_i;
        break;
    }
    rb_block_call(obj, id_each, 0, 0, iter, (VALUE)memo);
    if (memo[0] == Qundef) return Qnil;
    return memo[0];
}