Nextus-Web/src/main/java/dev/coph/nextusweb/server/ratelimit/RateLimitConfig.java

package dev.coph.nextusweb.server.ratelimit;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Immutable mapping from request paths to the {@link Rule rate-limit rules} that apply to them.
 *
 * <p>Three kinds of rules can be configured, resolved with the following precedence by
 * {@link #rulesFor(String)}:</p>
 * <ol>
 *     <li>an optional <strong>global</strong> rule that applies to every request;</li>
 *     <li><strong>exact-path</strong> rules matched by exact path equality;</li>
 *     <li><strong>prefix</strong> rules matched by path prefix, evaluated longest-prefix-first.</li>
 * </ol>
 *
 * <p>A request is subject to the global rule (if any) plus the single most specific path rule
 * that matches. Instances are built through the nested {@link Builder}.</p>
 */
public final class RateLimitConfig {

    /** Rule applied to every request, or {@code null} if no global rule is configured. */
    private final Rule globalRule;
    /** Rules matched by exact path equality, keyed by path. */
    private final Map<String, Rule> exactPathRules;
    /** Prefix rules, pre-sorted longest-prefix-first so the most specific match wins. */
    private final List<PrefixRule> prefixRules;

    /**
     * Builds an immutable configuration from a {@link Builder}, copying the exact-path rules
     * and sorting the prefix rules by descending prefix length.
     *
     * @param b the builder carrying the configured rules
     */
    private RateLimitConfig(Builder b) {
        this.globalRule = b.globalRule;
        this.exactPathRules = Map.copyOf(b.exactPathRules);
        this.prefixRules = b.prefixRules.stream()
                .sorted((a, c) -> Integer.compare(c.prefix.length(), a.prefix.length()))
                .toList();
    }

    /**
     * Creates a new, empty {@link Builder}.
     *
     * @return a fresh builder
     */
    public static Builder builder() {
        return new Builder();
    }

    /**
     * Returns the ordered list of rules that apply to the given path.
     *
     * <p>The list contains the global rule first (if configured) followed by at most one
     * path-specific rule: the exact-path rule if one matches, otherwise the longest matching
     * prefix rule. The returned list may be empty if no rule applies.</p>
     *
     * @param path the request path
     * @return the applicable rules, in evaluation order
     */
    public List<Rule> rulesFor(String path) {
        List<Rule> rules = new ArrayList<>(2);
        if (globalRule != null) rules.add(globalRule);

        Rule exact = exactPathRules.get(path);
        if (exact != null) {
            rules.add(exact);
            return rules;
        }
        for (PrefixRule pr : prefixRules) {
            if (path.startsWith(pr.prefix)) {
                rules.add(pr.rule);
                return rules;
            }
        }
        return rules;
    }

    /**
     * A single rate-limit rule: a limiter, the key resolver feeding it, and a name used to
     * namespace keys and aid diagnostics.
     *
     * @param limiter     the limiter that enforces the quota
     * @param keyResolver resolves the per-request key the limiter buckets on
     * @param name        a human-readable label (e.g. {@code "global"} or a path/prefix)
     */
    public record Rule(RateLimiter limiter, KeyResolver keyResolver, String name) {
    }

    /**
     * Internal pairing of a path prefix with the rule that applies to paths starting with it.
     *
     * @param prefix the path prefix
     * @param rule   the rule to apply for matching paths
     */
    private record PrefixRule(String prefix, Rule rule) {
    }

    /**
     * Fluent builder for {@link RateLimitConfig}.
     */
    public static final class Builder {
        /** Accumulated exact-path rules, keyed by path. */
        private final Map<String, Rule> exactPathRules = new HashMap<>();
        /** Accumulated prefix rules. */
        private final List<PrefixRule> prefixRules = new ArrayList<>();
        /** The global rule, if configured. */
        private Rule globalRule;

        /**
         * Creates a builder with no rules configured. Obtain instances via
         * {@link RateLimitConfig#builder()}.
         */
        public Builder() {
        }

        /**
         * Sets the global rule applied to every request.
         *
         * @param limiter the limiter enforcing the global quota
         * @param keys    the key resolver for the global rule
         * @return this builder, for fluent chaining
         */
        public Builder global(RateLimiter limiter, KeyResolver keys) {
            this.globalRule = new Rule(limiter, keys, "global");
            return this;
        }

        /**
         * Adds a rule that applies only to requests whose path equals {@code path} exactly.
         *
         * @param path    the exact request path
         * @param limiter the limiter enforcing the quota
         * @param keys    the key resolver for this rule
         * @return this builder, for fluent chaining
         */
        public Builder forPath(String path, RateLimiter limiter, KeyResolver keys) {
            exactPathRules.put(path, new Rule(limiter, keys, path));
            return this;
        }

        /**
         * Adds a rule that applies to requests whose path starts with {@code prefix}. When
         * several prefixes match, the longest one wins.
         *
         * @param prefix  the path prefix
         * @param limiter the limiter enforcing the quota
         * @param keys    the key resolver for this rule
         * @return this builder, for fluent chaining
         */
        public Builder forPrefix(String prefix, RateLimiter limiter, KeyResolver keys) {
            prefixRules.add(new PrefixRule(prefix, new Rule(limiter, keys, prefix + "*")));
            return this;
        }

        /**
         * Builds the immutable {@link RateLimitConfig}.
         *
         * @return the configured instance
         */
        public RateLimitConfig build() {
            return new RateLimitConfig(this);
        }
    }
}