// Functions for operating on polymers from day 5.
package polymer

import (
	"bytes"
	"unicode"
)

func ApplyReactions(data []byte) []byte {
	result := data
	changed := true

	for changed {
		result, changed = React(result)
	}

	return result
}

// React finds substrings of the form "xX" or "Xx" and removes them, returning
// the resulting bytes.
func React(data []byte) ([]byte, bool) {
	result := bytes.Buffer{}
	changed := false

	for i := 0; i < len(data); i++ {
		if i < len(data)-1 &&
			data[i] != data[i+1] &&
			string(bytes.ToLower([]byte{data[i]})) == string(bytes.ToLower([]byte{data[i+1]})) {

			// we've found a reduction; skip over it without writing
			//			return append(data[:i], data[i+2:]...), true
			i++
			changed = true
			continue
		}

		result.WriteByte(data[i])
	}

	// No changes possible.
	return result.Bytes(), changed
}

// StripElement removes all instances of letter or its upper-case counterpart
// from the input.
func StripElement(data []byte, strip rune) []byte {
	return bytes.Map(func(r rune) rune {
		if r == strip || r == unicode.ToUpper(strip) {
			return -1
		}
		return r
	}, data)
}