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Zero-Logic Templates for Java Programmers
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package org.klojang.templates;

import org.klojang.check.Check;
import org.klojang.collections.WiredList;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.HashMap;

import static org.klojang.check.CommonChecks.gte;
import static org.klojang.templates.Setting.TMPL_CACHE_SIZE;
import static org.klojang.templates.Template.ROOT_TEMPLATE_NAME;

final class TemplateCache {

    private static final Logger LOG = LoggerFactory.getLogger(TemplateCache.class);

    public static final TemplateCache INSTANCE = new TemplateCache();

    private final HashMap cache;
    private final WiredList entries;
    private final int maxSize;

    private TemplateCache() {
        maxSize = TMPL_CACHE_SIZE.getInt();
        String s = maxSize == 0
                ? " (caching disabled)"
                : maxSize == -1 ? " (unlimited)" : "";
        LOG.trace("Template cache size: {}{}", maxSize, s);
        Check.that(maxSize, TMPL_CACHE_SIZE.property()).is(gte(), -1);
        if (maxSize == 0) {
            cache = null;
            entries = null;
        } else if (maxSize == -1) {
            cache = HashMap.newHashMap(32);
            entries = null;
        } else {
            cache = HashMap.newHashMap(maxSize);
            entries = new WiredList<>();
        }
    }

    Template get(TemplateLocation location, String name) throws ParseException {
        if (maxSize == 0 || location.isString()) {
            logTemplateRetrieval(location, name);
            return new Parser(location, name).parse();
        }
        logCacheSearch(location, name);
        Template tmpl = cache.get(location);
        if (tmpl == null) {
            LOG.trace("--> not found");
            logTemplateRetrieval(location, name);
            tmpl = new Parser(location, name).parse();
            if (maxSize != -1 && entries.size() >= maxSize) {
                TemplateLocation eldest = entries.removeLast();
                LOG.trace("Cache overflow. Evicting {}", eldest.path());
                cache.remove(eldest);
                entries.add(location);
            }
            cache.put(location, tmpl);
        } else {
            LOG.trace("--> found");
        }
        return tmpl;
    }

    private static void logTemplateRetrieval(TemplateLocation location, String name) {
        if (LOG.isTraceEnabled()) {
            if (name == ROOT_TEMPLATE_NAME) {
                if (location.path() == null) {
                    LOG.trace("Loading template {}", name);
                } else {
                    LOG.trace("Loading template {} from {}", name, location.path());
                }
            } else if (location.path() == null) {
                LOG.trace("Loading included template \"{}\"", name);
            } else {
                LOG.trace("Loading included template \"{}\" from {}", name, location);
            }
        }
    }

    private static void logCacheSearch(TemplateLocation location, String name) {
        if (LOG.isTraceEnabled()) {
            String fmt = "Searching cache for template \"{}\" (source={})";
            LOG.trace(fmt, name, location);
        }
    }

}