As you type

A search box that answers while its reader types sends a word typed halfway: crypt on its way to cryptography. With prefix, Search reads the last word of the text as the start of a word too, so the box finds every document with a word that starts with it, as the reader types each letter.

const courses = store.index<{ name: string }>('courses', { fields: { name: 1 } })
await courses.setMany([
	{ id: 1, value: { name: 'Cryptographic methods of information security' } },
	{ id: 2, value: { name: 'Cryptanalysis' } },
	{ id: 3, value: { name: 'Cryptography' } },
	{ id: 4, value: { name: 'Foundations of cryptography' } },
])
const { hits } = await courses.search({ text: 'crypt', prefix: true })
hits.map(hit => hit.id) // ['2', '3', '4', '1']

courses = store.index("courses", Course, fields={"name": 1})
await courses.set_many([("1", Course("Cryptographic methods of information security")), ("2", Course("Cryptanalysis"))])
result = await courses.search("crypt", prefix=True)

courses, err := search.OpenIndex[Course](store, "courses", search.Fields{"name": 1})
result, err := courses.Search(ctx, "crypt", search.Prefix())

let courses = store.index::<Course>("courses").field("name", 1.0).open()?;
let found = courses.search("crypt").prefix().run()?;

Without prefix, crypt is a word of its own: it finds a course named Crypt, and none of the four above.

#Which word is a prefix

Only the last word, and only while the reader may still be typing it:

A word is read as Search reads every word: in lower case, accents off, ё as е. Crypt and crypt are the same prefix, in any script.

#How a prefix ranks

The words a prefix begins count as one word in each field: a document holding two of them counts both, as it would one word written twice. They weigh half their field's weight, less than the stem of a Cyrillic word, which weighs three quarters, so a document holding the word typed comes before one holding only a longer word, unless that word is far rarer or its field far shorter.

Among the words a prefix begins, the shortest field ranks first, as BM25 ranks any word: cryptograph finds Cryptography, then Foundations of cryptography, then Cryptographic methods of information security.

#Typos and a prefix

A word typed halfway is no typo while it begins a word the index holds: cardi is one letter from cards, and still finds cardigan rather than being read as cards. A last word that begins nothing the index holds is corrected as any word is, and corrected says so: cryptograhpy reads as cryptography.

A typo inside a prefix is not forgiven as a prefix: a last word that begins nothing is read only as the whole word nearest it, under the rules of typos, and its prefix is then that word.

#Limits and defaults

A prefix reads the postings of every word it begins before the search scores, so a short prefix on an index of many documents reads more than a whole word would. The 64 most held words bound it.