package gopart

// IdxRange specifies a single range. Low and High
// are the indexes in the larger collection at which this
// range begins and ends, respectively. Note that High
// is exclusive, whereas Low is inclusive.
type IdxRange struct {
	Low, High int
}

// Partition enables type-agnostic partitioning
// of anything indexable by specifying the length and
// the desired partition size of the indexable object.
// Consecutive index ranges are sent to the channel,
// each of which is the same size. The final range may
// be smaller than the others.
//
// For example, a collection with length 8 and
// partition size 3 yields ranges:
// {0, 3}, {3, 6}, {6, 8}
//
// This method should be used in a for...range loop.
// No results will be returned if the partition size is
// nonpositive. If the partition size is greater than the
// collection length, the range returned includes the
// entire collection.
func Partition(collectionLen, partitionSize int) chan IdxRange {
	c := make(chan IdxRange)
	if partitionSize <= 0 {
		close(c)
		return c
	}

	go func() {
		numFullPartitions := collectionLen / partitionSize
		var i int
		for ; i < numFullPartitions; i++ {
			c <- IdxRange{Low: i * partitionSize, High: (i + 1) * partitionSize}
		}

		if collectionLen%partitionSize != 0 { // left over
			c <- IdxRange{Low: i * partitionSize, High: collectionLen}
		}

		close(c)
	}()
	return c
}