attrrange

// Caller has a slice of some attributes, like a cell color that applies
// to a portion of text.
attrs := []cell.Color{cell.ColorRed, cell.ColorBlue}
redIdx := 0
blueIdx := 1

// This is the text the colors apply to.
const text = "HelloWorld"

// Assuming that we want the word "Hello" in red and the word "World" in
// green, we can set our ranges as follows:
tr := NewTracker()
if err := tr.Add(0, len("Hello"), redIdx); err != nil {
	panic(err)
}
if err := tr.Add(len("Hello")+1, len(text), blueIdx); err != nil {
	panic(err)
}

// Now to get the index into attrs (i.e. the color) for a particular
// character, we can do:
for i, c := range text {
	ar, err := tr.ForPosition(i)
	if err != nil {
		panic(err)
	}
	log.Printf("character at text[%d] = %q, color index %d = %v, range low:%d, high:%d", i, c, ar.AttrIdx, attrs[ar.AttrIdx], ar.Low, ar.High)
}