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

import org.klojang.templates.x.Lazy;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintStream;
import java.util.List;

import static org.klojang.templates.RenderErrorCode.UNEXPECTED_ERROR;
import static org.klojang.templates.RenderUtil.stringify;
import static org.klojang.templates.TemplateUtils.getFQN;

final class Renderer {

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


  private final RenderState state;

  Renderer(RenderState state) {
    this.state = state;
  }

  void render(OutputStream out) {
    Appendable appendable = out instanceof Appendable a ? a : new PrintStream(out);
    try {
      render(state, appendable);
    } catch (IOException e) {
      throw new RenderException(UNEXPECTED_ERROR, e.toString());
    }
  }

  void render(StringBuilder sb) {
    try {
      render(state, sb);
    } catch (IOException e) {
      throw new RenderException(UNEXPECTED_ERROR, e.toString());
    }
  }

  private void render(RenderState state0, Appendable out) throws IOException {
    if (LOG.isTraceEnabled()) {
      log(state0.getSessionConfig().template());
    }
    List parts = state0.getSessionConfig().template().parts();
    for (int i = 0; i < parts.size(); ++i) {
      Part part = parts.get(i);
      if (part instanceof TextPart tp) {
        out.append(tp.text());
      } else if (part instanceof VariablePart vp) {
        if (state0.getVar(i) != null) {
          Object val = state0.getVar(i);
          if (val instanceof Lazy lazy) {
            out.append(eval(lazy, state0, vp));
          } else {
            out.append(val.toString());
          }
        }
      } else /* TemplatePart */ {
        renderNestedTemplate(out, (NestedTemplatePart) part, state0);
      }
    }
  }

  private void renderNestedTemplate(Appendable out,
        NestedTemplatePart part,
        RenderState state) throws IOException {
    SessionData sd = state.getSessionData(part.getTemplate());
    if (sd != null) {
      Template t = part.getTemplate();
      if (t.isTextOnly()) {
        // Then the RenderSession[] array will contain only null values
        // and we just want to know its length to determine the number
        // of repetitions
        String text = ((TextPart) t.parts().get(0)).text();
        for (int i = 0; i < sd.sessions().length; ++i) {
          if (i > 0) {out.append(sd.separator());}
          out.append(text);
        }
      } else {
        for (int i = 0; i < sd.sessions().length; ++i) {
          if (i > 0) {out.append(sd.separator());}
          render(sd.sessions()[i].state(), out);
        }
      }
    }
  }

  private static String eval(Lazy lazy, RenderState state, VariablePart part) {
    StringifierRegistry reg = state.getSessionConfig().stringifiers();
    Object val = lazy.value().get();
    Stringifier stringifier = reg.getStringifier(part, lazy.varGroup(), val);
    return stringify(val, stringifier, part, lazy.varGroup());
  }

  private static void log(Template t) {
    if (t.getParent() == null) {
      if (t.path().isEmpty()) {
        LOG.trace("Rendering string template");
      } else {
        LOG.trace("Rendering  {}", t.path().get());
      }
    } else if (t.location().isString()) {
      LOG.trace("Rendering inline template \"{}\"", getFQN(t));
    } else {
      LOG.trace("Rendering included template \"{}\"", getFQN(t));
    }
  }


}