alphabet.go

package spellchecker

import "fmt"

type Alphabet struct {
	// Letters to use in alphabet. Duplicates are not allowed
	Letters string
	// Length bit count to encode alphabet
	// If it is less than rune count in letters then
	// several letters will be encoded as one bit.
	// It decreases database size for a bit
	// but drastically reduces search performance in large dictionaries
	Length int
}

var DefaultAlphabet = Alphabet{
	Letters: "abcdefghijklmnopqrstuvwxyz",
	Length:  26,
}

type alphabet map[rune]uint32

// newAlphabet create a new alphabet instance
func newAlphabet(str string, length int) (alphabet, error) {
	runes := []rune(str)
	if len(runes) == 0 {
		return nil, fmt.Errorf("unable to use empty string as alphabet")
	}

	if length > 63 {
		return nil, fmt.Errorf("alphabets longer than 63 are not supported (yet?)")
	}

	result := make(alphabet, length)
	for i, s := range runes {
		index := i % length
		if _, ok := result[s]; ok {
			return nil, fmt.Errorf("duplicate symbol %q at position %d", s, i)
		}
		result[s] = uint32(index)
	}

	return result, nil
}

func (a alphabet) encode(word []rune) bitmap {
	var b bitmap
	for _, letter := range word {
		if index, ok := a[letter]; ok {
			b.or(index)
		}
	}

	return b
}

func (a alphabet) len() int {
	return len(a)
}