# Search

NIP-50 adds one field to the filter from the previous chapter: a `search`
string. A relay that advertises the capability reads the string as a
human-readable query — `best nostr apps` — matches it against event content,
and returns results ordered by relevance rather than by `created_at`, with
`limit` applied after ranking.

Search is opt-in and implementation-defined. Relays decide whether they index events
at all, what matches, and how ranking works. The query may also carry
`key:value` extensions — `domain:`, `language:`, `sentiment:`, `nsfw:`,
`include:spam` — and a relay honors only the ones it understands, ignoring the
rest. There is no global index and no guarantee of completeness: a client
queries the relays it believes support search and accepts a partial view.

Search may be implemented relay-side, or it may be performed on a client in some
situations. This chapter provides utilities for parsing search terms along with
a very basic model for implementing search that is decoupled from filter matching
itself and entirely opt-in.

## The module

```rust {file=coracle-lib/src/lib.rs}
pub mod search;
```

```rust {file=coracle-lib/src/search.rs}
//! NIP-50 full-text search queries.
//!
//! A [`SearchQuery`] holds the terms of a search string and computes a
//! best-effort relevance score against event content — for the case where
//! search runs on the client, over events already in hand, rather than on a
//! relay.

use std::fmt;
```

## The query model

A `SearchQuery` is just the query's terms: the words split out of the search
string. NIP-50 also defines `key:value` extensions, but their meaning is
relay-defined, and the local scorer has no way to evaluate `sentiment:negative`
or `domain:example.com` without data it doesn't have. Rather than model
extensions we can't honor, we treat every token as a term. A relay that
understands an extension still sees it verbatim in the query string; the local
scorer simply matches it as text like any other word.

```rust {file=coracle-lib/src/search.rs}
/// A parsed NIP-50 search query: the terms of the query string.
///
/// NIP-50 `key:value` extensions are not modeled separately — their semantics
/// are relay-defined and cannot be evaluated locally, so each is kept as an
/// ordinary term.
#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub struct SearchQuery {
    /// The query's terms, in order.
    pub terms: Vec<String>,
}
```

### Parsing

Parsing splits the query on whitespace. Every token becomes a term, including
anything that looks like an extension. There is nothing to reject, so parsing is
total — it never errors.

```rust {file=coracle-lib/src/search.rs}
impl SearchQuery {
    /// Create an empty query.
    pub fn new() -> Self {
        SearchQuery::default()
    }

    /// Parse a raw query string by splitting it on whitespace. Every token,
    /// extension-like or not, becomes a term. Parsing never fails.
    pub fn parse(input: &str) -> Self {
        SearchQuery {
            terms: input.split_whitespace().map(str::to_string).collect(),
        }
    }

    /// True when the query has no terms.
    pub fn is_empty(&self) -> bool {
        self.terms.is_empty()
    }
}
```

Rendering joins the terms back into a query string. It is the inverse of
parsing: feeding the output of one into the other gives an equal query, modulo
runs of whitespace collapsing to single spaces.

```rust {file=coracle-lib/src/search.rs}
impl fmt::Display for SearchQuery {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(&self.terms.join(" "))
    }
}
```

## Scoring

NIP-50 returns results in descending order of relevance, so a boolean "matches
or not" is the wrong shape for a local implementation. The scorer instead
returns a number in `0.0..=1.0`, which can drive both inclusion (anything above
zero is a hit) and ordering.

The score has two parts. The base is the fraction of the query's terms that
appear in the content, compared case-insensitively — three terms, two present,
gives `2/3`. On top of that, repeated occurrences add a small, diminishing
bonus, so that among events matching the same set of terms the ones that mention
them more often rank higher. The bonus is bounded below `1/total`, which means
it can reorder events *within* a fraction but can never push a partial match up
to a full one: a missing term always costs more than any number of repetitions
can recover. An empty query — no terms — scores `1.0`, since there is no text to
constrain.

```rust {file=coracle-lib/src/search.rs}
impl SearchQuery {
    /// Score `content` against this query's terms, in `0.0..=1.0`.
    ///
    /// The base score is the fraction of the query's terms found in the content
    /// (case-insensitive substring). Repeated occurrences add a diminishing
    /// bonus, strictly less than one term's worth, so a partial match never
    /// reaches `1.0`. An empty query scores `1.0`: there is no text to match.
    pub fn score(&self, content: &str) -> f64 {
        let total = self.terms.len();
        if total == 0 {
            return 1.0;
        }

        let haystack = content.to_lowercase();

        let mut matched = 0usize;
        let mut extra = 0usize;
        for term in &self.terms {
            let needle = term.to_lowercase();
            if needle.is_empty() {
                // An empty term imposes no constraint; treat it as present.
                matched += 1;
                continue;
            }
            let count = haystack.matches(needle.as_str()).count();
            if count > 0 {
                matched += 1;
                extra += count - 1;
            }
        }

        let base = matched as f64 / total as f64;
        let bonus = (1.0 - 1.0 / (1.0 + extra as f64)) / total as f64;
        (base + bonus).min(1.0)
    }
}
```

Lowercasing uses `to_lowercase`, which folds case across Unicode rather than
only ASCII. That allocates, but nostr content is multilingual, and correctness
on non-Latin text is worth more than avoiding a copy in a best-effort matcher.

## Connecting queries to filters

The previous chapter gave `Filter` a `search` field but no way to set it. The
setters follow the established `add_*` / `clear_*` vocabulary.

```rust {file=coracle-lib/src/filters.rs}
use crate::search::SearchQuery;

impl Filter {
    /// Set the NIP-50 search query.
    pub fn add_search(mut self, search: impl Into<String>) -> Self {
        self.search = Some(search.into());
        self
    }

    /// Remove the search query, leaving no search constraint.
    pub fn clear_search(mut self) -> Self {
        self.search = None;
        self
    }
}
```

Scoring an event against a filter is then a matter of parsing the field and
delegating to `SearchQuery::score`. With no search set the method returns `1.0`,
so an unsearched filter never penalizes an event. This is purely the search
dimension — it is independent of the structural `matches` check from the
previous chapter, and the two are meant to be composed by the caller, not folded
together. A consumer that wants search-ranked results filters with `matches`,
scores with `search_score`, and sorts as it sees fit.

```rust {file=coracle-lib/src/filters.rs}
impl Filter {
    /// Best-effort local relevance score for `event`, in `0.0..=1.0`.
    ///
    /// Parses the `search` field and scores it against the event's content,
    /// returning `1.0` when there is no search. This considers *only* the
    /// `search` field; it is independent of [`matches`](Filter::matches).
    pub fn search_score(&self, event: &Event) -> f64 {
        match &self.search {
            Some(query) => SearchQuery::parse(query).score(&event.content),
            None => 1.0,
        }
    }
}
```

## What's next

Search depends on routing the query to a relay that actually supports it.
Discovering which relays advertise NIP-50, and choosing among them, is a
networking and relay-metadata concern — the subject of the Domain and Networking
sections, where relay selection is built on top of the filter types assembled
here.